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bg7tbl gpsdo master reference
MIS42N:
I don't know the design of the bg7tbl GPSDO, but most designs are phase locked to the GPS - i.e. on average the frequency will be exact within the limits of the GPS system. The GPSDO tries to minimise the phase difference between the local oscillator and what it thinks the GPS is telling it. However, what it thinks the GPS is telling it and the actual GPS time can differ slightly due to conditions between the satellites and the receiver.
What this means is the actual frequency of the GPSDO will wander between slightly high to slightly low due to changing conditions.
Comparing two GPSDOs of the same nominal frequency should yield nothing more than a phase difference between the two. If the phase difference is changing, then the instantaneous frequency of the two is different by the rate of change of the phase difference. It doesn't say if either is better than the other.
Two similar GPSDOs running in the same location from similar or (as some people do) the same antenna are going to react similarly to changes in conditions between the satellites and the receiver. Their frequency variations will track together so the phase difference won't show much.
One way of finding out which GPSDO is better is phase comparison with a local oscillator known to be stable. It doesn't need to be accurate. If the phase change over time (e.g per second) is constant, then the GPSDO is also stable. And knowing it is accurate in the long term confirms that it is also accurate in the short term (which is the desired outcome). This may be beyond the capability of a hobbyist as the phase change variation second to second can be small and not detected by many instruments.
Maybe the best the hobbyist can do is try to minimise external variability. A good quality OCXO in a stable environment (temperature, supply voltage, loading) controlled by a stable control voltage derived from a timing GPS receiver with a well placed antenna. Just how far one is prepared to refine the accuracy of a GPSDO is determined by what it is used for. If it is used for nothing more than the joy of getting ever better accuracy then the rabbit hole goes very deep. If the use is more practical, most GPSDOs are far more accurate than most hobbyists need.
Electro Fan:
Here is the Agilent generator using it's internal oscillator providing a 10 MHz signal to the counter with the counter taking Ref from the BG7BL GPSDO 1.
Electro Fan:
--- Quote from: MIS42N on June 20, 2024, 11:27:11 pm ---I don't know the design of the bg7tbl GPSDO, but most designs are phase locked to the GPS - i.e. on average the frequency will be exact within the limits of the GPS system. The GPSDO tries to minimise the phase difference between the local oscillator and what it thinks the GPS is telling it. However, what it thinks the GPS is telling it and the actual GPS time can differ slightly due to conditions between the satellites and the receiver.
What this means is the actual frequency of the GPSDO will wander between slightly high to slightly low due to changing conditions.
Comparing two GPSDOs of the same nominal frequency should yield nothing more than a phase difference between the two. If the phase difference is changing, then the instantaneous frequency of the two is different by the rate of change of the phase difference. It doesn't say if either is better than the other.
Two similar GPSDOs running in the same location from similar or (as some people do) the same antenna are going to react similarly to changes in conditions between the satellites and the receiver. Their frequency variations will track together so the phase difference won't show much.
One way of finding out which GPSDO is better is phase comparison with a local oscillator known to be stable. It doesn't need to be accurate. If the phase change over time (e.g per second) is constant, then the GPSDO is also stable. And knowing it is accurate in the long term confirms that it is also accurate in the short term (which is the desired outcome). This may be beyond the capability of a hobbyist as the phase change variation second to second can be small and not detected by many instruments.
Maybe the best the hobbyist can do is try to minimise external variability. A good quality OCXO in a stable environment (temperature, supply voltage, loading) controlled by a stable control voltage derived from a timing GPS receiver with a well placed antenna. Just how far one is prepared to refine the accuracy of a GPSDO is determined by what it is used for. If it is used for nothing more than the joy of getting ever better accuracy then the rabbit hole goes very deep. If the use is more practical, most GPSDOs are far more accurate than most hobbyists need.
--- End quote ---
Hi MIS42N,
Thanks for the info and explanations.
In my case I'm not yet at the point of looking at phase change variation, I'm just trying to figure out how to set up what I have so I can reasonably know frequency, ideally to at least 1 Hz and preferably a couple or a few digits better. Once I'm pretty sure I have some frequency dialed in I'd like to try to watch for phase drift.
I'm thinking that any of the 3 GPSDO's I have are likely to be the most accurate clock providers I have - more so than the oscillators in my generators and counters, with the BG7TBL counter being more accurate than my two HP counters.
So, that brings me full circle to Ringmodulator's post that had/has me thinking I'm maybe misconfiguring my setup by using the GPSDO to provide ref inputs to both my Agilent generator and the BGTBL counter, but after traveling around the rabbit hole I've gone full circle and I'm thinking that's my best setup. Or maybe not?? Thx
Electro Fan:
Back to square 1. (Generator taking ref from BG7TBL GPSDO is providing 10 MHz signal to counter which is also taking ref from BG7TBL GPSDO.)
Electro Fan:
Here is a ~$14 Amazon 10 MHz OCXO feeding Channel 1 on the BG7TBL counter with the counter taking the ref from the BG7TBL GPSDO 1 unit. The Amazon OCXO can be very stable around 10,000,000.7 Hz for 10-40 or more 10 second gates and then it will be off on one gate count by about 10 Hz. I think this might be something related to the power supply as it seems to happen when the air conditioning comes on. My thought was that if it was stable I could use it for some benchmark testing but it might be mostly a decoration in the rabbit hole.
Update - as I'm writing this the Amazon OCXO is now remaining stable to 100 1215 7164 10 second gates, so maybe it just needed longer to warm up and/or I need to keep the AC off. :)
As it warmed up with a 12V power supply the current draw dropped to ~193mA at ~150 gates and since then it's been running steady. If it keeps going it might be a deal for $14, or maybe with the luck of the draw one of these units could do better?
One of my thoughts in all this is that more test equipment should have a 10 MHz ref input. :)
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