Products > Test Equipment
bg7tbl gpsdo master reference
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:
--- Quote from: mojoe on February 10, 2024, 06:37:58 am ---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.
--- End quote ---
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?
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