Unknown GPSDO but BG7TBL logo on the circuit board and OCXO !

[hector.pascal]

Hi,
It seems to be a very good general purpose, reasonably quick locking in my location, value for money reference.  The only con is no documentation
E.g,  is the TXD NMEA output connector a mono or stereo jack?  The little picture on the back panel could be read either way.
And is that TXD output a genuine +/- RS232  (whether +12/-12V or +5/-5V), or is it the "TTL" version (~0/+5V)?
Also does anyone know if the version with green end panels is significantly different from the latest one (marked 2017/12/05) with the black ends?   Regards H.

[charly724]

Hi,
attached is the measurement of the serial output without load, it's 5 V.
I could not find a different between green and black panel version...
Regards Charly

[hector.pascal]

My September? 2017 "green" version has a 5V OCXO and 0-5V data (see attachments).  The 10 MHz w/f is 1.84Vptp into hi Z. The data connector is the tip of a 3.5mm stereo plug.  It locks in less than 4 minutes with the antenna sitting downstairs close to the unit. Sadly I have no equipment to verify its accuracy!

[hector.pascal]

Hi,
The NMEA "RS232" 0/5V data from my unit was inverted, presumably due to Q1, R7, R8. To get valid data, I inserted a simple outboard transistor inverter at the "RS232" output.
(2N3904, input from the tip of the stereo jack plug via 10k resistor to base, and 1k collector resistor from 5V. The inverted output is then available from the collector.   But note that 0/3.3V data would need collector connected via 1k to 3.3V, instead of 5V.)
Worked fine after that!  Presumably the valid data could be also tapped directly from the GPS module at R8, but I preferred a bit of buffering to allow for any "accidents"!
H-P

[Diabolo]

Hello,


Could you post a full photo of the component side?
Next to the Oxco CTS 970-2178-46 there should be a li-ion battery (BT1, 3-volt LIR MS621FE-FL11E), it is absent on your model.

The GPS module is a model VK1612U7M3L :
- http://www.worlduc.com/FileSystem/18/5370/178261/dfa73b3b55474546895252a20d2c6f61.pdf

Ocxo CTS-970-2178-46-10MHZ-5V is used in post # 386:
https://www.eevblog.com/forum/testgear/bg7tbl-gpsdo-master-reference/375/

Regards,
Diabolo

[hector.pascal]

Hi,
I'm not really sure what the battery is for - maybe it's not needed with this OCXO or the V.Kel GPS module. But certainly no battery is fitted in this version, and no date or "BG7TBL" text exists anywhere on the pcb.
The GPS module seems to be a 7M equivalent as the part number says it has a UBX G-7020 engine.  But note that there is no UBlox logo on the label!
Thanks for the datasheet - a pity I can't read Chinese!
Photo as requested.
H-P

[Diabolo]

Thanks for the circuit photo.
----
The BT1 battery is used to keep the latest information received on the satellites to facilitate / accelerate the locking of the GPS. On pin 22 (BCKP) of VK1612U7M3L, the datsheet indicates:
- "It’s recommended to connect a backup battery to V_BCKP in order to enable Warm and Hot Start features on the receivers. Otherwise connect to GND".

All GPSDOs BG7TBL have a battery connected BT1, 3-volt LIR MS621FE-FL11E.


Diabolo

[Hans_18T]

I got my GPSDO-PLL this week, the (circuit)board is Labeled as "PLL GPSDO BC7TBL 2018-02-10. GPS receiver has no label, The OCXO indicates: 1042/0394C - Fo=10MHZ - TOC0713A (includes a battery)
Front panel green (no indication BC7TBL)

[Ohm_My]

- frequency accuracy is higher than my measuring accuracy

If you use the TF930's computer interface  then you can extend the gate time to get more precision though with longer gate times the two GPSDOs will agree with each other more anyway so you might not gain a lot. The counter keeps the counts going continuously so if there is a frequency difference and the phase difference reaches 20 nanosecs then the frequency value will change at that point and you could estimate how long it took for a 20nsec shift.

This is interesting. What is the command to measure beyond the 100s gate time (M4)? I just recently purchased this unit and came across your post.

[jpb]

- frequency accuracy is higher than my measuring accuracy

If you use the TF930's computer interface  then you can extend the gate time to get more precision though with longer gate times the two GPSDOs will agree with each other more anyway so you might not gain a lot. The counter keeps the counts going continuously so if there is a frequency difference and the phase difference reaches 20 nanosecs then the frequency value will change at that point and you could estimate how long it took for a 20nsec shift.

This is interesting. What is the command to measure beyond the 100s gate time (M4)? I just recently purchased this unit and came across your post.

Sorry, I was a little lax in my statement. There isn't a specific command to extend the gate time but if you measure continuously then it doesn't reset between gates. I was measuring longish time periods (for testing 10MHz oscillators which I had mixed with a reference) and what I did was measure period and in software recover the count of the 50MHz clock. Then you can treat n gates as one super gate n times as big and use the total count.

The key point is that the counter, though it doesn't interpolate, it doesn't reset either.

Unfortunately they don't give the count readings directly so you have to recover it from the floating point number that is returned.

Though you're trying to get two numbers from one it is not impossible. Say you're measuring a period and using a gate of 1 second. The counter will return the ratio of the change in the internal 50MHz clocked counter and the nearest integral number of periods of the frequency being measured. You know the average number of periods from the measured period and the gate time, call this m periods.

Then what I did was assume that the true number was m-1, m or m+1 and for each I would calculate what the 50MHz clock count must be to give the measured period. This should be an integer so I'd choose the case m-1, m, m+1 that gave the closest to integer answer.

[Ohm_My]

How is this unit wired for RS-232? The phono jack looks a little confusing to get it wired correct.

[Hans_18T]

Can be either a mono or stereo jack (3.5mm)

Tip <-> 9 pin SUB-D pin 2
Ring <-> 9 pin sub-D pin 5

No issue seen so far on various computers, could be an "level" issue though on some computers as the interface provide only TTL levels.

[texaspyro]

Can be either a mono or stereo jack (3.5mm)

Tip <-> 9 pin SUB-D pin 2
Ring <-> 9 pin sub-D pin 5

No issue seen so far on various computers, could be an "level" issue though on some computers as the interface provide only TTL levels.

If it is TTL level, you may need to add inverters or a RS-232 driver chip that inverts the signal levels.  Scope the TX line... if it is usually high (> 1V or so) you will need to invert the signals.

[Electroalice]

Hi Purchase one of these units dated 2018-06-05. Compared the output to an XLI GPS. The 10 MHz is good but the 1 PPS has a constant jitter approx. 20ns. Every 1 second it shifts 3  ns then on the 6th second jumps back and repeats continuously.
Is this common to all units or is mine faulty?

[Hans_18T]

I never looked at the 1PPS (except for its presence). As the the 10MHz is derived from the 1PPS and provides a stable 10 MHz I would say your device is fine.
See also https://www.eevblog.com/forum/chat/how-accurate-is-a-gps-1pps-signal/msg170798/#msg170798

[Leo Bodnar]

Somebody was badgering me about comparing our mini GPS clock phase noise to "BG7TBL NEW PLL-GPSDO GPS Tame Disciplined Watch Sine GPS Receiver 10M" so I got one to test off eBay https://www.ebay.co.uk/itm/123519078474

Disclaimer: I have not done any tweaking or setting it up.  I just took it out of the box, used its supplied PSU, connected the antenna and let it sit for a few hours after ALM LED went off.

Cheers
Leo

[jpb]

I like the bottles of wine in the lab - essential equipment!

[Leo Bodnar]

Here is BG7TBL phase noise  plot taken on a different system (Agilent E5500) I use at home.
It's pretty much identical apart from 900Hz spur is absent this time - it might be an MCU correction loop artefact that comes and goes.
This measurement system has much flatter response between 60kHz and 100kHz - HP3048A has a few dB fall-off there.
Leo

[jeffsf]

The 10 MHz is good but the 1 PPS has a constant jitter approx. 20ns. Every 1 second it shifts 3  ns then on the 6th second jumps back and repeats continuously.

Good to know about the behavior of the PPS output. I'm not sure I'd call it "jitter" as it sounds like it's deterministic in behavior.

Might be similar to, for example, the Trimble Resolution T timing GPS that uses a 12.504 MHz internal clock and the PPS output is always coincident with an edge of that clock. See page 34-35 (36-37 of the PDF) of Resolution T GPS Embedded Board.

Edit: Checking https://www.u-blox.com/sites/default/files/products/documents/NEO-6_DataSheet_(GPS.G6-HW-09005).pdf shows a 21 ns granularity in the "timepulse" signal of that unit, used in several of the BG7TBL units, from what I've read in the "master thread" (linked above).

[JBooth]

Hi Purchase one of these units dated 2018-06-05. Compared the output to an XLI GPS. The 10 MHz is good but the 1 PPS has a constant jitter approx. 20ns. Every 1 second it shifts 3  ns then on the 6th second jumps back and repeats continuously.
Is this common to all units or is mine faulty?

The 10 MHz output on mine is really ugly.  Not clean at all.  There is jitter on the signal when the GPS is locked and the alarm light is off.  When the GPS lock is lost and the alarm light comes on, the frequency jumps down much as 10 KHz within seconds and drifts erratically around!  I'm wondering if the internal oscillator is bad since the 10 MHz output is so noisy when locked to GPS, and so far off frequency as soon as the GPS signal is lost.

Here are some screen captures from my oscilloscope to show you guys what I'm talking about.  Unfortunately I can't show you the jittering motion in a still picture, but you can sort of tell that it is definitely not a stable sine wave.  The last 2 pictures help show the amplitude of the low frequency that is modulating the 10 MHz output signal:

[cdev]

JBooth, congrats on your first post.

If it wasn't sold as a BG7TBL, my thinking is, it very likely isn't one.

A product could end up with a PCB that was literally copied from BG7TBL, but populated with different parts, that might have issues, like for example, those (vendor name omitted) OCXOs that in 2016 were supposed to be crushed but ended up being sold to people. Or ublox gpss that were sold as M8Ns but which actually were an M8 series but lacking a TCXO, and flash chip, so not firmware upgradeable and not really an M8N, more of a franken-ublox.

[Leo Bodnar]

I don't know of any 10MHz OCXO that can be pulled 1000ppm.
More importantly - do you trust your scope to do these measurements? I would expect good OCXO to have ET steering range of only few Hz.
Leo

When the GPS lock is lost and the alarm light comes on, the frequency jumps down much as 10 KHz within seconds and drifts erratically around

[JBooth]

I don't know of any 10MHz OCXO that can be pulled 1000ppm.
More importantly - do you trust your scope to do these measurements? I would expect good OCXO to have ET steering range of only few Hz.
Leo

When the GPS lock is lost and the alarm light comes on, the frequency jumps down much as 10 KHz within seconds and drifts erratically around

Yes, 10KHz momentary deviation is very unusual.  I believe it was because I had multiple instruments connected to the output via an improvised set of jumper wires on a breadboard (since I don't have a BNC "T" connector at the moment).  I suspect that the unshielded wires may have picked up some noise that caused a false count.  With the GPSDO connected directly to the counter only, the count drops about 10Hz when GPS lock is lost and the red alarm LED comes on.

But there still remains a strange mystery:  When I had the oscilloscope and the frequency counter connected to the GPSDO via oscilloscope probes to breadboard jumper wires, the GPSDO was constantly losing GPS lock every few minutes and triggering the >.1Hz error alarm LED.  With the GPSDO connected directly to just the frequency counter with coax cable, it never lost GPS lock for several hours running.  I even took the GPS antenna out of the window and put it under the workbench for several minutes and it never indicated that it lost GPS lock!  Weird.

I'm wondering if the GPSDO 10 MHz output is not isolated or buffered adequately?  That might also be a factor causing the jitters and pulsing on the output waveform.

[Hans_18T]

Do you have a picture of the board (component side), just for comparison.
Also mind that for instance TurboTom reported that the 10MHz BNC terminal had not to be soldered to the PCB at all.
I use the antenna under the roof, the device "sees" 5 or 6 satellites with a SNR of 35 db or better.

[cdev]

Photos would be extremely helpful in figuring out things like this- often taking much less time to figure them out with a picture than any other way.