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.