BG7TBL FA1 and FA2 frequency analyzers

[texaspyro]

The FA1 is a small, USB powered frequency counter.   You supply it with a 10 MHz reference and an input frequency of 1 .. 80 MHz.   It outputs a text string of the measured frequency every second.  They cost around $80 .. $120.

Apparently there is a PLL inside... the text string has a "PLL unlock" flag.  There is no info on how it works and I have not opened mine up.

I did a simple noise test on one where the input and ref clocks were from a Tbolt.  It looks like the measurement noise is around +/- 0.0002 Hz.

Lady Heather can now read it.  Attached is a screen capture of the noise test.

[ramon]

So how good is this device compared to others? I guess that it might be a very impressive number, otherwise you wouldn't had post this.
I can foresee a price increase on ebay of 200% due to eevblog effect.   

I have a 53131A, how do I need to configure Lady Heater to make this same test?
If I don't get a better noise ... I will be start looking the auction site to buy this new toy    

[texaspyro]

Ok, I opened it up...                                                                                       

Circuit uses:
a couple of 74LVÇ1G04 chips as input squarers.
Altera MAX-II EPM570T100CSN FPGA
Atmel Mega328 processor
FTDI USB interface
Analog Devices 8028AR dual op amp.
Some 10 pin chip with unreadable markings,
1117 voltage regulator
There are places for ISP and JTAG headers.

[jpb]

On the face of it, it would seem to be a relatively cheap means of getting ADEV plots. (Very cheap for the given accuracy.)

The PLL aspect worries me though. There is an established technique of getting accurate ADEV measurements by locking the oscillator under test to the reference and then tracking the Voltage needed to maintain lock.

http://www.ke5fx.com/tpll.htm

My guess is that it has a rational number type digital division in the PLL and maintains lock by changing the divisor or multiplier by a few bits up or down. It would be interesting to know how well it works with two separate sources - when doing a noise measurement with a split source it probably can maintain lock indefinitely but for two separate sources it may show steps or a bit of drift from time to time. For ADEV measurements the lock needs to be maintained but for a steady state measurement of frequency it will be less important.

The output indicates lost lock with an asterix (*) according to the ebay page I looked at. Texaspyro can you say how often in the output the stars appear? I suppose with a noise measurement where the same signal is applied to both terminals it should never lose lock.

[Yansi]

Interesting they could not implement it all within a 570 LE  FPGA and need an arduino to help.

[texaspyro]

I had it connected to an old Trak 8820 GPSDO that can output a programmable 1/5/10 MHz signal.  Switching between freqs caused 2 seconds of unlock messages.

The tight pll technique usually requires a DAC and ADC... there are none on the board.

As far as the FPGA vs Atmega chip...   I'm pretty sure there is floating point math and ASCII formatting going on in ATMEGA that would be a real pain in the FPGA.

Attached is a plot in measuring the Trak 8820.

[texaspyro]

I did a comparison of the performance of the TAPR-TICC/TADD2-Mini divider and the BG7TBL FA1 frequency analyzer performance.  The reference clock for both devices was a 5071A cesium.   The device being tested was a Nortel NTPX GPSDO 10 MHz output.  Attached are screen dumps of the two Lady Heather runs.

The noisy orange plot in the TICC run is the measured frequency offset from 10 MHz.   The noisy blue plot in the FA1 run is the measured frequency offset from 10 MHz.  (Note the scale factor differences)    Looking at the SPAN value (the difference between the max and min frequency measurements) shows that the FA1 noise level is around 4 times higher than the TICC.  The FA1 ADEV measurements are around 3 time higher.   The FA1 may be better than the TICC at lower (<50 second) tau,  but I have no way to verify that.  The FA1 screen dump includes a histogram of the frequency measurements.

One issue with the FA1 is that it seems to have an inherent frequency measurement bias of around -0.0002 Hz.  See the "avg#" value in the lower left corner of the plot.  I get nearly the same bias values when measuring 1, 5, and 10 MHz signals.   (I added a setting in Lady Heather to specify a measurment bias correction value).  It is interesting that the same -0.0002 Hz bias was present in the original BG7TBL GPSDO.

TAPR TICC pros:
high performance
lots of measurement and configuration options
open source design

TAPR TICC cons:
more expensive
requires external dividers to measure frequency (and a second power supply)
no case

FA1 pros:
inexpensive (around $100), decent bang for the buck
no frequency divider needed, 1 .. 80 MHz range
very simple operation (no configuration needed)
nice small unit with an extruded metal case.

FA1 cons:
only measures frequency (no way to test 1PPS signals)
around 3X-4x less performance than the TAPR TICC.
closed source "black box"
inherent minor frequency measurement offset

Lately BG7TBL has released the FA2.   This appears to be similar to the FA1, but it has some VERY nice features including a basic 1 Hz .. 200 MHz range plus a 30:1 pre-scaler allowing measurements to 6 GHz.  It as an LCD display, an internal (adjustable) OCXO and external freq ref input, reference output, selectable 0.1/1/10 second timebase.  Costs around $120 (with power supply)    If the FA2 measurement performance matches the FA1, it would be greatly preferable over the FA1.   I have an FA2 on the way.

[edigi]

From where have you ordered the FA2?
I've ordered mine close to 2 weeks ago and I don't see anything shipped yet..

By based on the description it seems to have indeed nice features, although statistics does not seem to be very well worked out, at least I'd expect some ADEV value shown + statistics of significantly more measurements not just 4 or 5 that are shown on all of the pictures that I've seen.

[texaspyro]

I did a measurement of the arrival times of the freq measurement data....  around 1110 msecs... so it looks like the consecutive measurement cycles have around a 110 msec dead time between them.

[harha]

Hi
Are you sure the fa-2 will work with lady heather-timelab?
Anyway have ordered one --when you get yours please tell us how it works
Regards
Hardy

[edigi]

It seems that FA2 units have started to ship this week (at least this has happened with what I've bought) so the lucky ones can maybe check it already next week (unfortunately shipping to my country is usually rather slow).

An interesting question to be answered would be that what the USB port at the back can be used for? None of the descriptions that I've found describe it...

[2N3055]

Hi
Are you sure the fa-2 will work with lady heather-timelab?
Anyway have ordered one --when you get yours please tell us how it works
Regards
Hardy

Are you're talking to teksaspyro ?
If yes, then yes,he should know. He wrote Lady Heather . 

[texaspyro]

An interesting question to be answered would be that what the USB port at the back can be used for? None of the descriptions that I've found describe it...

If it's like the FA1, it outputs an ASCII stream of the freq measurements.

My status show "import customs clearance completed".

[Theboel]

its a very nice equipment I hope someday they will release a box that can measure phase noise with less than 150USD tag price 

[Electro Fan]

its a very nice equipment I hope someday they will release a box that can measure phase noise with less than 150USD tag price 

Maybe BG7TBL will introduce such a product

In the meantime it will be interesting to hear from early users of the FA-2 about how it performs.

[harha]

Hi
Mine arrived at copenhagen airport--so maybe i can test it tomorrow

[texaspyro]

Miy FA2 came in today.   A very quick test shows that it works.  It does not appear to have the -0.0002 Hz freq measurement bias that the FA1 has.

The 0.1 second gate time mode is useless... there appears to be  a 300 msec dead time between measurements in 0.1 (and 10 sec) gate time modes so you only get a new reading every 400 msecs.  10 second gate time produces a reading every 10300 msecs.  1 sec gate time outputs readings every 1100 msecs (like the FA1).

It auto-detects the presence of an external reference... no way to manually switch between internal and exterrnal references.  I have not done any testing on the internal ref or the high freq channel.

The screen shows a LPF setting and a 50 ohm setting... no idea if these are switchable.... there is no documentation. 

The screen shows the input signal level.

[edigi]

The screen shows the input signal level.

Do you mean for the BNC input only or for the SMA input as well?
Even if it works only for the BNC (as I suspect), it's a nice addition...

[texaspyro]

The screen shows the input signal level.

Do you mean for the BNC input only or for the SMA input as well?
Even if it works only for the BNC (as I suspect), it's a nice addition...

I don't know... I haven't tested the SMA high freq channel yet.  I've only tried 10 MHz in.  Another thing left to try is a 1PPS signal

[texaspyro]

This helpful info from a Time Nuts user:

press and hold RST button, then MODE toggles LPF (low pass filter), GATE toggles input
impedence.

[texaspyro]

Attached is the FA2 noise floor test.  The FA2 was clocked by a HP-5071A 10 MHz output.  The FA2 frequency input was the FA2 reference output signal (which should be the 5071A output, maybe with some buffering distortion).   I didn't have the cables and T-adapter handy to split the 5071A output into the FA2 ref and signal input connectors.

The plot also shows the phase measurement of the input signal.

I am now looking at the FA2 internal oscillator performance.  Mine was shipped with it set 0.1 Hz high.  I used the tweak pot to get it within 0.003 Hz (with a little patience and twiddling you should be able to get it to +/- 0.001 Hz.   I am measuring the FA2 internal osc performance by running the FA2 with its internal osc as the reference and measuring the 5071A 10 MHz signal.

[texaspyro]

I did a test of the FA2 internal OCXO oscillator.  The FA2 uses this as the reference if you don't connect an external reference.  You can adjust the oscillator frequency with a small pot on the back panel.  As shipped the oscillator was 0.10 Hz off.  I adjusted to be within 0.003 Hz (I didn't try to fiddle it to be spot on).

I did the oscillator test by having it measure an HP-5071A cesium oscillator which is assumed to be (and is) spot on and very high quality.  Any deviations from the noise floor test that I did earlier are mostly due to the FA2 internal oscillator.

The wobbles in the frequency plot are due to air conditioning.  The oscillator changes freq by around 0.00025 Hz / degree C.  The oscillator drifted at the rate of 0.0065 Hz / day, but the drift rate is slowing down (it's now around 0.005 Hz / day).   It will be interesting to see how it ages in over the next few weeks.

[The Plumber]

Hi Tex,

Thanks for taking the time to do these measurements.  Cesium, eh? The more pedestrian among us have to settle for GPS!
Sadly I have no 10MHz output from my GPS, not without solder/surgery.
I took delivery of an FA-2 myself in the past week and noticed that I'm measuring a GPS's PPS at about 20ppb slow.
How much adjustment (turns clockwise or widdershins) did you need to apply in order to correct your OCXO?

I also notice a couple of hours before the OCXO seems to warm up to stability, speeding up  perhaps 3 ppb.
(I'm looking at ppb relative to the PPS measurements).  I'll set up some extended graphing once the garage door is fixed ;-)

[texaspyro]

How much adjustment (turns clockwise or widdershins) did you need to apply in order to correct your OCXO?

I don't really know.   The FA2 was in a bad place to get to and my screwdriver was not the right size, so I just started tweaking it while looking at my screen until the reading got close to 0.003 Hz off.  I could have got it closer to 0, but I figured that would be a waste of time until the unit settled in.   It was probably less than a turn.

I was unable to measure a Tbolt 1PPS with the FA2.   I would output intermittent 4 Hz readings, probably due to very narrow 1PPS pulse / low duty cycle.

This morning the FA2 oscillator drift is now around 0.001 Hz / day.  It's settling in faster than I expected.

[The Plumber]

I don't really know.   The FA2 was in a bad place to get to and my screwdriver was not the right size, so I just started tweaking it while looking at my screen until the reading got close to 0.003 Hz off.  I could have got it closer to 0, but I figured that would be a waste of time until the unit settled in.   It was probably less than a turn.

Thanks, I'll just have a go at making the adjustment.

I was unable to measure a Tbolt 1PPS with the FA2.   I would output intermittent 4 Hz readings, probably due to very narrow 1PPS pulse / low duty cycle.

Interesting. If your pulse is too narrow, maybe the "LPF" can actually help you.  The folks I bought the unit from claim that the magical key sequences are:

• RST key: Restart test
• RST+GATE:Enable / Disable CH1 LPF (150kHz low pass filter)
• RST+MODE,CH1 select 50ohm/1Mohm

I actually tried out the LPF myself in order to try to get rid of an artifact I see when measuring PPS.  Once in a while, (<1% of measurements) measuring PPS with a gate of 10s, I see a jump of N*(1/22) Hz, that is, 0.0909..., 0.181818..., 0.363636... added to the measured frequency.   I'm assuming that this *must* be systemic, and must have something to do with the selection of events relative to the gating window.

[cncjerry]

I wonder if that 300msec is related to the baud rate across the USB.  I thought it was sending 9,600 but I only used the arduino serial log and didn't see what it was set at, I thought 9,600. 

[texaspyro]

One big question is why the 1 Hz gate time has a dead time of 100 msec and the 0.1 Hz and 10 Hz gate times have a 300 msec dead time.

[cncjerry]

I hear there is a manual for the FA2.  Anyone have it?

[texaspyro]

I hear there is a manual for the FA2.  Anyone have it?

Manual?  For a device from China?   

So, no manual for you!   But pretty much everything has been worked out.

[cncjerry]

Mark, my expectations for this device are not higher than any others but a time-nut said someone in eevblog was sent a manual so I thought I would try.  It was probably written in Chinese anyway...

[The Plumber]

Manuel

[The Plumber]

One big question is why the 1 Hz gate time has a dead time of 100 msec and the 0.1 Hz and 10 Hz gate times have a 300 msec dead time.

I wonder whether this is an attempt to fix the apparent (1/11)Hz bug I seem to have found...

[cncjerry]

what's this 1/11 hz bug?

[texaspyro]

I did a test of the FA2 internal OCXO warm up time.  I powered mine down for 4 fours and then powered it up and measured the frequency as it warmed up.  It looks like it takes 45 minutes to stabilize close to its ultimate value. 

Note that it took a week of constant power for the oscillator drift rate to settle down.  The drift on mine seems to be around 0.00025 Hz/day.  I need to do a longer test to better weed out temperature sensitivity.

BG7TBL internal oscillator warm up time:

Minutes   Freq error
0         221 Hz
1         17 HZ
2         0.2760 Hz
3         0.0500 Hz
4         0.0350 Hz
5         0.0240 Hz
10        0.0130 Hz
15        0.0080 Hz
20        0.0060 Hz
25        0.0050 Hz
30        0.0042 Hz
35        0.0040 Hz
40        0.0034 Hz
45        0.0021 Hz <--- stable
50        0.0025 Hz
55        0.0022 Hz
60        0.0020 Hz

[FransW]

Any internal pictures available of the FA-2, indicating used components?

Thanks,

Frans

[The Plumber]

what's this 1/11 hz bug?

When measuring a GPS-referred PPS on CH1, I see intermittent measurements that appear to be small integer multiples of 1/11Hz above the actual frequency, e.g.:

[2019-10-04 22:23:46]    F:0000000001.181818179
[2019-10-04 22:25:10]    F:0000000001.090909087
[2019-10-04 23:17:22]    F:0000000001.090909088
[2019-10-04 23:17:58]    F:0000000001.181818178
[2019-10-05 00:18:58]    F:0000000001.090909088
[2019-10-05 00:22:10]    F:0000000001.090909087

[texaspyro]

The first time that I tried to set the FA2 internal oscillator frequency I tilted the case so that the screen was facing down and the adjustment screw was facing up.  I got it on frequency, but when I placed the unit with the bottom of the case facing down the oscillator was 0.032 Hz off.  So I did some more testing with the case in various orientations.  It looks like the frequency is fairly insensitive to orientation except for tilt in the front-back axis where it is around +/-1 milliHertz per 3 degrees of tilt (which seems rather high to me).  If you are using the FA2 in a mobile application and using the internal oscillator  pay close attention to the front/back tilt of the case.  Oscillator sensitivity to orientation is a well known "feature".

   F:0010000000.001619430  bottom down
   F:0010000000.001687723
   F:0010000000.001946380
   F:0010000000.001902348
   F:0010000000.001692710
   F:0010000000.001663366
   F:0010000000.001712156
   F:0010000000.001760899
   F:0010000000.001590295
   F:0010000000.001648737

   F:0010000000.001922004 bottum up
   F:0010000000.002214679
   F:0010000000.002209653
   F:0010000000.002063463
   F:0010000000.001912133
   F:0010000000.002556087
   F:0010000000.002775519
   F:0010000000.002053697
   F:0010000000.002014587
   F:0010000000.001570725

   F:0010000000.033927950 screen down
   F:0010000000.033435257
   F:0010000000.032562128
   F:0010000000.032625399
   F:0010000000.032098574
   F:0010000000.032225335
   F:0010000000.032391257
   F:0010000000.032571790
   F:0010000000.032922928

   F:0009999999.971298357  screen up
   F:0009999999.967820455
   F:0009999999.967859533
   F:0009999999.967986393
   F:0009999999.967800925
   F:0009999999.967947314
   F:0009999999.968327835
   F:0009999999.968252960
   F:0009999999.968030373

   F:0009999999.998658455  left edge down
   F:0009999999.998985308
   F:0010000000.000419484
   F:0010000000.002063510
   F:0010000000.003292634
   F:0010000000.003956045

   F:0010000000.002448722  left edge up
   F:0010000000.002029197
   F:0010000000.001917045
   F:0010000000.001985357
   F:0010000000.002012571
   F:0010000000.001980588

[texaspyro]

what's this 1/11 hz bug?

When measuring a GPS-referred PPS on CH1, I see intermittent measurements that appear to be small integer multiples of 1/11Hz above the actual frequency, e.g.:

Most PPS signals have a very skewed duty cycle which can screw up a lot of counters.   I you can, try a PPS with a 50% duty cycle.  Also, try enabling the Low Pass Filter.    My Thunderbolt PPS shows up as intermittent 4 Hz readings.

[The Plumber]

what's this 1/11 hz bug?

When measuring a GPS-referred PPS on CH1, I see intermittent measurements that appear to be small integer multiples of 1/11Hz above the actual frequency, e.g.:

Most PPS signals have a very skewed duty cycle which can screw up a lot of counters.   I you can, try a PPS with a 50% duty cycle.  Also, try enabling the Low Pass Filter.    My Thunderbolt PPS shows up as intermittent 4 Hz readings.

The vendor also suggested the LPF.  I've been running for a few days with LPF off and just switched it on a few hours ago.  I still see a couple of 1.090909...-type measurements, but time will tell whether they are less frequent.  I'll put the PPS on the scope, too.  I'd be very surprised if it were anything like square.

Intermittent 4Hz is also pretty weird, though.

[The Plumber]

The first time that I tried to set the FA2 internal oscillator frequency I tilted the case so that the screen was facing down and the adjustment screw was facing up.  I got it on frequency, but when I placed the unit with the bottom of the case facing down the oscillator was 0.032 Hz off.  So I did some more testing with the case in various orientations.  It looks like the frequency is fairly insensitive to orientation except for tilt in the front-back axis where it is around +/-1 milliHertz per 3 degrees of tilt (which seems rather high to me).  If you are using the FA2 in a mobile application and using the internal oscillator  pay close attention to the front/back tilt of the case.  Oscillator sensitivity to orientation is a well known "feature".

It's amazing that you can observe this effect using a little mail-oder counter of O(100) USD.
You'd probably need to spin a smartphone in a centrifuge for days before being to be certain of observing the same effect.
If ~1mHz works out to 0.1ppb, then yeah, that does seem rather high.  Applications under lots of acceleration need to account for this one way or another.  Speaking of which, if you're planning to corner aggressively in your FA2 mobile application, you'll suffer from a bit of offset there too.  Be sure to keep an ongoing acceleration profile if any of this matters to you :-)

[cncjerry]

As I reported on time-nuts, measuring the 1pps from my 5061b resulted in a 3hz count until I put an attenuator on/through it.  Then it dropped to 1hz.  I know the 5061b 1PPS is hot so that has something to do with it.  I also noticed that the power measured by the device didnt correlate with the before and after expected by the attenuator.  Probably something to do with the spikes.  l have to look at that 5061b signal again.  I bought a bunch of those SMA inline attenuators a while back and they work ok for this stuff.

Anybody know who BG7TBL actually is?  Have an  email?  If he/she (heard it was the latter) can do the FA2 for so cheap I can think of some other equipment for this person to make...like a DMTD comes to mind.

Jerry

[harha]

Hi
I got mine fa-2,seems to work great--upper limit with -20dBm at 7.6Ghz--Wutong Elektronics seems to be the name for the company???.
Where can i download Ladyheather supporting the fa-2?
Regards
Hardy

[texaspyro]

Where can i download Ladyheather supporting the fa-2?

I hope to get a new version out in a week or two...

[edigi]

Finally I've also received FA2 and hardly even starting the testing I've found immediately something strange.

The input impedance of the CH1 seems to be fixed to 50 Ohm (actually pretty precisely around 100 MHz checked with NanoVNA)  and the input impedance of the CH2 seems to be fixed to high impedance.
With pressing RST+MODE I can change what is shown on the display that is 50R or 1MR (50 Ohm or 1 MOhm) both for CH1 and CH2 (not supposed to be changeable for CH2, in fact by default it's not shown only after RST+MODE) but nothing is actually changed in the input impedance.
(The CH1 I've used with a BNC-SMA converter that has no 50 Ohm termination, double checked that),

This is actually pretty much the opposite how I'd expect the input impedance to be by default and that it cannot be changed is really annoying.
If CH1 would be fixed to 1MOHm and CH2 would be fixed to 50Ohm that would be still a better setup even if cannot be changed...

Can someone cross check this?

[edigi]

Actually the first clue that something is not right with the impedance of FA2 was that CH2 is showing a frequency even without anything connected. I've checked this even with a 50 Ohm termination and it's still showing some frequency that is actually not even varying too much (I'm far from anything to cause such interference).
Is this issue present only with my copy?

I've also made a longer (overnight) and a shorter measurement of it's own reference (OCXO is still in an early phase of aging). Results attached.

[harha]

No
I have the same signal--but the measured input impedances are ok.
Hardy

[edigi]

Strange. Do you mean that both CH1 and CH2 have the correct impedance and CH1 is switchable and CH2 is fixed to 50 Ohm?

It's quite easy to explain why one channel does not have the correct impedance (some manufacturing error) but it's increasingly hard to find an explanation why both channels are not OK like in my case.

[Electro Fan]

Probably/maybe not Wi-Fi but that frequency seems to be pretty close to the center frequency for Channel 108 on 5 GHz.

[harha]

Yes mine have  a fixed 50 ohm input for ch 2.works ok
Channel 1  have 50/1M ohm-- very usefull for lf.
Hardy

[edigi]

I've disassembled FA2 to see what's inside and what can be the reason for wrong impedance.
The prescaler is on a separate PCB and does not seem to have on the input side any resistor so I'm not surprised. The BNC side has some kind of switch but with so many connection points that it probably has some extra function as well.

I've made lots of high res photos (none of the components have the type scratched off, they seem to be well above that level) and I must say that I'm impressed. The whole thing is very neat and tidy and there are are lots of good quality components inside (BOM cost must be huge compared to the price). it's visible that quite much attention is paid into details (despite it seems they were in a rush to push this out).

I don't have time now to process the photos but I still attach some.

[cncjerry]

In addition to the FA2 I'm looking at the FSA3011 frequency stability analyzer.  But then I ran across this thing:

https://www.ebay.com/itm/DC5V-0-2A-1M-100M-simple-phase-noise-tester-noise-tester/122376396168?hash=item1c7e339d88:g:RbwAAOSw3v5YtmXN&redirect=mobile

This is listed as a "simple phase noise tester".  It has an input and output sma and is used with a spectrum analyzer.   I doubt very much it is really a phase noise tester.  I've used John's PN.exe with my spectrum analyzer but that is very limited to the SA noise floor unlike real PN test sets that go down to -170dBm.  So what is this thing doing?

Thanks,

Jerry

[jpb]

In addition to the FA2 I'm looking at the FSA3011 frequency stability analyzer.  But then I ran across this thing:

https://www.ebay.com/itm/DC5V-0-2A-1M-100M-simple-phase-noise-tester-noise-tester/122376396168?hash=item1c7e339d88:g:RbwAAOSw3v5YtmXN&redirect=mobile

This is listed as a "simple phase noise tester".  It has an input and output sma and is used with a spectrum analyzer.   I doubt very much it is really a phase noise tester.  I've used John's PN.exe with my spectrum analyzer but that is very limited to the SA noise floor unlike real PN test sets that go down to -170dBm.  So what is this thing doing?

Thanks,

Jerry

That has been around a while and has been discussed on this forum before, though I now can't find the relevant post. It is a frequency multiplier (with corresponding phase noise multiplication) so you can view the phase noise of say a 10MHz oscillator up at 2.6GHz.
Unfortunately it is very hard to get any details from the seller. I tried sending the seller a message and got no response.

I'm interested in the FSA3011 but it is rather pricey so it would be good to know if it works as well as advertised. If it was around half the price I might take a punt on it but at £400 plus import duty I'd prefer to spend money on a good quality USB audio interface and put together my own system. I'm a bit frustrated at present because my current setup with a good counter and using a DDS as a local oscillator is a bit too noise at around 10^-11 at 1s.

[cncjerry]

I think I am going to have to just bite the bullet and purchase a phase noise test set of some type.  I'll have all the stability test equipment I need shortly as I have another board coming.  But I also have a Cs, about 10 Rb units and two GPSDOs so stability-wise I'm covered. But I wish there was a way to measure close-in phase noise, like from 0 to 10Khz (but out to 100Khz) within the HF spectrum for less than $500.  Every time I look for a test set they are mucho $$$$.  I have used John's PN tool with my spectrum analyzers but they don't compare to the oscillators used today.  This all started when I built the N2PK VNA and I started a second one using a better oscillator.  But I couldn't measure the improvement other than a slightly better noise floor.

Not to get off topic, but if someone has an idea on how to inexpensively measure close in PN for less than $500, I would appreciate a PM.  I looked at a filter method where you used the signal in test (or another oscillator) out of phase to cancel the primary leaving just the noise.  I cant remember the challenges around that system.

By the way, I thought TVB and Corby on time-nuts tested the FSA3011.  I think the comments were inline with those here that it was a little expensive by 2x and he couldn't get it to meet the specs but I thought that was an early comment.  I would have to go back and see if there were other notes.

The FA2 is a great unit for the money.  Not good for 1pps measuring or comparison a/b of low frequencies unless there are hidden features.

Jerry

[jpb]

Re the FSA3011
Thank you for the reference to time nuts - I found this :
https://lists.febo.com/pipermail/time-nuts_lists.febo.com/2019-August/097304.html
I don't know if there is any further info later than August.

Going back a bit earlier the main one is in July:
https://www.mail-archive.com/time-nuts@lists.febo.com/msg04082.html

It does look as if it is over-priced for the claimed results. Also, if it doesn't live up to the specs then there is a danger that it fails to be good enough so it is a lot of money to  spend on something that may not be able to measure because the noise floor is too high.

[bnz]

Some info on the FSA3011 can be found at:
www.leapsecond.com/pages/FSA3011

[edigi]

Some more photos and measurement of own reference with 10s gate time.
I've noticed that OCXO is very sensitive to any vibration or movement but if it's kept stable it's pretty good.

[Electro Fan]

Those displayed numbers look pretty good.

On a related note, not sure that the info is reliable or even that it matters but fwiw, I asked an eBay seller (who sells/sold both the FA-2 and a 2018-06-05 BG7TBL GPSDO) which provides greater accuracy for a 10 MHz signal - and the answer was the BG7TBL GPSDO. 

I'm pretty happy with the BG7TBL GPSDO and I'm going to try the FA-2 using the BG7TBL GPSDO to provide the FA-2 with a 10 MHz reference and then see what the FA-2 displays for various frequencies fed from other source devices and compare those results to a couple other counters also taking the 10 MHz ref signal from the BG7TBL GPSDO and the other same source devices. Kinda hard to know what causes what but maybe it will be possible to isolate a few variables and learn something. When TEA syndrome strikes it can be hard to shake. 

[Diabolo]

Hello,


On the PCB front, we notice that D1 to D6 are not in place, and that the board has the location of 3 unassembled switches.

What can these missing elements serve?

Datasheet OCXO CTS 196xxxx : http://www.xtal.cc/UploadFiles/Product/20161101163222_40783.pdf


Cdlt,
Diabolo

[picburner]

@ edigi: the buzzer will be very weak with the protective label still attached.

[mark03]

I think I am going to have to just bite the bullet and purchase a phase noise test set of some type.  I'll have all the stability test equipment I need shortly as I have another board coming.  But I also have a Cs, about 10 Rb units and two GPSDOs so stability-wise I'm covered. But I wish there was a way to measure close-in phase noise, like from 0 to 10Khz (but out to 100Khz) within the HF spectrum for less than $500.  Every time I look for a test set they are mucho $$$$.

Something like this, perhaps?  http://www.wenzel.com/documents/measuringphasenoise.htm

[edigi]

Buzzer is my least concern (btw. it's loud enough even with protective seal on, unless you work in very noisy environment that is not too healthy).

The CH2 input impedance can be fixed with a 56 Ohm through hole resistor (we need through hole here to have some inductance that cancels it out on higher frequencies as the chip used for prescaler has 50 Ohm input impedance there; I've used a 1/8W version with short leads but I could not check the exact crossover point). While this fixes the input impedance of CH2, there is still some frequency shown for CH2 even nothing connected. In the prescaler output there should be a schmitt trigger but it's either not doing correctly its job or its something else and while driving the divided down signal connected to the main PCB it picks up noise from the prescaler.
Note: If you've checked my photos the meter is very well shielded (including front and back side PCB) so the signal that is measured cannot originate outside of the box (I've tested it even in the cellar, where even my mobile has trouble picking up any signal).
The prescaler is a /256 one (checked with DSO as I could not identify the chip). 6 wires are connecting the prescaler with the main PCB, 2 for power, 1 for the divided signal and 3 I can guess only. 2 probably is used to control division ratio (typically this kind of control is used with high divide ratio chips; probably not used here) and 1 wire that has a very strange, long burst of pulses like PWM. I suspect this has something to do with the shown frequency and probably this is what picked up by the prescaler chip. Otherwise I don't know the purpose.

The input impedance of CH2 was easy to fix (although requires some soldering experience) but I don't know what to do with CH1 without schematic or any kind  of documentation. CH1 is actually more problematic from this perspective as worst case I can swap the prescaler for CH2 (I have PCB that goes up to 11 GHz and probably even more from HMC chip series so I could even gain in the upper range with this kind of swap).
CH1 can handle quite low level signal till 270 MHz (even if frequency cannot be measured power is shown thanks to the AD8307) and a bit more if signal level is increased with sensitivity for frequency measurement dropping quite sharp from 280 MHz upwards.

Note: I've updated this post to reflect correct information based on what is learned from the standalone BG7TBL prescaler (see later posts).

[cncjerry]

I think I am going to have to just bite the bullet and purchase a phase noise test set of some type.  I'll have all the stability test equipment I need shortly as I have another board coming.  But I also have a Cs, about 10 Rb units and two GPSDOs so stability-wise I'm covered. But I wish there was a way to measure close-in phase noise, like from 0 to 10Khz (but out to 100Khz) within the HF spectrum for less than $500.  Every time I look for a test set they are mucho $$$$.

Something like this, perhaps?  http://www.wenzel.com/documents/measuringphasenoise.htm

The challenge with this and others is that you need a reference source VCO that is as good as or better than the DUT or OUT, ha.  I've played with these as they are easy to wire up on a breadboard.  Really easy to test 10Mhz as I have a bunch of decent, better than average, HP 10811's plus the one in my Cs beam for a reference.  Then you just need two switch the PLL loop constants to test 1hz, 10, 1K, 10K, etc. and using an external sound card adapter, you can get out to 100k using Spectrum Lab.

There is another version that uses a Johnson counter or quadrature hybrid (twisted wire quad hybrid by Breed et al, if IIRC) and then you get the I/Q advantages and you only need the source.  Now that one I haven't wired up but was thinking about it lately.

I guess what I'm looking for is some type of external board, already put together with high quality, high-speed ADCs, all the transformers and mixers architected like the one you suggested or the one I mentioned, for less than $500.  I can put it all together but the drawings are way over simplified because you should condition the input (those circuits are available) and I don't know if you need zero crossing for this or not.

Thanks

[jpb]

you should condition the input (those circuits are available) and I don't know if you need zero crossing for this or not.

For my experiments I've been conditioning the inputs using LTC6957 demo boards. They are expensive relative to the core chips but you get nice sma inputs/outputs.
https://www.digikey.com/product-detail/en/linear-technology-analog-devices/DC1766A-A/DC1766A-A-ND/3973495

[edigi]

I could get my hands on second copy of FA2 and naturally I've immediately started to check the input impedance.
1) CH2 is not 50 Ohm but some higher impedance so it's the same (but this is easy to fix)
2) The impedance displayed for CH2 cannot be changed with RST+MODE
3) The impedance displayed for CH1 can be changed with RST+MODE, clicking can be heard (which I could not hear earlier) and indeed the impedance is changed

Based on this it seem that my copy has some SW issue (has probably earlier wrong SW).
Is anyone aware of SW update possibility for FA2? Was it ever possible for FA1?
I fear the whole SW is closed and not even binary is available...
It seems that I have bad luck.

I've attached pictures of my wrong FA2 variant showing impedance for CH2.

[cncjerry]

you should condition the input (those circuits are available) and I don't know if you need zero crossing for this or not.

For my experiments I've been conditioning the inputs using LTC6957 demo boards. They are expensive relative to the core chips but you get nice sma inputs/outputs.
https://www.digikey.com/product-detail/en/linear-technology-analog-devices/DC1766A-A/DC1766A-A-ND/3973495

Looking at those boards, are there two inputs outputs?  It looked like from the data sheet it was an in+ and in- with corresponding outputs.

thanks - a little pricey.

[jpb]

you should condition the input (those circuits are available) and I don't know if you need zero crossing for this or not.

For my experiments I've been conditioning the inputs using LTC6957 demo boards. They are expensive relative to the core chips but you get nice sma inputs/outputs.
https://www.digikey.com/product-detail/en/linear-technology-analog-devices/DC1766A-A/DC1766A-A-ND/3973495

Looking at those boards, are there two inputs outputs?  It looked like from the data sheet it was an in+ and in- with corresponding outputs.

thanks - a little pricey.

Yes the inputs are + and - (so only one of them). I put a 50 ohm termination on the one I don't use. They are pricey (sadly) - it would be much cheaper to do your own pcb for the chips - I wanted to have something to go quickly. I was only planning to use one of them but then later I decided I needed a second (for source and reference).
There are two outputs (for the CMOS version I have) so you can supply two instruments or get a noise floor for the ref/dut inputs.

[wolfp]

My FA2 arrived yesterday. The first thing I did was translating the manual into understandable language.
Then I connected the FA2 to a Leo Bodnar GPSDO waited an hour and started measurements at 100MHz on CH1. The deviation was 1.4Hz - not bad for such a cheap device. The test is still running to observe the stability.
The next I will do is to compare the power-measurement of CH1 to a precision-generator.

BTW: The display of my FA2 seems only to be illuminated after warm-up of the VCOCXO.

Wolfgang

[edigi]

BTW: The display of my FA2 seems only to be illuminated after warm-up of the VCOCXO.

That's yet another (3rd variant) then.

Btw. The aging of the internal OCXO in the first week is around 50-100mHz (in the first days above 10mHz/day) thus precise measurement can be done only with external reference.
I didn't even bother to set the compensation in the first 2 weeks (partly because the hole at the back is not perfectly aligned with the adjustment potentiometer that makes harder to use the flat type of screwdriver).
With aging the frequency of the internal reference tends to decrease.

[Diabolo]

Actually the first clue that something is not right with the impedance of FA2 was that CH2 is showing a frequency even without anything connected. I've checked this even with a 50 Ohm termination and it's still showing some frequency that is actually not even varying too much (I'm far from anything to cause such interference).
Is this issue present only with my copy?

I've also made a longer (overnight) and a shorter measurement of it's own reference (OCXO is still in an early phase of aging). Results attached.

Hello,

I received an FA-2. Channel 2 also gives me the frequency 56xx mhz. If I shunt the CH2 input this frequency is always displayed. I think of an internal phenomenon of radiation captured when CH2 has no signal at the entrance. Maybe a shield is needed on some of the electronics. Overall this FA2 works very well and suits me.

Cdlt,
Diabolo

[wolfp]

Imho it is not unusual that these prescalers oscillate near the upper frequency if no signal is connected. I observed this also with other devices.

[wolfp]

Concerning the illumination of the display: Its not a feature but a bug. There seems to be a bad connection to the display.

[edigi]

It's pretty much common with FA2 that it displays something using CH2, maybe it's possible to get rid via connecting some small capacitor to the input of the next chip.
I have however many Fuji and Hittite prescalers and all of them work without any such phenomenon.

I've managed to make progress with the help of the seller in the wrong impedance of CH1 (which is more an issue than what signal is shown when nothing is connected).
Most probably I have a faulty relay switch. It does not switch no matter if it gets 5V (in case of 50R) or 0V (in case of 1MR).
Maybe the package had a harsh shipping or the relay switch was faulty already when it was built in and went unnoticed as even if someone checks if the unit works, it requires a bit more check if input impedance is correct or not...

I hope that the seller comes up with some reasonable solution, as even if I could replace the relay switch (probably as I have desoldering station) maybe it's not so cheap and easy to get 1 of such relay switch.

[edigi]

Actually for the record if someone faces a similar issue later on I attach a picture of the PCB and a short description that contains a significant amount of guessing.
 
Both the low and the high side of the  relay switch is controlled (the 0 volt is actually zero, if you add some load as otherwise it's more like floating). The low side control comes via R76 and Q3 (this is certain).  The high side control comes from Q1 and that ugly long line. D6 is probably for reverse current protection (the relay switch has coil).
The AD8307 measuring the power is connected together with the 50 Ohm load so it's not independent. My guess is that D9 switches the LPF.
I was wrong earlier that the relay switch has also some other purpose. No, its sole purpose is switching on the AD8307 together with the 50 Ohm input load.
U11 is probably for input signal amplification (could be easily checked with scope but sorry I was lazy).

[Diabolo]

Hello,

Here is the datasheet of the relay Takamisawa A5W-K.
- https://datasheetspdf.com/pdf-file/625329/FujitsuMicroelectronics/A5W-K/1
The relay A5W-K is for sale on Ebay, Aliexpress, etc ... for 0.50€ p/u
- https://www.ebay.fr/itm/5pcs-A5W-K-RELAIS-TAKAMISAWA-RELAY-2X-UM-5V-AUDIO-SIGNAL-DIP-10/272098686427?_trkparms=aid%3D555018%26algo%3DPL.SIM%26ao%3D1%26asc%3D20131003132420%26meid%3D12628388c8634e7bacc141318552629d%26pid%3D100005%26rk%3D1%26rkt%3D12%26mehot%3Dpf%26sd%3D272098685481%26itm%3D272098686427%26pmt%3D1%26noa%3D0%26pg%3D2047675&_trksid=p2047675.c100005.m1851
----------------
- https://fr.aliexpress.com/item/32832028594.html?spm=a2g0o.productlist.0.0.68ac75a548ZNeL&algo_pvid=75dc4962-e420-48a2-96e6-8d31d4d85755&algo_expid=75dc4962-e420-48a2-96e6-8d31d4d85755-20&btsid=9b984665-ea3a-45b5-acd9-43d1ee8716b4&ws_ab_test=searchweb0_0,searchweb201602_8,searchweb201603_52

Diabolo

[edigi]

Thanks. I've already bought a replacement in the local shop that I regularly use (+ more spares of them in Ebay, as it's cheaper but it takes weeks til it arrives). Local shop was more expensive, I've got 2 for around the same price as in your link for 5.
We'll see during the evening, as what still troubles me why is it possible change the impedance shown on the LCD for SMA input in my case? 
That's why I actually didn't want to post an update till I know more.
Btw. I never know what's available in these local shops as they are so old school that they don't even have a web page.
When I was looking for a 0603 SMD resistor they looked on me like I'm from the moon. This time when I've presented my need for this relay switch I've got it like when a roll is requested in the bakery shop...

[Diabolo]

Hello,

With the FA-2 I received, on CH2 I can not change the impedance and LPF because these 2 options are not displayed on the display in CH2 mode. So, if I press RST + Mode and RST + Gate are not enabled on CH2 with my FA-2.
-----
On the board of the facade, there are 3 switches, but the board is provided for 6 switches, what can serve the 3 switches (+6 diodes D1 to D6) uncabled, and what function would they have had if they had been wired ?


Diabolo

[Diabolo]

If one refers to the BG7TBL 6 ghz prescalator, there should be a HSMS8102 ? (HSMS8202 is ok) clipping diode on the CH2 input stage. The front panel has the slot (D7), but HSMS8202 SOT-23 is missing.


------
Topic to read, and download the PDF prescaler BG7TBL for general information, it may help.
- https://www.eevblog.com/forum/rf-microwave/bg7tbl-6-ghz-rf-prescaler/

Datasheet HSMS8102, HSMS8202 :
- http://www.farnell.com/datasheets/47811.pdf
- https://pdf1.alldatasheet.vn/datasheet-pdf/view/527215/AVAGO/HSMS-8102.html

Purchase HSMS 8202 :
- https://www.ebay.com/itm/10PCS-Diode-AVAGO-AGILENT-SOT-23-HSMS-8202-HSMS-8202-TR1-HSMS-8202-BLK-2R-2RX/162776443991?_trkparms=aid%3D1110001%26algo%3DSPLICE.SIM%26ao%3D1%26asc%3D60668%26meid%3D595ca2b77d7140cebd6f3d93df63cae5%26pid%3D100010%26rk%3D1%26rkt%3D12%26sd%3D183851485844%26itm%3D162776443991%26pmt%3D0%26noa%3D1%26pg%3D2047675&_trksid=p2047675.c100010.m2109


Diabolo

[Diabolo]

@Edigi

On your prescaler card, your resistance R35 is not Ok, you must have 10K, that is marking 103.
Look at my prescaler card in attached photo.




Diabolo

[edigi]

I've managed to fix the wrong non-switchable CH1 impedance problem.

The culprit was indeed the relay switch but it was not faulty.

After removing the switch you can see that the PCB has absolutely clear marking about the orientation.
Naturally someone totally unqualified for this kind of job managed to solder it with the wrong orientation (it makes me wonder how many percentage of the units has this mistake...)

I've attached a picture with the correct orientation (sorry for the shade, this was after the second re-solder and I could hardly wait to finish it). If you check back my earlier photo it's just the opposite way on them.

Diabolo
I don't have more time to check the prescaler now, it works at least . Based on the schematic that you've linked the chips seem to be pin compatible, but to me those don't appear as genuine ADI chips based on the marking and this prescaler seems to me a heavily simplified version (or one that is engineered down to cost).
Based both on product description and PCB the LPF and impedance change works only for CH1. I don't know why but I can still change what is displayed for both of those even for CH2 but it does not mean that it works or can be expected to work.
So far I haven't made any hacking, I've just tried to fix the product that I've received to meet its spec.

[edigi]

Topic to read, and download the PDF prescaler BG7TBL for general information, it may help.
- https://www.eevblog.com/forum/rf-microwave/bg7tbl-6-ghz-rf-prescaler/

When  you mentioned yesterday that prescaler it occurred to me that probably I have some variant of that prescaler (actually the /1000 version), although I've never bothered to check what's inside the box.
And yes I have it.
It's clearly a more complete version with genuine ADI chip.

In the FS2 prescaler the resistor you've mentioned is indeed 1k not 10k, but probably it does not matter much as it sets the charge pump current and in these prescalers only the scaled RF is used in the muxout output.
This explains also the extra 3 wires from the PCB. It's the SPI interface. The long pulse that I saw is the clock, the other is the data (I don't know why I did not see it) and the 3rd one is the LE.
The prescaler part is pretty clear now I think. They've re-used some of their older design but made it significantly simpler and cheaper.

[FransW]

Edigi & Diabolo,

A question:
did you check these experiences with either BG7TBL or the seller?

I am also wondering how your comments propagate to other buyers experiences.

Thanks,
Frans

[edigi]

I've contacted the seller with the relay switch issue (wrong/non-switchable CH1 impedance). He was helpful but at the end I had to find it myself. He offered a discount (but nothing concrete) if I buy next time in his shop.
I have no idea how to contact BG7TBL.
Also as no input protection was mentioned in the spec, we can't complain about that and after all CH2 input impedance is also 50 Ohm but just around 6GHz...

Btw. I've tried to fix it again using the new information I've learned here (also updated the original post not to mislead people; through-hole resistor is used to have some inductance that cancels it out where the chips starts to have the correct input impedance). A photo attached. I know it's not the nicest job, but I did not want to stretch this anymore after several solders (first starting with SMD).
Note: If someone has access to 6GHz VNA or 6GHz SA + directional coupler + FA2 (not a too likely combination I know) I'd not mind knowing what is the correct lead length for proper crossover...

However if the question is if am I planning to directly complain, the answer is no. In fact if you are happy with relatively precise frequency measurements this device is great deal (especially if you get one that doesn't need too many fixes).
At this price point it's probably hard to find a better package for hobbiest use. It's also possible to build one, but it's even more time (I've calibrated FA2 with my own developed GPSDO so I'm quite aware of many challenges already, actually I'm still in search of an OCXO that has good phase noise despite I have already some double oven, SC cut and Japan made you name it version).

Sure, if you're willing to spend the money (or your job depends on that), there is no question that one has to settle a branded device (even if it cost significantly more) that meets its spec out of the box and you can have a proof of calibration (another cost factor that hobbiest are not necessarily willing to take).
Btw. talking about double oven, when using this device unboxed for a while I've noticed that its reference has shown a 10mHz offset (probably due to better airflow). Thus its temperature sensitivity would be another lengthy exploration...

[Diabolo]

In the FS2 prescaler the resistor #35 you've mentioned is indeed 1k not 10k

Hello,

A marked resistance 103 is 10K. The 3rd digit indicates the number of zero, either 10 + 3 zero, or 10 000 ohms, or 10K. A resistance of 1K will be marked 102.
 ̶T̶h̶e̶ ̶r̶e̶s̶i̶s̶t̶a̶n̶c̶e̶ ̶#̶ ̶3̶5̶ ̶m̶a̶r̶k̶e̶d̶ ̶0̶1̶0̶ ̶t̶h̶a̶t̶ ̶y̶o̶u̶ ̶h̶a̶v̶e̶ ̶o̶n̶ ̶y̶o̶u̶r̶ ̶p̶r̶e̶s̶c̶a̶l̶e̶r̶ ̶i̶s̶ ̶w̶o̶r̶t̶h̶ ̶1̶ ̶o̶h̶m̶.̶
Update : I searched for resistance marked 01C. I had seen 010 on Edigi blurry picture. This resistance 01C would be a 10K, because 01C equals: 100 x 100 = 10 kOhms, so it is R35 compliant with a value of 10 kOhms marked 103.
------
I'm waiting the HSMS8202 diodes to install 1 and see how the CH2 input behaves.
------
My FA-2 works very well, but the ocxo installed seems to be an ocxo that has already been used. BG7TBL using only used ocxo in all its devices, a new ocxo costing more than the devices sold by BG7TBL. .
------
For contact BG7TBL : Email: BG7TBL@GMAIL.COM/BG7TBL@QQ.COM/BG7TBL@126.COM
------
FA-2 Service Manual: https://www.eevblog.com/forum/metrology/bg7tbl-fa1-frequency-analyzer/?action=dlattach;attach=846500


Diabolo

[Kean]

The resistance # 35 marked 010 that you have on your prescaler is worth 1 ohm.

Nope, I believe it is actually the correct value.
It is marked "01C" which is the EIA-96 marking code for a 10k 1% SMD resistor
http://kiloohm.info/eia96-resistor/96C

[edigi]

The resistance # 35 marked 010 that you have on your prescaler is worth 1 ohm.

I've used the device in the attached photo to measure (for good reasons). Since it was showing 1k and I did not solder the resistor out, it can be higher value but cannot be lower.

but the ocxo installed seems to be an ocxo that has already been used.

Unlikely, as a used OCXO has a different aging characteristics, but cannot be totally excluded. True for GPSDO modules.

[Electro Fan]

Received an FA-2.

At first I thought the FA-2 had some problems but after about an hour of playing around with it (after not having read the specs closely or the manual at all) I found that the FA-2 is a very nifty device.  The ability to see the dBm power reading on the 50 Ohm setting on Channel 1 is very instructional - mostly I purchased the FA-2 to look at frequencies but I think the power reading feature might be my favorite feature.   The LPF feature on Channel 1 is something I’m still trying to get a grip on – I think the specs say 150 kHz but I haven’t figured out how to get it to perform in predicable manner – might be operator error.

I don't have any good way to check the FA-2's accuracy but it seems to show values that are consistent with what other users here have published, and the values seem consistent with what my generator and other counters indicate.  So when I say "the device seems to work ok" that just means I can't measure with the accuracy needed to confirm the exact displayed measurements.  (There are a lot of digits displayed to the right of the decimal point on the FA-2; accuracy and resolution are two different things, of course.)

The device seems to work ok with an external 10 MHz reference signal or the internal oscillator, and it seems to output a 10 MHz reference signal ok.  I'm not sure if the 10 MHz reference output is derived from the internal oscillator or the external reference signal but it outputs the 10 MHz reference signal both when the device is attached to an external reference and when it is operating with the internal oscillator.

Channel 1 toggles between 50 Ohms and 1 MOhm.  Initially I couldn't figure out why Channel 2 seemed to show a 10 MHz input signal is about 21 MHz but I figured out that Channel 2 doesn't operate below 20 MHz. (Duh.)  When fed a signal of 20 MHz or greater it seems to operate ok (even though the spec says it only starts at 30 MHz).  Channel 2 doesn't appear to offer the ability to toggle the impedance or the LPF on and off - as per the other posts here I guess that is by design.  Also, as others have reported, with nothing attached to Channel 2 it shows a frequency - on my unit about 3.95 MHz - maybe not ideal but I can live with it.

Overall, it seems like another good and fun product offering a lot of value from BG7TBL.

[Diabolo]

Hello,

I searched for resistance marked 01C. I had seen 010 on Edigi blurry picture. This resistance 01C would be a 10K, because 01C equals: 100 x 100 = 10 kOhms, so it is R35 compliant with a value of 10 kOhms marked 103.
-----
It's not easy with all these kinds of SMD markings, because BG7TBL uses 2 kinds of SMD markings in its resistances.

Sorry
Diabolo

[Electro Fan]

Just checking to see if any FA-2 users might be up for comparing the dBm power readout on Channel 1 at 50 Ohms to see how it compares with respect to accuracy on any known power measurement devices you might have?  I've tried to account for my short cable and connectors but I'm getting lower readings on the FA-2 than I would expect.  Might have fed it a tad too much power...? 

Edit:

Or maybe I've got my red apples (dBm) confused with my green apples (dB).

The cable I'm using is a Tektronix 012-0117-00 which I think is RG223.  The spec on RG-223 indicates that the loss on the 2.5 foot cable at 20 MHz should be 0.1 dB (according to the Times Microwave calculator but let's say maybe 0.2 dB total with the two BNC connectors).

https://www.timesmicrowave.com/Calculator?Product=RG-223&RunLength=2.5&Frequency=20

So that's ~0.2 dB of cable plus connectors loss, but the the FA-2 and my generator both specify dBm (dB's relative to a milliwatt).  So if the generator says it's set for 3.0 dBm (while outputting a 20 MHz signal - on the 50 Ohm setting) and the FA-2 says it is seeing 1.9 dBm (while reading 20 MHz - on the 50 Ohm setting), is it possible that the loss of 1.1 dBm between the generator and the FA-2 is roughly equivalent to (ie, caused by) ~0.2 dB of cable and connector loss?  If I understand correctly, 1.0 dBm at 50 Ohms is 1.25 mW.  Is it possible that 2.5 feet of RG223 absorbs approximately 1.25 milliwatts?  Seems plausible but I think I need some help understanding the calculations even if that's where the 1.1 dBm went missing. 



Thanks
 

[FransW]

Hi,

Also the connectors play a role:
theoretical representation of a matched impedance connection (50 ohms) and VSWR < 1.35:1.  Under these circumstances, approximately 97.7% of incident power will be delivered to the load, thus resulting in minimal reflected power.

VSWR
The voltage standing wave ratio (VSWR) is a measure of the effective impedance of the mated connector. The higher the VSWR, the more power is reflected from the connector due to impedance mismatches. Note that VSWR is a function of frequency, and connector VSWR values should only be compared at the same frequency.

The attachment shows the max power for connectors as a function of frequency.
I could not unearth the loss per connector table. Sorry.
Anyway, the connector contributes to the total loss.

Frans

[FransW]

Found something:
https://www.digikey.com/en/articles/techzone/2018/jun/how-to-select-use-maintain-coaxial-connectors-rf-applications

[edigi]

The CH1 input impedance of FA2 is very well matched in 50 Ohm mode except at the edges of its range (lower edge: capacitive -a higher DC blocking capacitor would do good here-, upper edge: inductive; no surprises). The reflected power is at least 30dB below of the incident power in most of the range (definitely so in 20MHz) or at least with my copy. Thus impedance mismatch cannot be the source for the lower power shown.
The source for the lower power shown is either the generator (I don't know what kind of generator you've used) or the FA2 interpretation of the ADC value (AD8307 output value converted).
So far I didn't bother to check at all if it's precise at this level or not as for power measurements I have significantly better tools (that also work for a wider frequency range).

Note: For 20 MHz, the connector is of negligible significance.

[Electro Fan]

Frans, thanks for the charts.  I think they show the impact of the connectors and even any VSWR is likely to be relatively small.
I'm guessing it's something in the FA-2.
EF

[Electro Fan]

The CH1 input impedance of FA2 is very well matched in 50 Ohm mode except at the edges of its range (lower edge: capacitive -a higher DC blocking capacitor would do good here-, upper edge: inductive; no surprises). The reflected power is at least 30dB below of the incident power in most of the range (definitely so in 20MHz) or at least with my copy. Thus impedance mismatch cannot be the source for the lower power shown.
The source for the lower power shown is either the generator (I don't know what kind of generator you've used) or the FA2 interpretation of the ADC value (AD8307 output value converted).
So far I didn't bother to check at all if it's precise at this level or not as for power measurements I have significantly better tools (that also work for a wider frequency range).

Note: For 20 MHz, the connector is of negligible significance.

edigi,

Thanks.  The generator is an Agilent 33522B and it produces signals with measurements that are consistent with an Agilent 8648C, so I'm thinking the discrepancy is in the FA-2 - but I might have fed it too much power when using it.  I'm hoping someone here can compare the FA-2 dBm readings from a generator to some other dBm measuring device.

[edigi]

I've made a quick check and in case of my FA2 copy the power level indicated for CH1 is just the opposite way, that is, it's quite consistently 1.1 dB or so above the real value.
I've checked it using 2 different methods thus changing the generator output level and using SMA attenuators (that I trust) with the same level.

It would be interesting to know if this is consistently off (depending on the FA2 version; remember my FA2 copy can change both the impedance and LPF even for CH2 even though it does nothing else just the display is changed so it has its own SW issues as well) or there is a varying level of error in the displayed value depending the actual FA2 (showing the lack of any calibration).
The best would be though if the SW allowed some compensation (not just for this but also for the frequency; the current potentiometer based solution is far from ideal).

Somewhat related: There is a pretty good material related to precision frequency standards (without any math and with simple and clear terms so well understandable by anyone not even deeply technical) that I'd recommend almost to anyone (OK excepts for experts of this topic) to read in order to know what to expect from FA2 or similar precision frequency meters (using OCXO or external reference).

https://apps.dtic.mil/dtic/tr/fulltext/u2/a248503.pdf

[Electro Fan]

I've made a quick check and in case of my FA2 copy the power level indicated for CH1 is just the opposite way, that is, it's quite consistently 1.1 dB or so above the real value.
I've checked it using 2 different methods thus changing the generator output level and using SMA attenuators (that I trust) with the same level.

It would be interesting to know if this is consistently off (depending on the FA2 version; remember my FA2 copy can change both the impedance and LPF even for CH2 even though it does nothing else just the display is changed so it has its own SW issues as well) or there is a varying level of error in the displayed value depending the actual FA2 (showing the lack of any calibration).
The best would be though if the SW allowed some compensation (not just for this but also for the frequency; the current potentiometer based solution is far from ideal).

Somewhat related: There is a pretty good material related to precision frequency standards (without any math and with simple and clear terms so well understandable by anyone not even deeply technical) that I'd recommend almost to anyone (OK excepts for experts of this topic) to read in order to know what to expect from FA2 or similar precision frequency meters (using OCXO or external reference).

https://apps.dtic.mil/dtic/tr/fulltext/u2/a248503.pdf

edigi, thanks for the measurement and the link.

So based on a very small sample size (2) it looks like the dBm power reading is about +/- 1.1 dBm of actual

[texaspyro]

I know a guy that tested a FA2 with a very accurate generator and was very happy with the results. 

Also, the FA2 has two commands for calibrating the power meter:

select CH1=50R,input 10M 0dBm
$E4040*   

select CH1=50R,input 10M -20dBm
$E4141*

[Electro Fan]

I know a guy that tested a FA2 with a very accurate generator and was very happy with the results. 

Also, the FA2 has two commands for calibrating the power meter:

select CH1=50R,input 10M 0dBm
$E4040*   

select CH1=50R,input 10M -20dBm
$E4141*

That's cool!  Thanks for command info texaspyro.

[Electro Fan]

So looking at the manual and the commands:

select CH1=50R,input 10M 0dBm
$E4040*   

select CH1=50R,input 10M -20dBm
$E4141*

Just guessing but the process is to run the $E4040* command while the FA-2 is powered up and connected on Ch1 at 50 Ohms to a reliable generator outputting 10 MHz at 0 dBm?  Then save and exit?  Then repeat with $E4141*while the generator is running at -20 dBm?  Then save and exit? 

Save and exit might be $E2020* ?

Thanks

[texaspyro]

So looking at the manual and the commands:

select CH1=50R,input 10M 0dBm
$E4040*   

select CH1=50R,input 10M -20dBm
$E4141*

Just guessing but the process is to run the $E4040* command while the FA-2 is powered up and connected on Ch1 at 50 Ohms to a reliable generator outputting 10 MHz at 0 dBm?  Then save and exit?  Then repeat with $E4141*while the generator is running at -20 dBm?  Then save and exit? 

Save and exit might be $E2020* ?

Thanks

Not quite... the $Exxxx commands do the work and save the results.  So just connect the proper signal and issue the appropriate $Exxxx command.

[Electro Fan]

So looking at the manual and the commands:

select CH1=50R,input 10M 0dBm
$E4040*   

select CH1=50R,input 10M -20dBm
$E4141*

Just guessing but the process is to run the $E4040* command while the FA-2 is powered up and connected on Ch1 at 50 Ohms to a reliable generator outputting 10 MHz at 0 dBm?  Then save and exit?  Then repeat with $E4141*while the generator is running at -20 dBm?  Then save and exit? 

Save and exit might be $E2020* ?

Thanks

Not quite... the $Exxxx commands do the work and save the results.  So just connect the proper signal and issue the appropriate $Exxxx command.

So I can see all the 10 MHz counter readings scroll by in terminal mode on the PC (via USB connection).  I've turned off the FA-2 and entered the $E4040* command while the generator is outputting 10 MHz at 50 Ohms on Ch1 and then turned the FA-2 on but that didn't work.  What's the process for uploading the commands from the PC to the FA-2?  Thx

[texaspyro]

So I can see all the 10 MHz counter readings scroll by in terminal mode on the PC (via USB connection).  I've turned off the FA-2 and entered the $E4040* command while the generator is outputting 10 MHz at 50 Ohms on Ch1 and then turned the FA-2 on but that didn't work.  What's the process for uploading the commands from the PC to the FA-2?  Thx

WTF?   How can one possibly expect it to accept commands or do anything when  it is powered off?   

You enter the $Exxxx commands while the counter is powered on and spewing measurement data.  Yes, the terminal display will look like confusing crap,  but the FA2 doesn't care.

[Electro Fan]

So looking at the manual and the commands:

select CH1=50R,input 10M 0dBm
$E4040*   

select CH1=50R,input 10M -20dBm
$E4141*

Just guessing but the process is to run the $E4040* command while the FA-2 is powered up and connected on Ch1 at 50 Ohms to a reliable generator outputting 10 MHz at 0 dBm?  Then save and exit?  Then repeat with $E4141*while the generator is running at -20 dBm?  Then save and exit? 

Save and exit might be $E2020* ?

Thanks

Not quite... the $Exxxx commands do the work and save the results.  So just connect the proper signal and issue the appropriate $Exxxx command.

So I can see all the 10 MHz counter readings scroll by in terminal mode on the PC (via USB connection).  I've turned off the FA-2 and entered the $E4040* command while the generator is outputting 10 MHz at 50 Ohms on Ch1 and then turned the FA-2 on but that didn't work.  What's the process for uploading the commands from the PC to the FA-2?  Thx

Edit:

Figured it out.... Seems to be working!  Thanks!!

Looks to now be within 0.1 dBm; fluctuates between 0.0 and 0.1 dBm of mirroring the generator settings for both 0 dBm and - 20 dBm.

On a scale of 1 to 10 that is about a 10.

For anyone else trying the process, a few details.

Connect the FA-2 on Ch1 at 50 Ohms to your generator outputting 10 MHz at 50 Ohms at 0 dBm.
Connect the FA-2 via a USB port.
Open a terminal program on your PC.
If it's working on the right com port you will see the 10 MHz counter readings scroll by.
Open MS Word and enter $E4040*
Save the file as a .txt file named E4040 (or some acceptable characters).
In the terminal program use the "send a text file" command to upload the E4040 text file.
You will see an acknowledgement of the calibration text file going by among the scrolling counter info in the terminal program.
Notice that the dBM reading on the FA-2 is now very close to 0 dBm.

Repeat the process at -20 dBm.

You should be good to go with not only pretty accurate frequency counting on the FA-2 but also pretty accurate dBm power readings.

BG7TBL Rocks!  and texasypro too!

EF

[Electro Fan]

Happiness is test equipment where frequencies, amplitudes, and phases all read pretty closely across the gear.   

[texaspyro]

There is no need to send the $Exxxx commands from a text file.   You can enter them from the keyboard while the FA2 is spewing frequency readings.  Yes, it looks horrible and confusing but works just fine.

The next version of Lady Heather supports the FA2.  It has keyboard commands (in the "P" menu) for changing most of the device configuration options,  but I did not include the calibration commands (to minimize the chance of somebody accidentally screwing up their calibration).   But you can use the !u keyboard command that sends command strings to the FA2 in order to send the $Exxxx cal commands.

[edigi]

My FA2 copy has probably some kind of beta SW as when I did the power measurement calibration it did not take any effect until a power cycle (btw. I was using Putty and right clicking sent the text to FA2 that I've copied earlier to clipboard; text is not echoed back though).

What was even stranger that at the end of the power measurement calibration I've noticed a huge change in its reference frequency (~120mHz; even retrace around the time when I replaced the relay switch was only a fraction of this), so I had to calibrate the frequency of the reference again. I simply don't have an explanation for this jump (maybe I've slightly knocked the box during the power measurement calibration or is this calibration also impacting the compensation voltage of the OCXO?).

[texaspyro]

You power cycled it...   could cause oscillator retrace error.

[neil]

Ebay has a bunch of different sellers with what appear to be the same FA2 device (photo's of some show a date code printed on the front panel after the BG7TBL name, others don't - significant?). And AliExpress and probably other sources also offer the device.

I'd like to get one, and so I'd appreciate vendor recommendations.  At this price point I'm not too concerned over the small price differences of the various sources, but more interested in quality and latest version.

[Diabolo]

Hello,

I bought my FA-2 from this seller. Quick expedition. The received FA-2 operates without defects or problems. I asked PSU EU version.
- https://fr.aliexpress.com/item/4000120075382.html?spm=a2g0s.9042311.0.0.1ec66c378LS7JX

Regards.
Diabolo

[Jarl]

Thanks for the hint, Diabolo - I ordered my FA-2 from the seller you indicated. The instrument has now arrived and seems to work fine except for the missing 50 ohms resistor in the CH2 input which should be easy to fix.

As the proud owner of a few BNC T-adapters and some  pieces of coax I initially set up a noise floor test at 10 MHz. With assistance from TimeLab I recorded the ADEV at 1 and 10 sec gate time as shown in the attached file below. I find the result quite remarkable even compared to professional equipment.

I also attach my edited version of the User Manual. The chinese language has been left out in order to make reading easyer for non-chinese readers.

I now look forward to see further documentation from BG7TBL, i.e. block diagram, circuit schematic etc.

[hgl]

Probable measurement principle of the FA-2.
A VCO (SI5351A) converts the 10Mz to 100Mhz. Resolution is further increased by delay elements (MAX II LCELL) and flip-flops. Principle like the FPGA solutions.
The delays probably calibrated and stored in the EEPROM. The question is: are the devices individually calibrated during production or was a single characteristic recorded once for all?

How the delay chain works for MAX II CPLD, see here: https://iopscience.iop.org/article/10.1088/1742-659

[Vojtech]

Hello Hgl, Your link doesn't work.

[hgl]

Seems to be a temporary link ...

Search IOP for "Design of High-Precision Frequency Measure System Based
on CPLD Time Delay Unit" and use the first hit.

[Jarl]

You are probably right in your analysis of the circuit topology inside the FA-2, hgl.
 
I can add that it seems that the FA-2 has a small stationary offset of approx 0,1 mHz at 10 MHz. The offset varies with the shape of the external clock input and with the test frequency -  among probably other factors.

But just to illustrate the merits of the FA-2 in connection with ADEV tests I attach a collection of ADEV noise floor tests at 10 MHz with 1 sec gate time and external clock. In the tests I compare the SR620 counter, the TAPR TICC with 10MHz to 1 pps prescaler and the FA-2, all with the same setup and using the same external clock input. As only one laptop was available for the test, the testing was done sequentially.

[hgl]

I've done some reengineering and that's the result. As expected, the cpld increases the resolution, makes the switching of the reference sources and is used as a level converter. The first picture shows the I2C communication between MCU and SI5351 VCO.

[hgl]

When used with a terminal program FA-2 shows on Power up:
FREQ COUNTER
BG7TBL
V20190922
READ EEPROM!

Version 2.0 22.sept.2019 ?

I have also made some measurements
Noise internal ref
Noise external Rb ref
Measurement Rb with internal ref




 

[hgl]

This is a more detailed circuit. What is still missing ? power supply U1 and U3,
prescaler, relay and impedance switching, voltage reference U2 for the ocxo trimming pot,  TLC3702 dual comparator U7 where I dont know what he is good for and the chicken feed.

[Jarl]

I am very impressed by your results so far, hgl.

If possible, be specific with the exact design of the circuit of the connections into and out of the counter, since no information is available about their sensitivity to overload in the "manual".

In short:  are all input and output connections protected in any way and if so, how well protected ?

[hgl]

The BNC inputs are AC coupled and have protective resistors and diodes. The reference output is also protected. At 6 GHz input, I can't see any protection. I can see right now there are two versions and the CH1 input is different.

[hgl]

The board posted by edigi ends with 190823 and mine with 190920. Bottom right in the CH1 input area are differences.

[FransW]

These differences are not very obvious.
Except for the board numbers.
Can you please indicate what they are?

Thanks, Frans

[hgl]

The 190823 board has a 6 pin IC and the other board did not have it.

[FransW]

Spotted,
Thanks

[bnz]

For me, the noise floor by looping back the internal reference is noticeable better than the noise floor with an external reference. For the external reference I tried various setups (T-adapter, two buffered outputs) and oscillators, they give similar results.

Could that be due noise of the reference input circuity?

[hgl]

I have also observed and the external reference should have 50% duty cycle. For example, 30/70 does not work.

[bnz]

The oscillator I used with the T-adapter had sine output, the other slightly deformed sine.

[Jarl]

I get similar results as bnz when measuring Noise Floor with an external oscillator versus measuring Noise Floor with the internal oscillator.

I guess that it might be due to the short rise time of the 4 V square wave output from the CTS OCXO within the FA-2 - a rise time which could be much shorter than the rise time and noise content of the signal from the external oscillator input after it's internal processing within the FA-2.

Since the lack of long term stability of the internal oscillator seems to prevent its use for ADEV measurements beyond 100 s or so you might consider adding a phaselock circuit to the FA-2, disciplining the internal OCXO to an external, more stable reference in order to exploit the advantages of the internal OCXO for longer test periods.

This could be made with just 3 IC's and find it's place within the FA-2 case. It could improve the ADEV resolution as outlined in the test result shown by bnz and in addition compensate for the internal oscillators strong sensitivity to changes in the gravitational field. But it will unfortunately not be able to correct for the small, but notable counting offset when using the FA-2 to measure frequency.

[jpb]

The good results for the measurement of the internal oscillator against itself are interesting but, unfortunately they don't really measure a noise floor in the sense of how low a measurement of another oscillator can go as they are measuring a good correlation of a relatively noisy source. The measurements by TexasPyro show much higher ADEV when the internal source is measured relative to an external reference.
The noise floor measured with an external reference are more directly applicable as that is the setup you'd want to use with a good reference oscillator. The results are still pretty good given the price of the instrument.

According to this data sheet (CTS 196 series) :
https://www.ctscorp.com/wp-content/uploads/2015/12/OCXO196.pdf

The ADEV of the internal reference at 1s is ~ 10^-11 so though the internal correlated measurement seems to go down to 10^-12 the measurement of an uncorrelated source will be at least 10^-11 even if the source being measured is only 10^-12.

[Jarl]

Thanks for the heads up. jpb.

I have the phase lock of the internal OCXO in operation just now and the improvement in Noise Floor ADEV is as significant as could be expected, but only when using the internal OCXO to feed the CH1 input.

If I feed the CH1 input from the disciplining house standard - which has very low phase noise - the improvement in Noise Floor is much less impressive.

An interesting lesson learned....

[Jarl]

As a follow-up to jpb's comment and in order to illustrate the difference between ADEV measurements in loopback-mode and ADEV measurements on an uncorrelated source I attach a set of measurements made on a spare LPRO 101 rubidium oscillator which I have had in continuous operation for several years in my workshop. This oscillator has its own power supply and is in every way completely separated from my 10 MHz house standard.

The blue curve show the ADEV measured with the FA-2 using the 10 MHz house standard (+12 dBm, sinusoidal) connected to the external ref input.

The red curve show the same measurement but with the FA-2 OCXO being phase-locked to the house standard.

As can be seen, no visible advantage is obtained by phase-locking the OCXO to the house standard since virtually the same result is obtained by using the external ref. input directly - at least with the sort of real life signal obtained from a LPRO 101 rubidium.

Just for comparison the green curve show the noise floor of the FA-2 when looping back the undisciplined OCXO output to the CH1 input - that is just the unrealistic, correlated measurement described by jpb.

It would be of interest, if someone with access to two independent 10 MHz low phase noise sources could repeat the measurement as shown by the blue curve in order to establish the "real life limit" of ADEV measurements with the FA-2. 

[hgl]

This is a test with 4 free running OCXOs of the same type but not at the same time

[Jarl]

Many thanks to hgl for sharing your test results. They seem to be very close to the previous results of noise tests using the external reference input and the loopback-method discussed earlier.

Please tell us how your FA-2 during the present tests was controlled via it's external reference input - by what sort of reference oscillator ?. And please also tell us the type of, or technology behind the four test oscillators.

[hgl]

I have 50 Ocxo from a German company KVG produced around 2005. Parts are not labeled but the data sheet in the Attachment fits. Test oscillator was connected to external input and ref out to CH1.
I also have 9 Swiss made Rubidium Temex MCFRS01 with 4x10e-11 / month drift and can repeat the measurement but that takes time because they need a heat sink and a special connector and I can't just replace them.

[jpb]

I have 50 Ocxo from a German company KVG produced around 2005. Parts are not labeled but the data sheet in the Attachment fits. Test oscillator was connected to external input and ref out to CH1.
I also have 9 Swiss made Rubidium Temex MCFRS01 with 4x10e-11 / month drift and can repeat the measurement but that takes time because they need a heat sink and a special connector and I can't just replace them.

It would be interesting to know what the results are with the signal from the OCXO split and passed to the external input and the measurement (DUT) input.

The loop back tests give very low numbers but I'm still not sure if this translates to an equally low noise floor if, for example, two HP10811s with ADEV of less than 10^-12 were measured against each other.

[hgl]

I also tried a BNC T-coupler and a 75 Ohm distributor with my Rb and remember no relevant differences. Also no relevant differences between sine and square at the external ref input.

[edigi]

While I don't know how exactly the FA2 works or what is the exact reason for the worse ADEV value when external reference is used (I can also confirm that, checked with T-adapter), but my guess for the root cause is that the external reference is connected to a different pin of the Max II CPLD. Either the routing to that pin is noisier or the routing within the CPLD is noisier.

When measuring own reference most of the phase noise effect of the reference is taken out from the measurement. This is because when the interpolating part is measured (the time difference between the signal edge and the next reference edge) if the signal = reference (maybe with some shift) even if there is phase error, the phases for signal (= reference) and reference are moving together.

Thus when measuring own reference most of the phase noise of the reference is eliminated. That is what we're measuring is the noise of the measurement setup itself.
So while I maybe wrong with this whole theory, this means that the measurement setup for the external reference is worse.

This is unlucky, since in case of the external reference input a better precision reference can be used. However in the light of this result it seems that with external reference only the accuracy can be improved on the price of loosing some stability.
The good thing is that FA2 is still pretty good even when using external reference.

One way to cross check this theory is temporally removing the OCXO from its place (or maybe its signal output only to provide for the rest of the circuits the same load) and directly connecting an external reference there.

Some comments to older posts.

The 190823 board has a 6 pin IC and the other board did not have it.

True, but even the IC is in place, some DC blocking capacitors are not (missing also from my PCB) so net result is the same: That block probably planned to do some input signal shaping is not doing anything even in case of my variant of the board.

The delays probably calibrated and stored in the EEPROM. The question is: are the devices individually calibrated during production or was a single characteristic recorded once for all?

Probably not. In the FPGA solutions typically much better precision is aimed with a single measurement. Thus in FPGA case it matters a lot that the thermometer code is linear. In case of FA2, in statistical mode probably multiple measurements are done during the measurement period and the expectation is that even the thermometer code is not linear it averages out.

[Jarl]

Just to clarify: When using the FA-2 without external reference the signal from the internal OCXO is available on the reference output BNC, +6dBm, and sinosoidal - in spite of the oscillator itself delivering a square wave signal.

When you apply an external reference to the FA-2 - sinosoidal or square wave and with an amplitude within the limits specified in the user manual -  the signal from the external reference is available on the reference output as a sinusoidal + 7 dBm signal - instead of the signal from the internal OCXO.

This means that you unfortunately are not able to make an uncorrelated measurement of the instruments noise limit by using an external standard connected to the external ref. input and at the same time looping the signal from the external reference output back into the CH1 input because the two signals are essentially from the same source, just with som internal buffering and filtering added.

If one want to make an uncorrelated measurement of the noise level which can be obtained with the FA-2 in a "real life" measurement there is a need for two stable and independent low phase noise oscillators, one applied to the ref. input and the other applied to the CH1 input.

[edigi]

If one want to make an uncorrelated measurement of the noise level which can be obtained with the FA-2 in a "real life" measurement there is a need for two stable and independent low phase noise oscillators, one applied to the ref. input and the other applied to the CH1 input.

It's very unlikely that there is anyone on this forum who could do that measurement. Let me elaborate a bit.

Like I've explained above when correlated measurement is done (either looping back to CH1 the REF output or splitting external reference to EXT REF in and CH1) the reference phase noise impact is mostly eliminated.
That's why you see so outstanding ADEV figures. Mostly equipment measurement noise is measured (OK correlated), reference noise is excluded.
However if you'd like to measure uncorrelated noise level, the phase noise of the 2 references better to be at least one magnitude of order below of the phase noise of a typical OCXO since in that measurement you can measure only a combined noise level of FA2 and the 2 low phase noise oscillators.
That means something like BVA OCXO (maybe there is also something better/newer available but that's so much beyond my reach that I did not search that at all).
So while maybe there is somebody on this forum with a single copy of BVA OCXO (and willing to play with FA2) for that measurement you'd need 2 of them. Good luck finding someone with 2 copies of that OCXO and willing to play with FA2...

While I did not think too much of this but maybe using DDS that can output phase shifted and frequency shifted signal could be better. Not totally uncorrelated but checking parameter by parameter the impact of loosening (relaxing) the correlation a bit.
At least from equipment perspective it's more realistic, although more complex as well. Not something that I'd be willing to waste time on. After all we're talking about a 100USD frequency meter designed for and used by hobbyists.

[Jarl]

I am a bit more optimistic with respect to the resources available to members of this forum, edigi. After all we have already seen a report on a test of the FA-2 done with a Caesium-standard.

I look at the problem from a practical angle. Although the FA-2 is a cheap and not very well documented instrument, the correlated measurements show an interesting, theoretical limit of the instruments stability and resolution when doing ADEV tests with external references in loopback mode.

Now to the good question: What will be the noise limit of the FA-2 when used for some practical testing of ADEV ?

Using my 10 MHz house standard as external reference I have established the ADEV of the uncorrelated output from a spare LPRO 101 - which is well known for not-so-stellar performance with respect to phase noise. But it is an indication of the ballpark-figure to expect.  What I now would like to know is whether it would be worthwhile for me to invest in better instrumentation by comparing similar test results from other list members who has access to better references and test objects.

I respect your point of view - that it is a waste of time, but please allow me to find the problem interesting and worth further investigation.

[edigi]

An atomic frequency reference/standard is probably easier and cheaper to obtain than a really low phase noise oscillator despite that some organizations that can afford new technology (like cryogenic sapphire oscillator or alike) replace old equipments.

I at least don't have anything that could even come close to the phase noise requirements of the measurement that you've described (could try second FA2, SA reference output or some double oven OCXOs though).

[FriedLogic]

There are plenty of surplus oscillators with an ADEV well below 1E-12 at around 1s. Some of the HP10811s, Morion MV89s and many others perform that well, although most do not. Old oscillators often do not perform a well as they once did anyway, and the disposal and reclamation process that they go through can damage many more.

The problem of course is that there's no way to know for sure that they work - let alone work that well - before getting them. Anyone with a bunch of good oscillators like that which appear to work fine may well find that they have a couple of very good ones. The only way to know would be to try them.

There are some very expensive oscillators available as surplus, but unless the supplier can give recent test data for that particular oscillator, or accepts returns, it becomes a very expensive lottery.

[hgl]

https://www.ebay.com/itm/OSA-OSCILLOQUARTZ-BVA-8600-quartz-crystal-oscillator-FREQUENCY-STANDARD/174124227960?hash=item288a9cdd78:g:JnEAAOSwGzBdy6gM

[jpb]

https://www.ebay.com/itm/OSA-OSCILLOQUARTZ-BVA-8600-quartz-crystal-oscillator-FREQUENCY-STANDARD/174124227960?hash=item288a9cdd78:g:JnEAAOSwGzBdy6gM

One sold fairly recently (a few months ago) for around a third of that price (£1500 I think) - I was tempted but it was just too pricey and you have no guarantee that its performance is still at the 10^-13 level, especially as you'd need two of them to even get a measurement to find out!

[hgl]

if you have 2 and find a difference you need a third to find out which is bad 

[jpb]

Looking at the noise floor plots (for loop back) in this thread I notice something a bit different from my counter and from plots for other ADEV systems (and this is true of measurements from 3 different people), the slope of the curves go as -1/2 instead of -1.

Attached is a plot of a couple of my setups for my counter the FCA3100 which has a one shot resolution of 50 psecs.

The top curve (straight line) is for the counter just measured with a split buffer output - it starts at a bit over 10^-10 (2 x 50psecs = 10^-10 at 1 sec) and drops as a straight line with a slope of -1.
The lower curves are for two different measurements where I've used a mixer and an AWG to try and get heterodyne amplification. The noise floor is lowered but below around 10 seconds the slope is much less (nearer -1/2) and I find that real measurements in this area are a bit odd, the DUT and the Ref being swapped over give very different results.

The measurements for the FA2 have this same slope of -1/2.

According to W.J.Riley's Handbook of Frequency Stability Analysis, a slope of -1/2 corresponds to White FM while -1 corresponds to White PM. That is with a slope of -1 it indicates a fairly constant phase noise (measurement noise of phase) whilst -1/2 corresponds to noisy frequency. I guess this makes sense in that the FA2 measures frequency with a gap between (dead band) while my counter measures back-to-back phase even when measuring frequency.

[FransW]

if you have 2 and find a difference you need a third to find out which is bad 

Whay happens when you use 5 OCXO's (different make or same make), 3 CS's, 3 RB's and 2 hydrogen masers?

Selecting the most accurate one seems nearly impossible.
Just average and further rely on NIST/BIPM?

Or just extrapolate to the Planck time?

[jpb]

I think for standard time they use a collection of Caesium standards distributed about the country - but then you have the issue of transmitting time between them.

An additional problem is that if you have a collection of oscillators in the same lab then they are all subject to the same temperature/humidity and if they react in similar ways there must be a degree of correlation (in simplistic terms, if you had a bunch of OCXOs or GPSDOs and they all have similar F/T slopes) how do you get independent references? All your clocks might be in agreement but they might all be wrong together! 

[FransW]

The question was rhetoric.
The point is that absolute measurements do not exist.
You might want to have a look at:   https://de.wikipedia.org/wiki/Frequenzkamm,
a "frequency comb generator" from the Max Planck Institute for Quantum Optics.
The inventor ended up with a Nobel prize in Physics (2005).

[EV]

Here are pictures when same 10 MHz signal is connected to EXT-REF and CH1. With 10s gate time Avg is about 0.02 mHz high and with 1s gate time 0.2 mHz high.

[EV]

Here is the same test with Agilent 53131A counter.
- Number of samples 100
- Gate time 1 s
- Avg is 0.02 mHz low

Test with FA-2
- Number of samples 137
- Gate time 1s
- Avg is 0.23 mHz high

FA-2 needs longer gate time (10 s) to get same accuracy!

Has somebody else tested this?

[jpb]

Here is the same test with Agilent 53131A counter.
- Number of samples 100
- Gate time 1 s
- Avg is 0.02 mHz low

Test with FA-2
- Number of samples 137
- Gate time 1s
- Avg is 0.23 mHz high

FA-2 needs longer gate time (10 s) to get same accuracy!

Has somebody else tested this?

This is an interesting result.
The Agilent 53131A counter is 10 digits/sec according to its spec which I guess is around 10^7/10^10 = mHz but that is for a one off - you measured an average over 100 samples so it should improve to 0.1mHz or better (I think the scaling goes as sqrt(N) where N is the number of samples).
The FA-2's spec is 11 digits/sec for 10MHz - 99.999999MHz or 10 digits/sec for 1MHz-9.999999MHz so should be good for around 0.1mHz so for one off and down to 0.01mHz over 100+ samples.
But the FA-2's spec gives the "resolution" as 0.1mHz at 10MHz for a 1s gate so if this is the least significant digit then it is coming out a couple of counts high.

So on the face of it the Agilent is much better than spec (which is not surprising as I guess that they are conservative in their specs), while the FA-2 is around 2 to 3 times or more worse than spec or perhaps the spec may be accurate (the resolution and speed are as stated) but there isn't an accuracy spec.

What puzzles me is how this translates to ADEV measurements. The Agilent 53131A has a one-shot resolution of 500psecs, my counter - the FCA3100 has a one-shot resolution of 50psecs and this translates pretty closely to an ADEV noise floor of 5 x 10-11 at 1sec (which all makes sense).
The results of loop back tests on the FA-2 give a noise floor (apparently) of around 2.6 x 10-12 at 1 sec which implies an accuracy or one-shot resolution of less than 3 psecs which is ten times better than the top of the range Agilent counter (20psecs).
This doesn't make much sense to me.

I don't (yet) have an FA-2, my present ADEV setup seems to be good for around 10secs upwards and I'd like to get to around 1 sec upwards. The loop-back tests on the FA-2 implies this may be possible (what I want to measure is typically around 2 to 3 x 10^-12 at 1sec) but though the FA-2 is very cheap for what it is, I still don't want to spend £100 if it isn't going to do the job I want to do.
Similarly the much more expensive FSA3011 (around £400) promises less than 1 x 10^-12 at 1 sec but the results on time-nuts implies that this is more like 3 x 10^-12 which is much more borderline.

[EV]

Here is one test more. EXT-REF is ZYT GPSDO (10 MHz) and input signal 10 MHz is from Trimble Thunderbolt GPS.

- Gate time 10 s
- Number of samples 7953
- So test time is over 21 hours
- Avg is again 0.0246 mHz high

So last two digits are there for nothing! To get them accurate the gate time must be increased.

[edigi]

What puzzles me is how this translates to ADEV measurements. The Agilent 53131A has a one-shot resolution of 500psecs, my counter - the FCA3100 has a one-shot resolution of 50psecs and this translates pretty closely to an ADEV noise floor of 5 x 10-11 at 1sec (which all makes sense).
The results of loop back tests on the FA-2 give a noise floor (apparently) of around 2.6 x 10-12 at 1 sec which implies an accuracy or one-shot resolution of less than 3 psecs which is ten times better than the top of the range Agilent counter (20psecs).
This doesn't make much sense to me.

You cannot translate between the one-shot resolution and the ADEV value. Statistical counters make several thousands of time stamps of the signal within the measurement period in order to improve resolution beyond what is possible with a one-shot measurement.
Thus the real resolution will be much better than what is possible with a one-shot measurement (assuming that the signal measured has the needed amount of cycles).

[jpb]

Here is one test more. EXT-REF is ZYT GPSDO (10 MHz) and input signal 10 MHz is from Trimble Thunderbolt GPS.

- Gate time 10 s
- Number of samples 7953
- So test time is over 21 hours
- Avg is again 0.0246 mHz high

So last two digits are there for nothing! To get them accurate the gate time must be increased.

I guess that this is the drawback of the measurements not being back-to-back.
My counter (FCA3100 = Pendulum 95?) can do back-to-back measurements so that a high measurement will be cancelled out by a low measurement (if one period is measured too long then the next will be too short by a corresponding amount).
The FA2 measures frequency and then there is a gap before the next gate so that all the measurements can be high and it seems that this is the case, though not by very much.

The FA2 still is very good value for money, a counter < £100 compared to counters that cost £1500 or more but it is useful to know its limitations.

[jpb]

What puzzles me is how this translates to ADEV measurements. The Agilent 53131A has a one-shot resolution of 500psecs, my counter - the FCA3100 has a one-shot resolution of 50psecs and this translates pretty closely to an ADEV noise floor of 5 x 10-11 at 1sec (which all makes sense).
The results of loop back tests on the FA-2 give a noise floor (apparently) of around 2.6 x 10-12 at 1 sec which implies an accuracy or one-shot resolution of less than 3 psecs which is ten times better than the top of the range Agilent counter (20psecs).
This doesn't make much sense to me.

You cannot translate between the one-shot resolution and the ADEV value. Statistical counters make several thousands of time stamps of the signal within the measurement period in order to improve resolution beyond what is possible with a one-shot measurement.
Thus the real resolution will be much better than what is possible with a one-shot measurement (assuming that the signal measured has the needed amount of cycles).

My experience with my counter is that there is a pretty good relationship between the two.
It does do time-stamping and regression frequency measurement as well as back-to-back and I've done a lot of experimentation but even extending measurements to over a million seconds and using over-lapping ADEV does not affect the magnitude, though it does make for much smoother curves.

Measuring say 10MHz it will measure (or count 10^7) cycles in a second but the end points are only accurate to 50psecs (or 33psec resolution). The regression measurement should make the frequency a bit more accurate using time stamping but my practical experience is it doesn't improve it hugely.

Perhaps I should have said resolution rather than one shot resolution (though I think the two are closely related).
This page on leapsecond.com shows the general effect:
http://www.leapsecond.com/pages/adev-avg/

Averaging statistics will reduce the standard deviation (as sqrt(N)) and give a more accurate average value but ADEV statistics don't seem to work the same way.

[EV]

I guess that this is the drawback of the measurements not being back-to-back.
..
The FA2 still is very good value for money, a counter < £100 compared to counters that cost £1500 or more but it is useful to know its limitations.

Yes it is very useful device for the money and it has no fan. I must take power plug off if I don't want to listen Agilent's fan.

I am interested if someone can do same tests. Is this error same and always in the high side in other devices?

[edigi]

Here is one test more. EXT-REF is ZYT GPSDO (10 MHz) and input signal 10 MHz is from Trimble Thunderbolt GPS.

Have you tried reversing your setup? (Using as reference what you use as input currently, and using as input your current reference; just in case...)

[EV]

Have you tried reversing your setup? (Using as reference what you use as input currently, and using as input your current reference; just in case...)

I think it has no influence to the results. Look at Reply #147. There is same 10 MHz signal connected to EXT-REF and CH1.

[edigi]

There is same 10 MHz signal connected to EXT-REF and CH1.

I must have the golden copy of FA2 then (the one that I had to fix the relay switch  ) because doing the same test overnight it's precise even in the last digit.

[EV]

I must have the golden copy of FA2 then (the one that I had to fix the relay switch  ) because doing the same test overnight it's precise even in the last digit.

Congratulations! How have you connected the same signal to EXT-REF and CH1? I am using distribution amplifier.

If I connect the signal from REF output to CH1 I get Avg error +0.005 mHz with gate time 10 s and N=60. Maybe the error decreases with more samples.
This is much less than 0.023 mHz which I got using distribution amplifier.

[jpb]

I must have the golden copy of FA2 then (the one that I had to fix the relay switch  ) because doing the same test overnight it's precise even in the last digit.

Congratulations! How have you connected the same signal to EXT-REF and CH1? I am using distribution amplifier.

If I connect the signal from REF output to CH1 I get Avg error +0.005 mHz with gate time 10 s and N=60. Maybe the error decreases with more samples.
This is much less than 0.023 mHz which I got using distribution amplifier.

I have noticed a similar phenomenon with ADEV measurements on my counter.

I have two Oscilloquartz Star 4+ GPSDOs.

If I split the output from one and feed it to the Ref in and the input port of the counter then I find the ADEV at 1 sec is around 10^-10 or 100psecs whereas the counter is 50psecs.
If I do a loop back from the ref out to the input then I get an ADEV corresponding to 50psecs (in round numbers).

The interesting thing is, that when I measure the second GPSDO against the first the ADEV at 1 second is 5 x 10^-11 (50psecs) i.e. better than the single GPSDO when split!

My explanation (which is just a guess) is that the split case is too accurate so that it always measures one lsb too high (say) and then one lsb too low and averaging doesn't make any difference as there is no dithering because of noise. While when the second is measured there is some noise which allows the averaging to remove the quantization error. The loopback also has no noise to dither but is also got no delays etc so it doesn't matter.

[edigi]

How have you connected the same signal to EXT-REF and CH1? I am using distribution amplifier.

If I connect the signal from REF output to CH1 I get Avg error +0.005 mHz with gate time 10 s and N=60. Maybe the error decreases with more samples.
This is much less than 0.023 mHz which I got using distribution amplifier.

I just used an SMA T adapter (for the FA2 converted with BNC adapter to REF input and CH1 input). As the REF input is not 50 Ohm (but some high impedance) this means even some reflections...
As GPSDO aims for better accuracy and sacrifices noise for that (the GPS disciplining means small pushes and pulls and the controlling DAC may not have enough bits + the control wire may pick up noise) it's not a good source for this (this is why I have not used GPSDO output).
The error definitely decreases with more samples. It may actually require a huge amount of samples to go entirely to zero in some cases (or it may never happen, I don't know). I've made a new (and longer) run and what is in the attached picture is an example of that.
As the measurements results are assumed to be random (although this may not be true, could be cross checked with the distribution of measurement results, it may even worth especially in your case) in case of too few samples a short bad series can bias the average. In longer case it should not happen. The 0.005 mHz error with just N=60 is not a terrible result at all (although in my case it's probably in the range of 2-3 microHz in the worst case). Especially if you use a lesser stability source like GPSDO.
If you have very high expectations, probably you're better off with a branded device that specifies in much details what you can expect from it.
E.g. FA2 does not have the resolution for low frequency signals at all.

Update: Fixed soft photo

[FransW]

"If you have very high expectations, probably you're better off with a branded device that specifies in much details what you can expect from it.'

As far as I am aware, the forum members are busy creating the specifications within the limits of their knowledge, experience and used equipment.
So far I have seen nothing from the designer(s) but for a few relatively simple explanations.
No circuit info, no design specs, no detailed description(s), no test reports or reviews from cailbration labs and/or international standards institutes, etc.

Enjoy,  Frans

It is however a nice piece of equipment.
Time-Nuttery is a very limited subject for most of us.

[jpb]

The manual does have some vague specs as to accuracy and digits/sec on different ranges but given that the whole thing costs rather less than a calibration on an Agilent counter say it is not that surprising.

It would be nice though to have some sort of clue as to the theory of operation/circuit so we could understand its limitations or at least to satisfy our curiosity.

[mino-fm]

It would be nice though to have some sort of clue as to the theory of operation/circuit so we could understand its limitations or at least to satisfy our curiosity.

Reading the datasheet you get 8 digits/s for 1 Hz signals. I suppose it is a reciprocal counter with Fref between 100 - 200 MHz. User 'hgl' observed that Fin <= 0,6 Hz reduces the update rate to 10 s and at < 0,3 Hz display shows 0 Hz.
For higher input frequencies as 1 MHz linear regession promises three more digits, so 11 digits/s are possible. Because of CPLD's max. frequency of 200 MHz it could be possible to achieve >= 12 digits/s at this input frequency.
So far theory.

I tried to implement this theory using STM32H7xx with Fref = 240 MHz. Reciprocal counter function gives stable 8 digits/s. With higher sample rate up to 1 MS/s and linear regression over 1E6 samples (time stamps) I hoped to reach higher resolution: no succes at all! Maybe some day I'll find out what's going wrong or someone shows me the right way.
STM32F7xx and TDC7200 give 10 digits/s even at low frequencies. Decreasing update rate to 10 s - 100 s resolution will be 11 - 12 digits. This is a practicable compromise without 'magic', if you like to test other frequencies but 10.00000000000 MHz ;-)

[FransW]

There is a difference between the average, the mean, the expectation value and the "true" value.
Partly because they belong to different worlds: the classical mechanical world and the quantum mechanical world.

Resolution is not strongly connected. You can spend a lifetime (or more ...) to enhance the resolution, but this will not bring you closer tot the "true" value.

We have to accept that measurement accuracy has its limits. Primarily related to the purpose of these measurements.

Frans

[jpb]

It would be nice though to have some sort of clue as to the theory of operation/circuit so we could understand its limitations or at least to satisfy our curiosity.

Reading the datasheet you get 8 digits/s for 1 Hz signals. I suppose it is a reciprocal counter with Fref between 100 - 200 MHz. User 'hgl' observed that Fin <= 0,6 Hz reduces the update rate to 10 s and at < 0,3 Hz display shows 0 Hz.
For higher input frequencies as 1 MHz linear regession promises three more digits, so 11 digits/s are possible. Because of CPLD's max. frequency of 200 MHz it could be possible to achieve >= 12 digits/s at this input frequency.
So far theory.

I tried to implement this theory using STM32H7xx with Fref = 240 MHz. Reciprocal counter function gives stable 8 digits/s. With higher sample rate up to 1 MS/s and linear regression over 1E6 samples (time stamps) I hoped to reach higher resolution: no succes at all! Maybe some day I'll find out what's going wrong or someone shows me the right way.
STM32F7xx and TDC7200 give 10 digits/s even at low frequencies. Decreasing update rate to 10 s - 100 s resolution will be 11 - 12 digits. This is a practicable compromise without 'magic', if you like to test other frequencies but 10.00000000000 MHz ;-)

I think that the approach may not be the conventional one. For a reciprocal counter of conventional sort take the Hameg HM8123 - this has a 400MHz clock yet is restricted to 10 nsec or 9 digits/sec. My own counter, an FCA3100 which is based on a Pendulum 95(? I think) has interpolation with 50 psecs resolution and does do 12 digits a second but the ADEV noise floor is around 5x10^-11 (as I'd expect from 50 psec) whilst the FA2 seems to manage ten times better at around 3 x 10^-12.
Additionally the FA2 comes up with messages apparently (I don't have one, but this is what has been reported on this forum) indicating a loss of lock in a PLL - this may be lock to the reference of course but I think it is more than that, I think it may be how the FA2 works. That is the FA2 may have a digital PLL and locks its own reference to the DUT by adjusting R and N values (if it is an integer PLL) and then calculates the frequency for the R and N values giving lock.

Riley's system uses an approach like this:
https://www.wriley.com/A%20DDS%20Clock%20Measurement%20Module.pdf

[edigi]

I tried to implement this theory using STM32H7xx with Fref = 240 MHz. Reciprocal counter function gives stable 8 digits/s. With higher sample rate up to 1 MS/s and linear regression over 1E6 samples (time stamps) I hoped to reach higher resolution: no succes at all! Maybe some day I'll find out what's going wrong or someone shows me the right way.
STM32F7xx and TDC7200 give 10 digits/s even at low frequencies. Decreasing update rate to 10 s - 100 s resolution will be 11 - 12 digits. This is a practicable compromise without 'magic', if you like to test other frequencies but 10.00000000000 MHz ;-)

I also use TDC7200 in my own counter for the interpolating part.
Page 3 of http://n1.taur.dk/permanent/frequencymeasurement.pdf shows one way how resolution can be improved.

The resolution of FA2 is pretty impressive as it can show even small frequency changes happening much less than a 100sec (probably some kind of delay line TDC is used like shown in the code example https://cas.tudelft.nl/fpga_tdc/TDC_basic.html).

In practice however as even most of the low phase noise OCXOs have too high phase noise to fulfill the stability requirement of the 12 digit/s measurement multi-seconds averaging is needed...

[FriedLogic]

My own counter, an FCA3100 which is based on a Pendulum 95(? I think) has interpolation with 50 psecs resolution and does do 12 digits a second but the ADEV noise floor is around 5x10^-11 (as I'd expect from 50 psec) whilst the FA2 seems to manage ten times better at around 3 x 10^-12.

But do we know that the FA2 does not do any averaging or other processing? If it does, it's more comparable the averaging mode on the FCA3100 which manages a much better 12 digits/sec.

My FA2 has arrived, so when I get it up and running I'm thinking of measuring the weighting across the sample period, unless someone has already done that.

[FransW]

Did you see the upgraded version yet?
https://www.aliexpress.com/i/4000120075382.html

CH1 Frequency Accuracy (?, probably resolution):
- 0.1s Gate Time: 0.001Hz@10M
- 1s Gate Time: 0.0001Hz@10M
- 10s Gate Time: 0.00001Hz@10M
             
- CH1 Test Speed: 11 bits/second@10MHz
- CH1 Power Test Range: -50DBM to +20DBM, 1M to 550M@50 OHM
- CH1 Power Test Accuracy: 0.5DB@10MHz
- CH1 Connector Type: BNC

[jpb]

Thanks for the heads-up.
The upgraded version looks exactly the same on the outside as the non-upgraded version so if you bought one it would be tricky to know which you'd got without a frequency source above 6GHz to try it out with. (My frequency synthesizer only goes up to 3GHz.)

Edit: I notice that it has the date 20190622 (22nd June 2019) after the BG7TBL which the older model doesn't seem to have.

[EV]

I ordered also this new version from eBay by make offer method. 

[edigi]

In the upgraded version probably only the prescaler has been replaced because the rest of the specification seems to be the same.
As it's on a separate PCB, it's actually pretty easy to replace it.

[FransW]

Please keep us informed and when possible with internal pictures (see edigi's post).

Note:
BG7TBL's RF contributions over the past years are highly appreciated.
He (?) makes it possible to with limited means do measurements that would require
more expensive tools when not  made available by him.
I refer to GPSDO's, attenuators, filters, pre-scalers (1000:1 f.e.), noise generators etc.
Personally I appreciate the resolution of the FA-2. Digits!

[EV]

Please keep us informed and when possible with internal pictures (see edigi's post).

It will take about one month before I get it this new version.

[mino-fm]

For a reciprocal counter of conventional sort take the Hameg HM8123 - this has a 400MHz clock yet is restricted to 10 nsec or 9 digits/sec. My own counter, an FCA3100 which is based on a Pendulum 95(? I think) has interpolation with 50 psecs resolution and does do 12 digits a second.

To test the basic resolution of reciprocal counters you better use Fin = 1 Hz. So no statistics can embellish the results.
For HM8123 the datasheet shows resolution of one LSD = (1,25 x 10e-8s x frequency) / measurement time. At 1 Hz you can expect 8 digits/s and internal Fref should be 80 MHz I guess.
I don't know FCA3100 but an absolute resolution of 50 ps will lead to 10 digits/s @ 1 Hz.

I also use TDC7200 in my own counter for the interpolating part.
Page 3 of http://n1.taur.dk/permanent/frequencymeasurement.pdf shows one way how resolution can be improved.

You can give a practical example for regression? I know this document but I failed to get any improvement using a lot of timestamps.

[jpb]

You can give a practical example for regression? I know this document but I failed to get any improvement using a lot of timestamps.

The FCA3100 has smart frequency measurements which uses time stamps and regression for gate times between 0.1 and 100 secs. In theory it should increase the resolution (you're using all the points not just the end points), but my practical experience has been similar to yours. The results are a bit smoother so I tend to keep it turned on but it doesn't make a major difference. It might do so more at a gate of 100 secs rather than the 1 sec I use.

[edigi]

You can give a practical example for regression? I know this document but I failed to get any improvement using a lot of timestamps.

It\s not entirely clear to me what you mean by practical example. Is it about how it works or is it about what its used for?

Anyhow finding good (low phase noise) signal source is harder than the implementation itself.
This can be one very important factor why you cannot see improvement. Already 11 digits/s requires a good low phase noise SC cut OCXO (none of my TCXOs are good enough for this; OCXOs are OK).
I don't know what kind of reference and signal source of the author of that paper has used, as flicker free 12 digits/s requires clean signals that in my view are not easily obtainable on hobbyist level (I'm not willing to spend big amount of money on this). Let's not consider measuring own reference here as it can be misleading.

As for the implementation: Like described in the paper, huge amount of timestamps are needed for the measurement duration, let's say for 1 sec. If only one measurement is done, the time difference between the signal edge end the reference edge can be measured by the steps given by the TDC7200 that is ranging somewhere 54-57ps (actually the difference of 2 of such timestamps need to be used), influenced also by noise (that can kick some of the TDC measurement to near neighbor bin).  That's the resolution of the measurement.
If time-stamping is continuously done and the signal and the reference is not phase coherent, there will be semi-random time differences between the signal edge and the reference edge. Those are still measured in discrete steps due to the TDC limitation but when statistics of those sufficiently distant (corresponding to measurement nominal duration) timestamps are done, it can resolve beyond the discrete step possible by the TDC.

Several commercial/branded frequency analyzers that have time interval resolution only somewhere between the 20-100ps range can do significantly better resolution due to statistics. The single timestamp resolution of FA2 is significantly worse, thus the statistics improved resolution also doesn't come close to the state of the art equipments. For hobbyist use however in my view it's still good value (IMHO actually beyond the need of most hobbyist and even TDC7200 based ones are pretty good without any statistics), especially that hobbyist typically don't have access to very good phase noise signal sources.
As for practical use it allows easy and quick analyses of most signal (or frequency reference) sources.

[mino-fm]

It\s not entirely clear to me what you mean by practical example. Is it about how it works or is it about what its used for?

Maybe some kind of code that shows that it works. Many people are talking about timestamps and linear regression, but I want to see a positive result.  I'm still thinking that FA2 is working just that way.
As 'jpg' wrote above, in best case I get a little bit of smoothing the readings. But with fast sampling time of 1 MS/s I would expect two or three more digits. If you are interested I could show you my code for STM32H7, but it does not give any improvement.

Phase noise of TCXO or OCXO is not the problem, because I only want to improve 8 digits/s to 10 digits/s.
Using TDC7200 and STM32 I've no problem to get stable 10 digits/s - at any frequency.

[jpb]

The statistics of the variance of the slope is more complicated than I remember, e.g. see:
https://www.seas.upenn.edu/~ese302/extra_mtls/Regression_Notes.pdf
so it is not easy to see how much of an improvement it gives without sticking numbers in.

[Grandchuck]

My FA-2 arrived yesterday and it seems to work well.  Noticed something interesting:  tipping it about 30 degrees changes the displayed frequency about 8 mHz at 10 MHz.  A web search found this:  https://www.vectron.com/products/g_sensitivity/gsensitivity_index.htm

There is a link there with a slide series by John R. Vig.  I have attached one slide.

Wow, lots of metrologic fun for about $100!

[BiggieJohn]

My FA-2 arrived yesterday and it seems to work well.  Noticed something interesting:  tipping it about 30 degrees changes the displayed frequency about 8 mHz at 10 MHz.  A web search found this:  https://www.vectron.com/products/g_sensitivity/gsensitivity_index.htm

There is a link there with a slide series by John R. Vig.  I have attached one slide.

Wow, lots of metrologic fun for about $100!

Dave did a video about this in EEVblog episode #646

[Diabolo]

Hello,

Here is the explanation of the 3 switches not wired on the front of the FA-2.
BG7TBL seems to use the same front / display on several of its devices, including the 1Hz-8gHz generator WB-SG1 which has 6 switches on the front.

WB-SG1 generator link: https://fr.aliexpress.com/item/4000468601984.html?spm=a2g0w.12010615.8148356.22.11f0d5c7pfHHq1



Diabolo

[Grandchuck]

Learned some more:
 
1/ Other ovenized xtal oscillators are noticeably less sensitive to rotation when compared to the internal reference in the FA-2.
2/ Comparing a Morion unit to an Isotemp showed the Morion to be noticeably more stable.  The stability of the Morion is shown in the attached file.

The Morion is an XO00281M-CT-MV89 and the Isotemp is a OCXO44-12.

A rubidium reference was used for the data collected.

[ArthurDent]

The FA-2 looked very interesting to me so after searching on eBay for some time I found one about 400 miles from where I live in New Hampshire. At $104 delivered I couldn't pass it up and 3 days after buying it, I had it in hand. It doesn't look that bad but I do plan to add a dual diode and resistor to CH2. Here's a photo of it counting its own timebase @10S.

[Diabolo]

Hello,

CH2 signal input protection against excessive levels. Modification of the CH2 input level of the FA-2:
1x HSMS 8102 diode (dual diode schottky 10 GHz) on the space provided.
1x 47 Ohm (cms 0805) and the recovered ceramic input capacitor (cms 0603) welded in line above the board.
The 47 Ohm resistor was chosen at random without looking for any input impedance.

[][/img]

[][/img]

Diabolo

[Diabolo]

After.

Presentation of the WG-SG1 signal generator, from 1Hz to 8GHz, which can be modulated on CH2, and sweep mode on the 2 channels.
Locations are free of components, some for signal level adjustment options, etc.
The CH1 square wave (3.3V) and CH2 (0 dBm, 225mV) signal pure wave, or adjustable modulated wave, signal output level are fixed and not adjustable.

Main board:


Main board CH2:



Edit:
Following numerous requests, the WB-SG1 generator can be purchased from this serious seller for +/- 145 $ / 135 €, fast delivery without problem. Ask for the power supply adapter plug for your country.
Advice: also plan the purchase of some SMA attenuators to adjust the output signal according to your needs.
- https://fr.aliexpress.com/item/4000468601984.html?spm=a2g0w.12010615.8148356.22.11f0d5c7pfHHq1


Diabolo

[EV]

Have you found how this modulation frequecy is set? There is ON/OFF option on CH2 and modulation frequency should be set between 1Hz-1MHz, but how?

Presentation of the WG-SG1 signal generator, from 1Hz to 8GHz, which can be modulated on CH2, and sweep mode on the 2 channels.
...
Diabolo

[Grandchuck]

https://www.eevblog.com/forum/testgear/wb-sg1-wide-band-generator/

[Diabolo]

Have you found how this modulation frequecy is set? There is ON/OFF option on CH2 and modulation frequency should be set between 1Hz-1MHz, but how?

Presentation of the WG-SG1 signal generator, from 1Hz to 8GHz, which can be modulated on CH2, and sweep mode on the 2 channels.
...
Diabolo

Hello,

The CH2 modulation frequency is set via CH1. You normally enter a frequency on CH1 as if you wanted to use this frequency on CH1, and this frequency entered on CH1 will be that for CH2 modulation.

Regards.
Diabolo

[EV]

OK, Thanks Grandchuck and Diabolo!

[Diabolo]

Hello,

No one has anything new to announce about the FA-2?

Regards.
Diabolo

[EV]

I got this 12.4 GHz version today. There is no date text after BG7TBL on the front panel.
Here are pictures from the prescaler:

[Diabolo]

Hello,

The prescaler is different and its reference seems to have been deleted, and C4 is massing strangely

Diabolo

[EV]

This my new 12.4GHz version looks to be more accurate than my old 6GHz version. With 10s gate time Avg is only 0.0015 mHz high.

Here are pictures when same 10 MHz signal is connected to EXT-REF and CH1. With 10s gate time Avg is about 0.02 mHz high and with 1s gate time 0.2 mHz high.

[Diabolo]

Hello,

The CH2 input stage still has no overload protection.

Diabolo

[Jarl]

Hello,

The CH2 input stage still has no overload protection.

Diabolo

I agree, and what makes it worse: The input present some undefined high impedance at the CH2 input connector, similar to the problem with 6 GHz version, and the CH2 connector center pin is connected directly to the anonymous prescaler chip via a 100 pF series capacitor. 

As a safety measure I have soldered a 56 ohm SMD resistor directly across the pins of the CH2 input connector. It will of course not be 56 ohms at 12 GHz but hopefully it will be better than nothing.

According to the brief specification for the FA-2 in the advertisments of the 12 GHz version the CH2 input is designed for an input range of -20 dBm to + 13 dBm @ 0.5 to 5 GHz. Until further information is available you are free to guess what is permitted from 30 MHz to 500 MHz and from 5 GHz upwards...

Please also note, that the permitted input range for CH1 has been changed recently for the 6 GHz version. The max input is now specified as 2V RMS in both high impedance and in 50 ohm mode. The same values applies to the 12 GHz version.

As a final remark I can confirm the observation of EV: The 12 GHz version with firmware 2019 12 24 is indeed more accurate than the 6 GHz version. With an external standard connected to ref input and to CH1 input I get offsets in the single digit microhertz range when averaging 1 sec. readings for some time until a stable display is available. It will be interesting to see to what degree this accuracy is present when doing tests on independent sources.

Jarl

[apblog]

I just posted some general questions in the test equipment forum (edit: moved to the metrology forum) but I have some specific questions for you guys.

For a 12GHz FA-2:

How long does it take to get a reading at 0.01 Hz accuracy on a 25MHz signal?

How long does it take to warm up when paired with a BG7TBL GPSDO?

How difficult/inconvenient is it to use for general purpose work?

Thanks.

[mankan]

I started out adding input protection to channel 2 on my FA-2 6GHz. Added a HSMS8202 dual schottky diode and a 49.9 Ohm 0805 resistor but I slipped with the tweezers so now I've lost the input capacitor. What is a suitable value? I have 360pF and 1000pF available in 0603.

[mankan]

I took the 360pF one and measured channel 2 input sensitivity.

MHz	dBm
200	-21
1000	-29
3000	-28
4000	-25
5000	-15
6000	0

Then I added the 1000pF too, same results though. And I know now that I should have done these measurements before I started the whole thing. 

Still interested to hear about the original value.

[Jarl]

Mankan,

As stated in reply 195 above, the capacitor in series with the CH 2 input has a value of 100 pF.

Good luck with the repair.

Cheers

Jarl

[Jarl]

apblog,

From my experience with my single sample of the FA-2 12 GHz version you should expect around +/- 0,02 Hz "single shot" accuracy at 0,1 sec gate time and about 10 times better accuracy at 1 sec gate time when measuring a 25 MHz source on CH 1. These figures are based on the FA-2 with a GPS disciplined Rubidium oscillator as the external 10 MHz reference. The accuracy improves with time when using the statistical mode and eventually reaches the value spacified in the manual of 0.001 Hz at 10 MHz and 0,1 sec gate time, equal to 0,0025 Hz at 25 MHz.

If you plan to use a BG7TBL GPSDO as external reference you may need to consider the accuracy of this device, including warm up effects and GPS induced jitter (if any). The FA-2 itself has no warm up time if used with an external reference - at least not at the level of accuracy you are considering in your question.

Your question about using the FA-2 for general purpose work is a bit difficult to answer - it depends on your requirements.

- If you wish to explore the accuracy of the FA-2 to its full extent you need to use an external reference with at least 10 x the same accuracy.

- You would probably not want to use the FA-2 below 100 kHz due to its internal limitations below that frequency.

- You will have to accept the inconvenience of the small screen of the instrument which is a nuisance especially in the very useful statistical mode.   

- Lacking a diagram of the FA-2 we do not know at this time how it is protected from overload at the CH1 and CH2 inputs - this could turn into a problem in its daily use as a service instrument since overloads inevitably do happen from time to time.

My personal conclusion: The FA-2 is an excellent low cost instrument with remarkable data for its intended purpose: frequency analysis on a hobbyist scale. If you are after something else you may perhaps better look for a second hand, professional counter with specifications according to your measurement needs.

Cheers

Jarl

[mankan]

Thanks Jarl, 100pF 0603 is on order now. Will update as soon I've mounted one.

[apblog]

Jarl,

Thanks for helping me understand how the specs translate into real-world units, and for the info on the limitations.

I was torn between buying used pro equipment that might not be reliable and that I can't verify, or buying new hobbyist grade equipment that I could count on (haha) to work.

On the whole though it sounds like this setup will help me understand the performance of my design.

[Diabolo]

Hello,

New frequency generator 1hZ at 15 GHz from BG7TBL called WB-SG1-OPT15G 1Hz at 15 GHz! The output level setting is adjustable compared to the 8 GHz model with the output level fixed.
https://www.ebay.com/itm/WB-SG1-OPT15G-1Hz-15GHz-Signal-Source-Signal-generator-Power-ADJ-OCXO-BG7TBL/233507852253
-----
Characteristic:
overview:1Hz-15GHz wide band output,10M-15GHz with amplifiler adjust,OCXO inside，
support extern connectfrequency stanard
CH1 frequency range：1Hz-200MHz
CH1 step:1Hz-19.999999MHz/1Hz,20MHz-200MHz/10Hz
CH1 output amplifiler:3.3Vpp
CH1 plug type:BNC
CH2 frequency range：10MHz-15GHz
CH2 step:10Hz
CH2 reference output power：+5DBM- -7dBm@1G
CH2 output impedance:50 OHM
CH2 plug type:SMA
inside frequency stadnard:10MHz
inside frequency stadnard type:OCXO
inside frequency stadnard ageing:0.5Hz/year
10M frequency output power:5dBm
extern frequency input range:0dBm to +20dBm
run mode:CH1 frequency mode,CH2 frequency mode,CH1 sweep mode,CH2 sweep mode
front panel:MODE,change mode
ENT:input data
<>^V:left/right/up/down key
CH1:channel 1 output
CH2:channel 2 output
rear panel:10M REF INPUT:extern 10M input
10M REF OUTPU:10M output
USB:USB port
STD ADJ:inside OCXO frequency adjust
power switch
dc plug
power :DC11.7-12.5V,star:less than 0.5A,stabilize:less than 0.25A
size:L*H*D=106*55*105mm
weight:350g
accessory: DC12V adapter 1pcs

Regards
Diabolo

[Lemonizer]

Wow ! I don't know much in terms of high frequency signal generator, but looking at it's bare specs, if it really act like what's on paper, how much a comparable device from regular known brands cost ? 285 USD for it seems quite acceptable !

I don't have the need for something like that, but I surely want to add it to the BG7TBL little collection :>

I was wondering : Is this a good device to test the maximum bandwith of counters or scopes ? For example I have a tds 540, which goes up to 500Mhz. My best waveform gen goes up to 30Mhz (60, if I find how to hack it), so I have no way to see how would look a signal at that frequency.

[mankan]

I took the 360pF one and measured channel 2 input sensitivity.

MHz	dBm
200	-21
1000	-29
3000	-28
4000	-25
5000	-15
6000	0

Then I added the 1000pF too, same results though. And I know now that I should have done these measurements before I started the whole thing. 

Still interested to hear about the original value.

Ok, mounted a generic cheap 100pF 0603 50V MLCC:

MHz	dBm
200	-20
1000	-28
3000	-27
4000	-25
5000	-14
6000	1

So a dB less sensitive now.

[Electro Fan]

This my new 12.4GHz version looks to be more accurate than my old 6GHz version. With 10s gate time Avg is only 0.0015 mHz high.

Here are pictures when same 10 MHz signal is connected to EXT-REF and CH1. With 10s gate time Avg is about 0.02 mHz high and with 1s gate time 0.2 mHz high.

Hi EV, sorry for the late reply to this post but any chance you still have your 6GHz version to warm-up and try a similar test (same N samples with 10s gate and a similar DUT setup) so we can get a look at the results for comparison to your 12.4GHz version?  Thanks, EF

[Jarl]

Hi Electrofan

I happen to have ADEV recordings available that may answer your request.

Annexed you will find direct comparison of ADEV measurements of the FA-2, 6 GHz and 12 GHz versions. All tests are made on CH 1 with a stable 10 MHz external reference.

One set of recordings show the ADEV self test with 1 sec. gate time and the corresponding test measuring ADEV on an independent LPRO 101 Rb-standard which has been switched on and left to itself for many months.

The other set of recordings show the ADEV self test with 10 sec gate time.

It is worth noting that all the ADEV  measurements of the 12 GHz version are slightly worse compared to the corresponding measurements on the 6 GHz version. I guess that this is due to the wider CH1 bandwidth of the 12 GHz FA-2 (300 MHz vs. 200 MHz).

Cheers / Jarl

[EV]

I don't have this 6 GHz version anymore.

This my new 12.4GHz version looks to be more accurate than my old 6GHz version. With 10s gate time Avg is only 0.0015 mHz high.

Here are pictures when same 10 MHz signal is connected to EXT-REF and CH1. With 10s gate time Avg is about 0.02 mHz high and with 1s gate time 0.2 mHz high.

Hi EV, sorry for the late reply to this post but any chance you still have your 6GHz version to warm-up and try a similar test (same N samples with 10s gate and a similar DUT setup) so we can get a look at the results for comparison to your 12.4GHz version?  Thanks, EF

[Electro Fan]

Hi Electrofan

I happen to have ADEV recordings available that may answer your request.

Annexed you will find direct comparison of ADEV measurements of the FA-2, 6 GHz and 12 GHz versions. All tests are made on CH 1 with a stable 10 MHz external reference.

One set of recordings show the ADEV self test with 1 sec. gate time and the corresponding test measuring ADEV on an independent LPRO 101 Rb-standard which has been switched on and left to itself for many months.

The other set of recordings show the ADEV self test with 10 sec gate time.

It is worth noting that all the ADEV  measurements of the 12 GHz version are slightly worse compared to the corresponding measurements on the 6 GHz version. I guess that this is due to the wider CH1 bandwidth of the 12 GHz FA-2 (300 MHz vs. 200 MHz).

Cheers / Jarl

Hi Jarl,

Thanks for the data collection, graphing, and sharing.  Nice work!

I found the results different than what I expected given the impressive results EV showed with his 12 GHz FA-2 - but given your good testing maybe the explanation is that EV simply had a better DUT than what I've been using in my tests.   Assuming the 6 GHz FA-2 counter performs as well or better than the 12 GHz FA-2 counter the only other explanation beside a better DUT I guess could be that EV's 10 MHz reference was better than the GPSDOs I've been using, but it seems the more likely explanation would be that his DUT had better performance.  Thoughts?  Thx again

EV - if you happen see this, what DUT and 10 MHz reference were you using?  Thx

[Jarl]

Electro Fan

Thanks for your kind words.

I would like to add, that when making a simple self test on CH 1 on my FA-2 6 GHz and 12 GHz versions using the statistics mode I have noted that resetting the counter - take say 100 or 1000 readings - reset once again - repeat the measurement - show different offset results from one test run to the next run. My 12 GHz version is better in this respect compared to the 6 GHz version but in any case the offset is there and in my case it is quite unpredictable.

What annoys me is, that I cannot just make a self test and subtract the offset from the result of the following test on a independent source since I do not know the real internal offset value when I reset the counter to make a new measurement.

This problem do not show itself on the ADEV measurements since any initial offset do not enter into the ADEV calculations.

When using the FA-2 for precision measurements on independent sources one should take account of the accuracy and stability of the external time base of the counter. I suggest  the time base should have a precision and stability 10 times better than the object you are measuring in order to avoid influencing the result too much.

Since you get a lot of numbers from the FA-2 following the comma you may have to disregard some of the numbers since in reality they just reflect inaccuracies of the time base.

When using a simple GPS based reference you may need to extend the measurement period in order to even out the GPS reference's minor, slow variations in frequency. 

/ Jarl

[EV]

I am using ZYT QPSDO with Trimble unit as 10 MHz reference. What DUT means?

...
EV - if you happen see this, what DUT and 10 MHz reference were you using?  Thx

[jpb]

DUT = Device Under Test (in this case the oscillator being measured)

[EV]

Here is picture of the DUT attached. WB-SG1 is connected to FA-2. Both of them have ZYT GPSDO as reference connected from Extron distribution amplifier at right of them.

[Tazz]

Comparing Hgl and Diabolo boards picture posts, the PCBs of the counter and generator are common/the sames.
This is anyway clearly indicated on the pcb: F-CNT&F-SSG
Did someone see a counter with the 2019-10-20 PCB date ? (6Ghz Diabolo Gen).
Is the 15Ghz Gen version have new main PC Rev date ?

Very interesting devices, it already have a lot of history with all theses evolutions/variations. This is sad that it does not have official published schematics with revision history and binary firmware updates (at least).

[texaspyro]

I've had a BG7TBL FA2 measuring an HP 5071A cesium beam unit since September.  After a couple of months of initial warmup and aging, I adjusted the internal oscillator of the FA2 to get the reading to 10.000000000 MHz.  Six months later the reading had drifted around 0.009 Hz.   Over the last month the drift rate was less than 0.0002 Hz/month.

[mino-fm]

It would be nice though to have some sort of clue as to the theory of operation/circuit so we could understand its limitations or at least to satisfy our curiosity.

Reading the datasheet you get 8 digits/s for 1 Hz signals. I suppose it is a reciprocal counter with Fref between 100 - 200 MHz. User 'hgl' observed that Fin <= 0,6 Hz reduces the update rate to 10 s and at < 0,3 Hz display shows 0 Hz.
For higher input frequencies as 1 MHz linear regession promises three more digits, so 11 digits/s are possible. Because of CPLD's max. frequency of 200 MHz it could be possible to achieve >= 12 digits/s at this input frequency.
So far theory.

Today I'm sure FA-2 works just that way.
CPLD is able to add and multiply each single pulse up to some 10 MHz. Size of variables should be 80 or 96 bits integer to avoid overflow errors. At the end of gate time the µC has to perform math calculations with 64 bit double precision. Gaps between two samples are 0.1 - 0.3 s which is enough for a slow µC like ATmega328 to do this.
Finally you get 11 - 12 digits/s for 10 MHz signal without any magic.

Using a fast µC (STM32H750) combined with TDC7200 results in 10 digits/s. Double (64 bit) calculations are very fast, so several 10000 time stamps/s are possible, which will give further two digits/s. At last I was succesful using linear regression. Let me add one picture and a .xls-file to show results of some minutes with 1s gate time (gapless).

[vk3alb]

What am I missing? I have a FA-2 and LH windows version 6.14 beta. I've searched all over looking for a way to get LH to recognise the counter. The counter works and is sending stuff to a comport, I can select the comport / baudrate but LH is not picking up the counter. Is there a /rx to choose the FA-2?

[texaspyro]

What am I missing? I have a FA-2 and LH windows version 6.14 beta.

I have FA1/FA2 support in the next version of Heather.  Unfortunately the code is on a computer that is in a locked down building on a university campus.

[notfaded1]

I have FA1/FA2 support in the next version of Heather.  Unfortunately the code is on a computer that is in a locked down building on a university campus.

Awe bit by the plague virus again!!!  What kind of cesium do you have @texaspyro?

[bingo600]

I've had a BG7TBL FA2 measuring an HP 5071A cesium beam unit since September.

[Ringmodulator]

Tip for newbees like me:

TimeLab from KE5FX works wth the FA-2. No special configuration required.

Choose the Aquire menu: Aquire from counter in Talk-only Mode...
Choose comport
Start Measurement

(yes, kinda obvious for TimLab users)

http://www.ke5fx.com/timelab/readme.htm

Chris

[notfaded1]

Tip for newbees like me:

TimeLab from KE5FX works wth the FA-2. No special configuration required.

Choose the Aquire menu: Aquire from counter in Talk-only Mode...
Choose comport
Start Measurement

(yes, kinda obvious for TimLab users)

http://www.ke5fx.com/timelab/readme.htm

Chris

Thanks for the info!  I have an FA-2 and didn't realize timelab would recognize in talk only mode.  The 5071 Cs is a really nice design... must be for it to make it through 4 companies.

[Ringmodulator]

I was curious about a detail, that was not described in the maual:

What comes out of the 10Mhz output, when an external 10MHz signal is fed to the external 10MHz input?
Might be the 10MHz from the internal OCXO, or the external fed 10MHz, or nothig...

It turns out, that it is the external fed 10MHz, that is been sent to the extenal input. So If you feed in an external reference, you can dasychain this signal to another device without a spltter or distributor. At least worth a try.

[DM4DS]

... No special configuration required.

Choose the Aquire menu: Aquire from counter in Talk-only Mode...
Choose comport
Start Measurement
.
Chris

Any idea why this doesn´t work for me?

hterm is working.

I see :
F-CH1:0024999999.999956054
F-CH1:0024999999.999980506
F-CH1:0024999999.999990234
in the monitor.

Line Terminator 10 or 13 makes no difference, What am I missing?

Hardware is the 12Ghz version saying 20200705 at startup.

Timelab shows only: Initializing acquisition on the right side , but not Acquired etc shown.

I am the only one?

[Ringmodulator]

... No special configuration required.

Choose the Aquire menu: Aquire from counter in Talk-only Mode...
Choose comport
Start Measurement
.
Chris

Any idea why this doesn´t work for me?

hterm is working.

I see :
F-CH1:0024999999.999956054
F-CH1:0024999999.999980506
F-CH1:0024999999.999990234
in the monitor.

Line Terminator 10 or 13 makes no difference, What am I missing?

Hardware is the 12Ghz version saying 20200705 at startup.

Timelab shows only: Initializing acquisition on the right side , but not Acquired etc shown.

I am the only one?

Hi,

my unit is the 6GHz unit and the output strings have a different format.

I think, there is a possibility to taylor the input strings in timelab. I have not used this myself, so I leave the explanation up to users with better insight.

Chris

[DM4DS]

Nobody with the 12G version?

[Jarl]

Hi

I have checked with my 2019 12 24 version of the 12 GHz counter.  It has the same output format as the 6 GHz version, each line starting with
F: followed by the frequency readout in Hz.

When using TimeLab to measure frequency ADEV  I get a display as shown below. Please note the content of the various check-boxes.

It seems that BG7BTL has changed the format of the serial output in the new version. It would be of interest to know the reason for this change and to know if other features has been changed as well.

Cheers
Jarl

[rfclown]

Just received a FA-2 from eBay today. Have only measured 10 MHz so far. Ref Output was 0.11 Hz off compared to a Rubidium. So far so good. Tweeked pot to make it 0.01 Hz.

Found this forum thread and read through it. Nice to get the manual with serial commands. From what I read on the forum, I decided to measure input impedance. I haven't opened up my unit yet to see what CH2 in looks like. Calibrated to 3.5mm, so CH2 measurement is S11 at connector. For CH1, I put in a BNC to SMA adapter and didn't bother correcting for the offset since this is for lower frequencies. CH1 looks good below 500 MHz. CH2 doesn't look 50 ohms anywhere. I attached the touchstone files with .txt extension. Attached graph of return loss.

At some point I'll be doing a mod on the CH2 input.

[rfclown]

The FA-2 manual doesn’t specify any accuracy, only “resolution” (10s: 0.00001Hz@10MHz). I wanted to figure what I would consider as the accuracy for the CH1 input at 10S gate time. I connected a coax from 10 MHz REF OUTPUT to CH1. Set GATE=10S and IMP=50 ohms. Ran for one hour reading the serial port. Plotted the error (Reading-10MHz). The stated resolution is 1e-5. In the one hour run, the error for seven of 350 readings was greater than the “resolution”. For whatever reason, when reading from serial, the resolution is 1e-9. The LCD display resolution is 1e-5. I’m showing the error of the serial read resolution on the plot. I don’t know why they spew out so many digits when they are meaningless. Anyway, for my purposes based on this data, for a 10S gate time on CH1, I’m going to round to 1e-4. That gives me a 12 digit counter that’s dead on for $130. I bought this on eBay from a US seller so I wouldn’t have to wait. I tweaked the Ref pot on this (and my two Rubidiums) the other day based on a GPSDO that I finally got from China. For years I’ve been using an HP5386A that I got from eBay. The internal reference is crap, so I got some eBay LPRO 101s for a good deal, and I only use the HP if I have a rubidium for a reference. I just turned on my scope to see how close the FA-2 ref is to my GPSDO. I’m too impatient to wait for the result; it’s been 20 minutes, and it hasn’t slipped a half cycle yet. That’s better than 5e-11. I’m very happy with this FA-2. No need to fire up a rubidium to make a pretty decent frequency measurement. Some day I’m going to open it up and match the CH2 to 50 ohms.

[FransW]

You will need to contact NIST for accuracy.
Resolution is all you have now.
Further calibration to define accuray is (in my opinion) senseless or at least a waste of time.
Regards,
Frans

[tstp]

I also found that my 2020 version FA-2 does not work in either Lady Heather or TimeLab.
After reading reply #227 from Jarl, I realized that the change of frequency output format of FA-2 seems to be the cause of problem.

Trying my FA-2, I found that by changing just one setting, TimeLab 1.51 is able to read the new version FA-2:

Please fill in 3 instead of 1(default) in "Numeric Field #" within the [Acquire from counter in talk only mode] dialog box.

Then, it looks working fine now.

TimeLab is amazing and so flexible to configure! 
Reading the description in TimeLab dialog box, the explanation is:
The 3rd numeric field is just what we want, because the output e.g. F-CH1:0009999999.999912345 could be parsed to 3 possible numbers: "-", "1", and  "0009999999.999912345".


I still do not find a way to let 2020 version FA-2 work in Lady Heather which is powerful and can be used on non-PC platforms like Raspberry Pi .


Picture of output of FA-2 2020 version and the test of OCXO are attached below:  (gate time 1sec, the meaning of "*" sometimes appears before F-CHn is unknown) 

[FriedLogic]

  The FA-2 only uses measurements which are close to the previous one for calculating stats, and if they are more than a certain amount off it marks that measurement in the USB output with an asterisk. I don't remember exactly how large the difference needed to be, but IIRC it was something like a few PPM.

  Whether or not it's within this range also appears to change the dead time by a small amount. The dead time varies a little anyway, and also changes with the mode.

  The actual gate time on the one that I tested appears to be 10.24s on the 10s range, and then about 100-140ms dead time. I'll need to write up the tests that I did sometime...

[elwood]

Hi all,
got my FA-2 yesterday and see the problem. Connected the cable to the reference generator 10 MHz on the back and constantly blinks the difference of 1000 Hz. When the "star" on the top right shines, it shows the real value, but when the "P" appears, it shows less at 1000 Hz. What could be the problem? Unfortunately it is difficult for my understanding.

[rfclown]

I don't see such behavior on my unit. In 0.1S gate time I get about 0.002 Hz P-P. In 1S gate time I get 0.0002. I don't use 0.1S gate time much at all. My suggestions:

1. While in the statics mode (like shown in your picture), push the RST buttom to reset the stats. It could be that you plugged the cable in while stats had already started.

2. Use longer gate time (1S or 10S).

Someone posted an English version of the manual which explains the * and P. The * flashes with the gate. The P is high precision mode.

[elwood]

2. Use longer gate time (1S or 10S).

All modes have been tested. Time does not affect anything. The developer admitted that a chip on the board is faulty.

[elwood]

Hi all,

Today, just in case, I opened the frequency meter, took a photo, looked closely at the board and noticed a couple of solder hairs in the wrong places (near the sop-8 chips), cleaned and washed the board with alcohol. Now everything works fine, "P" mode is always on 

[Ringmodulator]

On eBay a new version of the FA-2 showed up, the FA-2-6G PLUS.
It has 6 pushbottons and displays 2021 01 31 as version (date).

[mankan]

There is also a 12G PLUS and a 26.5G PLUS. However, still no sensitivity spec over 10GHz.

[MegaVolt]

FA-3

https://www.ebay.com/itm/124669140586

[mankan]

Looks like larger screen and other button layout, also available here in multiple variants: https://www.aliexpress.com/item/1005002477751627.html

[Ringmodulator]

The 3.2"display is a really nice upgrade!

[Diabolo]

The 3.2"display is a really nice upgrade!

Hello,

The screen 3.2 is bigger, but the slab is placed behind the front facade and it is not pretty, it would be necessary to install a plexiglass plate perfectly adjusted in the window cutout.

Regards

[rfclown]

I just discovered something about my FA-2. The CH2 input needs about 0 dBm at 1-3 GHz to give proper readings (I've not characterized this at other frequency ranges). The unfortunate thing is, with a lower level (like -10 dBm) it gives a reading (in the ballbark), but incorrect. Took me a while to figure out why my measurements were off. When I first got this thing, I was mostly using the CH1 input for 10 MHz signals. I've been very pleased with the price/performance ratio of my FA-2. Since there are so many versions of this thing, it's impossible to know what the "specs" really are. That's part of what I have to put up with in buying a cheap instrument.

[Grandchuck]

I just checked my FA-2 at 1.5 GHz.  It works down to -29 dBm.

[Kosmic]

I just checked my FA-2 at 1.5 GHz.  It works down to -29 dBm.

Got similar results over here. At 1.5GHz the minimum is -27dBm, 2.0GHz it's -30dBm and 2.7GHz around -34dBm.

Specs of channel 2 from the manual:

CH2 Frequency range: 30MHz-6GHz
CH2 Input impedance: 50 OHM
CH2 Frequency resolution：0.1s: 1Hz@1GHz
 1s: 0.01Hz@1GHz
 10s: 0.001Hz@1GHz
CH2 Input power range:-20dBm- +13dBm@0.5-5GHz,
CH2 Connector type: SMA

you should at least get -20dBm on channel 2. If not, the prescaler is probably damaged.

[rfclown]

I just checked my FA-2 at 1.5 GHz.  It works down to -29 dBm.

Got similar results over here. At 1.5GHz the minimum is -27dBm, 2.0GHz it's -30dBm and 2.7GHz around -34dBm.

Specs of channel 2 from the manual:

CH2 Frequency range: 30MHz-6GHz
CH2 Input impedance: 50 OHM
CH2 Frequency resolution：0.1s: 1Hz@1GHz
 1s: 0.01Hz@1GHz
 10s: 0.001Hz@1GHz
CH2 Input power range:-20dBm- +13dBm@0.5-5GHz,
CH2 Connector type: SMA

you should at least get -20dBm on channel 2. If not, the prescaler is probably damaged.

Thanks for the responses. I guess it's my unit. I don't believe I've ever put a large signal in CH2.

When I had a smaller signal, the reading wasn't way off. The only reason I noticed is because I had both the FA-2 and the transmitter I was measuring using the same 10 MHz reference from a GPSDO, so I knew it should be spot on.

Edit: I must have been doing something wrong. I just double checked with a signal generator, and it's more like -21 dBm at 1G. I must have been mistaken as to the level coming out of the source I was measuring previously.

[burkm]

As the 1st few reviews of the FA3 are coming in it seems - at least to me - that the FA3 is just a FA2 with a somewhat larger display and casing. All the other "ingredients" including the prescaler of the F3 version and the stated specifications seem to be identical to the FA2 version. The elevated prices for the F3 version asked for right now on several market places (ebay, aliexpress etc.) therefore are quite overpriced because of this - from my point of view - probably because of a lack of reviews and detailed information on these units. I would wait awhile whilst the pricing will be approaching more realistic levels...

[Ringmodulator]

I have both, the FA-2 and FA-3.

I had a short look inside the FA-3 and it looks like it has the board of the FA-2.
(I did not compare them side by side)

Removing the protecitve foil on the display was a real pita and required disassembly to remove the display.

The front panel has 2 additional buttons, but they are for the impedance and the LPF, which can be switched
 on the FA-2 by pressing 2 buttons. So no additional function or features here.

According my tests, the performance is identical.

So, ift it is not for the bigger display, I suggest to go with the FA-2.

Chris

[burkm]

If the bigger display is the real difference between those two I would go for the bigger display (F3) but not at those prices asked right now...

[Ringmodulator]

Here you can see both versions FA-2 and FA-3 to get an impression of the displays.

Chris

[burkm]

From what I have read the FA-3 and the FA-2 are identical except the larger size of the display, which leaves some questions about the current prices asked for the FA-3 versions...

[Ringmodulator]

More info here:

[Nargun]

G'day all.

Recent purchase of the BG7TBL FA-2.  The instrument appears to be working fine but I am having difficulties with the serial output.

I'd like to be able to use the unit with Timelab / Lady Heather etc, however all such software seem to require the test unit to output a continuous stream of measured data, which is the appropriately parsed, formatted and used in the relevant determinations.

My FA-2 can be connected to using any given serial terminal and responds normally to the documented commands such as '$S*' which prompts the output of the selected channel stats, or '$D*' which outputs the selected channel freq reading.

Unlike others in this thread though, my unit does not output a continuous stream of measurements, which software like Timelab needs to parse.

The unit needs to be polled with a command as above, before it outputs a single line of data.

I have a sneaking suspicion that there may be an undocumented command i.e. '$nn*' or similar that may configure the device for continuous output.
I'll be damned if I can find it on the inter webs tho - despite all manner of Google mashing.

Does this make any sense to any of the good folk on this excellent and informative site?

I'd be indebted to hear from you if it does!

Cheers & stay safe all - many thanks!

Nev. B
Canberra

[Ringmodulator]

Hi Nev,

that seems odd.
I have the FA-2 and FA-3 and both output a string of measurement for each gate period.

There might be different firmware versions for at least the FA-2, as we have reports of different output string format (mentioned in this thread).

If you use Chan 1, make sure, that there actually is an inputsignal, otherwise it might not output a measurement.
Chan 2 produces "measurements" without an input signal.
If the gatetime is 10 sec, you have to wait up to 10 seconds for the string to appear (I know that this is obvious, but fell into this trap myself, because I thougt, that I set a shorter gate time)

Sorry, I cant help with undocumented commands.

Chris

[FriedLogic]

Recent purchase of the BG7TBL FA-2.  The instrument appears to be working fine but I am having difficulties with the serial output.

Hi,

Was it like that from new, before you tried sending any commands to it?
If so, it might be worth checking with the seller to see if it's faulty since as far as I know they should normally just sent the data.

If there is a specific command to send data, it suggests that the continuous data can be turned off/on.

[Nargun]

Thanks all

Yes, has always been the same since purchase.  I've tried all gate times and pretty much every other setting that I can change, to little effect.

Sending $S* prompt an immediate output of the selected channel signal - but just one string/line.

On boot, the unit displays:

FA-2-6GP
FREQ COUNTER
1Hz-6.0GHz
BG7TBL V20210511

Fairly up-to-date firmware version there by the looks.

If any other owners have the same firmware version I'd love to hear if your unit behaves normally (i.e. outputs a continuous data stream).

Anyway - thanks again for the feedback - I'll try and get back to the vendor.

Nev.

[Nargun]

Apologies - I had my commands mixed up.

When using a loopback arrangement with the 10 MHz ref out as an input to Ch.1,

$S* outputs:

N:0000000025
AVG:0010000000.000026773
MAX:0010000000.000539217
MIN:0009999999.998921569
P-P:0000000000.001617648
READ STATIS DATA SOK

$D* outputs:

$F-CH1,0010000000.000637256,+0000000081,DOK

Nev.

[Ringmodulator]

This is my FA-3 output after startup:


ÿREAD EEPROM!

FA-3-6G       
1Hz-6.0GHz
FREQ COUNTER
BG7TBL V20210401
*FA0010000001.762987798
 FA0010000001.765365926
 FA0010000001.166358810
 FA0010000000.607329410
 FA0010000000.077409839
 FA0009999999.576813777
 FA0009999999.093665327
 FA0009999998.648740140
 FA0009999998.242994464
 .....




This is my FA-2 output after startup:


[00]READ EEPROM!

FA-2 PRECISION
FREQ COUNTER
1Hz-6.0GHz
BG7TBL V20191224
*  F:0010000079.335765709
   F:0010000077.721252453
   F:0010000076.413177439
   F:0010000075.100230553
   F:0010000073.777609762
   F:0010000072.439697556
   F:0010000071.089028763
   F:0010000069.720054912
   F:0010000068.335290743
   F:0010000066.940123780
   F:0010000065.531985956
   .....


Chris


 

[HighPrecision]

Hi,

last year I have got a used FA-2, this unit is working from several months 24/24 hours, yesterday made a test for counter noise feeding reference 10MHz to EXT-REF and CH-1 inputs from a frequency distributor, two outputs of a RACAL 9480 driven through 'DIV' option from HP5065A Rubidium (5MHz).

[RadioTube]

I received my FA-2 6G PLUS last week. It came with firmware V20210215. The only difference I noticed between PLUS and previous variants are 3 extra buttons on the front panel. So no new features, but easier to use, no double key presses necessary. The format of the serial output (FA...) is compatible with TimeLab, no tweaking of any settings was necessary to get it running.
CH2 input impedance is far from 50 Ohms, so I use this input with a 3dB attenuator to get some basic matching. Does anyone have a datasheet of the CH2 prescaler? I am interested in the input impedance and the specified power levels at various frequencies.

[Leo Bodnar]

It turns out I had FA-2 (V20190922) for quite awhile but never used it until today.

So I have connected two 10MHz Rubidium oscillators - one to FA-2 reference input and the other - to CH1.  Levels of both are about 7Vpp.

It seems that 0.1s gate results are very noisy and also offset by about 1.5E-10Hz.  1s gate frequency measurements are offset by around 2E-11Hz.

I have used 53230A and 53100A to verify the results: 10s gate FA-2, 53230A and 53100A agree pretty well.

Are these 0.1s and 1s gate FA-2 frequency measurement offsets deterministic and documented or do you, guys, just ignore them?

Cheers
Leo

[HB9EVI]

I always had the impression that those values with 5-6 decimal places behind the '.' the device spits out just pretend an accuracy that simply doesn't exist, especially on low gate times

Leo's chart seems to me the proof of that estimation

[Leo Bodnar]

I have swapped the input and the Ext. reference signals (both are 10MHz Rubidiums) and here's the result.
I would have assumed that in the perfect frequency counter they should be symmetrical around zero.

I have tried inserting variable attenuator in both CH1 and EXT.REF inputs but it has not visibly changed the outcome.

Feeding the same signal into CH1 and EXT.REF produces 2E-11 offset at 1sec gate (2E-10 at 0.1sec and 2E-12 at 10sec.)

Leo

[hgl]

That's interesting, thank you 
I measured Rubidium, HP10811, and a Samsung GPSDO with the FA2 counter against each other. In all combinations there was noise of this magnitude. Unfortunately I don't have another counter with this resolution to verify this.

[mino-fm]

Not knowing anything about FA2 internal operation but playing arround with different versions of high-res reciprocal counters, I always see a small positive offset doing linear regression using 64-bit double calculations. This offset doesn't appear if 128-bit integer calculations are used.
Maybe this helps.