bg7tbl gpsdo master reference

[mojoe]

Will be interesting to see pics of the inside of that - appearently it might be the same PCB as my GPSDO w/LCD (no dates on front/end of case), so wonder if it would be a 'simple' modification.

Main PCB says: GNSS-D-OCXO(RB)  2021-06-10
Display board says: 20210527

And LCD says: BG7TBL V20220808 when powering up.

I've already opted for a different gps disciplined rubidium standard, though.

Also; for the Trimble OCXO inside my unit - date code 1337, is that YYWW, ie. Week 37 Year 2013?

That LCD pinout looks like a standard 16x2. The connector to the main board only has 6 pins. You'd need 2 for power/gnd and 4 for data. Are the control lines hard wired? Would you try to figure out how that display board is wired? I just ordered a GPSDO without display.

[Ringmodulator]

That LCD pinout looks like a standard 16x2. The connector to the main board only has 6 pins. You'd need 2 for power/gnd and 4 for data. Are the control lines hard wired? Would you try to figure out how that display board is wired? I just ordered a GPSDO without display.

My guess:
Most likely, the the dislpay board has a separate microcontroller, which receives serial data from the ublox chip via the 6-pin connector. The data is then processed for the display controller. 'I very much doubt, that you can directly connect a standard display.
I too have a unit without the display and if there is an easy way to retrofit a display, I would be interested.

On the other hand, you could use a raspberry pi with a dispay running Lady Heather ...

[mojoe]

It may just use an I2C adapter, too.

[mojoe]

The GPSDO I ordered from Amazon just arrived. The PCB is marked 2021-06-10, and looks exactly like the pics posted earlier. The same CTS OCXO and unmarked GPS. Yes, I took it apart before turning it on.

Now that I have a better look at the PCB, I can see that the LCD connector is actually labled "ISP-LCD". It is indeed connected to the SPI pins on the nearby ATmega 328p. So, in addition to programming the 328p, the uC is controlling the optional LCD via four digital I/O's (the other two pins are Vcc and Gnd).

Upon power up, it takes less than a minute to get a fix. The serial port uses a straight-through cable, at 9600, n, 8, 1.

u-center identifies the GPS as an M8030, 32 channels, with firmware 3.01 in ROM. I can't tell if there is any flash without trying to upgrade the firmware or taking the lid off the GPS (not going to happen). I believe v3.01 is the last version firmware, so there is no need to look for flash. I doubt that uBlox is going to release a newer firmware for M8 chips at this point. So, it's not a real uBlox GPS, but uses a uBlox M8030 chip. I don't think this is the Timing version GPS. The v3.01 firmware supports all of the current GNNS systems.

Looking at the config in u-center, only GPS is enabled (good for a GPSDO). NAV5 is set to Stationary, Auto 2D/3D. SBAS is set to Disabled. TP (timepulse) has a cable delay of 820 ns (I'll have to see if this corresponds to the length of the coax on the included antenna). TMODE is Disabled (probably best to set this to Survey-in for hours, once placed in its permanent location). The other settings look to be standard defaults.

Next, I tried Lady Heather. LH auto identified the GPSDO as standard NMEA. LH had a squashed display, but pressing F11 made it go full screen. The picture was taken after about half an hour since power on for the second time (first time was with u-center). I've never used LH before, so I'll have to read the docs to see what the graph means, and why it dings at me occasionally.

So far, so good. Next, I'll hook it up to my 11-digit frequency counter and see how it tracks with my HP 58540A GPSDO that has been running for about 2.5 years, since I moved.

Edit: I forgot to mention that the GPS is spitting out only NMEA sentences, no ubx. Of course, this can be changed in u-center.

[mojoe]

I should add the pinout of the 6-pin connector. There is no pin 1 mark, so I'll call pin 1 the top pin, when the 328p is to the left. The third column is the pin number on the 328p.

1 - Gnd
2 - MISO - 16
3 - SCK - 17
4 - /RESET - 29
5 - MOSI - 15
6 - Vcc

[mojoe]

I unplugged the power for a few minutes, to move it upstairs and hook things up to compare with my HP 58540A GPSDO. It has a battery, so I assume it didn't have to do a cold start (which didn't take long the first time). I don't know if any OCXO tuning parameters are stored or not.

Looks good so far. The alarm LED went out in just a few minutes. In less than ten minutes, it was within a few milliHertz of my HP 58540A GPSDO. I'll log the data for 24 hours and post a graph, later.

[mojoe]

I measured the length of the coax on the included antenna. The coax is unlabeled, but looks like RG174. Calculating the delay gives 18.7 ns. I don't know why the GPS module is programmed with a huge 820 ns delay. Does anone have a clue?

After reading the Lady Heather manual, it looks like it can measure all the delays. After I'm finished collecting data for the comparison to my HP GPSDO, I'll hook up LH and see what I get.

I'll probably also start LH in ubx mode, instead of the default NMEA mode. This is supposed to give me more info.

[mojoe]

I found the manual and just read it. It has a block diagram but no detailed information. I see that the PPS from the GPS goes to the CPLD. So, the question is, does the CPLD read from the u-blox GPS and do sawtooth correction before passing it on to the 328p and the BNC?

[Ringmodulator]

I measured the length of the coax on the included antenna. The coax is unlabeled, but looks like RG174. Calculating the delay gives 18.7 ns. I don't know why the GPS module is programmed with a huge 820 ns delay. Does anone have a clue?

After reading the Lady Heather manual, it looks like it can measure all the delays. After I'm finished collecting data for the comparison to my HP GPSDO, I'll hook up LH and see what I get.

I'll probably also start LH in ubx mode, instead of the default NMEA mode. This is supposed to give me more info.

It has been reported, that the BG7TBL has an offset of around 2mHz for the 10MHz output. It is mot clear, if this is true for all versions of the BG7TBL.
I have verfied this on my unit against a Samsung GPSDO with a TinyPFA phase frequeny analyzer.
For most application, this is probably a non-issue.

It is my understanding, that the cable delay is used to compensate the phase delay of the 1 pps output. It does not influence the accuracy of the 10MHz output oft the GPSDO.

And yes, running LH in ubx mode gives more info.

Chris

[mojoe]

After 12 hours, I'm getting from 0-3 mHz low on this model. I had read about the offset on earlier models, but was under the impression that it was fixed on recent models (I don't remember the year).

I'm still curious as to why mine came with such a huge delay. This is certainly not a default.

I also have a NanoVNA H4. It was my intention to flash the PFA firmware on it and test again, with that. I've not used Timelab. Are there any recommended settings for this type of measurement?

[Ringmodulator]

After 12 hours, I'm getting from 0-3 mHz low on this model. I had read about the offset on earlier models, but was under the impression that it was fixed on recent models (I don't remember the year).

I'm still curious as to why mine came with such a huge delay. This is certainly not a default.

I also have a NanoVNA H4. It was my intention to flash the PFA firmware on it and test again, with that. I've not used Timelab. Are there any recommended settings for this type of measurement?

I too did the conversion of a NanoVNA-4H to TinyPFA by reflashing it.
Getting it to work with Timelab was straigt forward, even for a Timalab noob like me.
I have the settings on another pc. If you have problems, I can check.

[mojoe]

Some results, after logging data for 24 hours. I was using my HP 11 digit counter (showing me single mHz) with it's external reference hooked to my HP 58540A GPSDO, which has been running continuously for about 2.5 years. Both GPSDO's were on the same HP timing antenna via a splitter (I plan on getting an amplified antenna splitter, but the passive one seemed to work fine). Data was collected using a Prolific GPIB-USB dongle and EZGPIB, setting the counter to a 10 second gate time. I have not done a survey-in yet.

If I make a graph with all 24 hours of data, it will be too huge, and still won't show the fine data. So, I'll summarize. At 140 seconds, it stabilized at +/- 0-4 mHz. At that point the Alarm LED went out. It stayed +/-0-4 mHz for several hours. Later, it stayed +/-0-3 mHz for several more hours. During the last several hours it settled on 10000000.000 and 9999999.999 (mostly 10000000.000). That LSD may just be due to my counter.

Looking at the data, it is interesting that the frequency is low much more often that it is high. I assume that has something to do with the firmware and how it tunes the OCXO.

Here is a small graph, showing the initial warm up. It gets within 4 mHz of 10 MHz quickly. However, this was not a total cold start, as earlier I had it running while I looked at the GPS module with u-center. It was then unplugged for 10-15 minutes.

I conclude that this revision hardware/firmware does not have the frequency offset reported for earlier revisions (at least to the mHz resolution).

Next, after I finish setting up a spare mini-PC, I'll run Lady heather on this GPSDO, using /rxu to force u-blox binary mode. I understand that this will make a difference in what is reported to the software vs the default NMEA mode that is autodetected. I'll also do a survery-in for a day or two with LH. I'm new to LH, so I'm not sure what I'll see.

Later, I plan on flashing my NanoVNA H4 with the PFA firmware. I have no idea how much better data I'll get with a PFA vs my HP counter. I hope I can get a few more digits, to see how this GPSDO really performs.

As mentioned above, my counter only lets me see to single mHz. If I did the math right, 1 mHz from 10 MHz is 1E-10. Good enough for anything I need, but a good GPSDO should be 100 times better than that.

[mojoe]

I got LH running. I had some trouble with the built-in Comm 1 port, so for now I'm using a USB-serial adapter. After a second read through of the docs, I set a bunch of command line options (Edit: I forced ubx mode).  I can't find how to turn off the satellite displays. I'd like to have the graph wider.

For some reason, I can't get any of the video resolution options to give me a full screen display. I'm using a 1080p TV via HDMI, but F11 and /vh crashes, /vf and /vc=1920x1080 gives me a smaller screen, and everything I tried has fonts too small to read. The best so far is the default of 1024x768.

After fooling around a bit, I turned on SBAS (GPS was already on) and started a 48 hour survey with SP. Shortly after, I did a screen capture.

I understand what most of the text means. Are the satellite signal levels (dBc) good? I'm going through a passive splitter with my HP timing antenna. I understand what the graph in the upper right is. I'm not so sure what the graph at the bottom is showing me. I did turn on ADEV.

If I understand things, after the survey is done, I can turn off SBAS, then do an EE to write the surveyed position back to the u-blox GPS. Then I may want to increase the elevation mask and save that.

Any words of wisdom or enlightenment about any of this is appreciated.

[mojoe]

Before starting LH, I fired up my HP 8920B to see what the 10 MHz sinewave looked like. The level measured about 12 dBm. The second harmonic was at -35 dBm (-47 dBc) and the other harmonics were in the noise. For harmonics, I set the span from 0-100 MHz.

I understand that the OCXO puts out a squarewave, so the LPF is doing an excellent job.

[FPSychotic]

Hi, I won a bid if this GPSDO with no display, CTS oscillator and original labeled Neo7 , I don't remember if 7n or 7m , but I can open again of needed.
It gets lock quite fast in indoor with my antenna that is triband and 3-V.

I made and db9 cable with TX to RX to a ftdi and displayed in ucenter , 8.1 old version used in Neo7 years and the last one 23.xx I think. With both U-center versions can see the sats it locks, 10 average in the room, and precision is absolutely enough with that, I wont put outside the antenna.

 The problem is that even reading the sats, it doesn't read the config, firmware version, or anything and I can change nothing, all the gnss tab boxes appears detached, the firmware empty...any idea of why this behaviour?
Maybe my cable is wrong and I should buy a commercial one?

[MIS42N]

I understand what most of the text means. Are the satellite signal levels (dBc) good? I'm going through a passive splitter with my HP timing antenna. I understand what the graph in the upper right is. I'm not so sure what the graph at the bottom is showing me. I did turn on ADEV.

If I understand things, after the survey is done, I can turn off SBAS, then do an EE to write the surveyed position back to the u-blox GPS. Then I may want to increase the elevation mask and save that.

Any words of wisdom or enlightenment about any of this is appreciated.

In my experience, signal levels over 35 dB are good. If there are reflected signals it can cause a problem, the elevation mask can help there. Only if your antenna is in a poor position. If you get a survey in less than a metre then it's not a worry.

I think, but someone will know better, that when survey in completes the GPS module writes the data to memory itself and enters timing mode automatically. I did it a year or two ago and that might be my faulty memory.

[mojoe]

In my experience, signal levels over 35 dB are good. If there are reflected signals it can cause a problem, the elevation mask can help there. Only if your antenna is in a poor position. If you get a survey in less than a metre then it's not a worry.

I think, but someone will know better, that when survey in completes the GPS module writes the data to memory itself and enters timing mode automatically. I did it a year or two ago and that might be my faulty memory.

Yes, that would be if I had the GPS do a self survey. I opted to let LH do a survey, as it says it will be more precise.

[mojoe]

The problem is that even reading the sats, it doesn't read the config, firmware version, or anything and I can change nothing, all the gnss tab boxes appears detached, the firmware empty...any idea of why this behaviour?
Maybe my cable is wrong and I should buy a commercial one?

That's weird. Perhaps it is your cable. You're obviously receiving data. Maybe you aren't transmitting to the GPS. When you use u-center, it sends commands to put the GPS in ubx mode.

My GPSDO is configured to NMEA mode on power up (I assume all the different revisions are). u-center should be receiving the NMEA data, which is why you're seeing satellites.

The cable needs to be straignt through. I initially used a cable made with ribbon cable and IDC connectors.

[mojoe]

Looking at the LH display (below), the GPS module is obviously reporting sawtooth corrections (the numbers change every second). Is there any way to tell if the GPSDO itself is using this to actually make the corrections to the PPS?

[FPSychotic]

I did the cable myself with a connector and two cables to the ftdi, can be seen in the picture. Maybe I need ground too.

[mojoe]

I'd say that's a definite yes.

[FPSychotic]

Something wrong, I connected PIN5 (gpsdo's GND I guess, as it's the GND pin of RS232) to the GND im the FTDI adapter and will stop even of receiving.

I bought a RS232 to USB , but I think what I have should work

[mojoe]

After letting LH run its precision survey for 48 hours, I used u-center to enter those fixed coordinates into the GPS receiver. I also turned off SBAS and set the elevation mask to 15 degrees. To make sure that the changes stuck, I power cycled the GPSDO and checked again.

After powering the GPSDO again, the LOCK LED came on in seconds. It took right at two minutes for the ALARM LED to go out. This time, I turned on ADEV in LH. I let it run overnight.

After running about nine hours since the last power up, I get the ADEV numbers shown in the picture. LH says this about ADEV:

"Heather calculates ADEVs based upon the PPS and OSCillator error values reported by the unit. These values are not derived from measurements made against an independent reference (other than the GPS signal) and may not agree with ADEVs calculated by measurements against an external reference signal."

So, what are these numbers really telling me about the performance of this GPSDO? Is it really that good? If a more accurate source is not the comparison reference, what can I infer about these numbers?

I also see that LH reports that the sawtooth accuracy has improved by 20 ns. From this, I gather that LH itself is making sawtooth corrections?

[mojoe]

Related to the last post, I see that the two plots (which I think are showing me oscillator tuning and PPS tuning?) are much less jumpy than before the survey. Is this due to the survey, or the fact that the oscillator is simply settling in after a few days (not counting a brief power down)?

[MIS42N]

Related to the last post, I see that the two plots (which I think are showing me oscillator tuning and PPS tuning?) are much less jumpy than before the survey. Is this due to the survey, or the fact that the oscillator is simply settling in after a few days (not counting a brief power down)?

I have no experience with Lady Heather, so no idea what the graphs tell you. The PPS improves after survey in, as it can now calculate corrections for ionospheric conditions etc. But it will still have a sawtooth component, due to the clock in the GPS module not being synced to anything. You will also get an improvement as an oscillator beds down, usually a slow drift in one direction which gets smaller as days go by. It is good practice to not move the GPSDO once it is running. Brief power down will have some short term effect, mainly due to the temperature of the oven dropping then going back up. Moving it can disturb the mounts, and that is a small but long lasting effect. I inverted my GPSDO once to see what effect it had, changed the control voltage by quite a bit.