If *frequency* accuracy is of interest, then ADEV (especially at 1 second) is probably not a good measure on a GPSDO. At 1 second, you are looking at the free running performance of the OCXO. You get great numbers, but the accuracy might only be as good as the OCXO's. You need to wait long enough that the GPS section has had time to get the OCXO settled down in order for the accuracy of the GPS system to transfer in a meaningful way to the OCXO.
Hi Bob,
The ADEV graph is still useful to show where and how the OCXO and GPS performance curves merge. Whether ADEV is necessary isn't clear to me since the main purpose of ADEV is to filter out OCXO parameters like aging that confuse normal mathematical variance calculations. The GPSDO cancels those out.
The assumption here is that the GPSDO being measured has had enough operating time for the GPSDO to correct any OCXO frequency error. i.e. the OCXO is locked to GPS. The OP wanted to know if that's all that's necessary. The answer is no, but the OP will have to decide whether the differences are significant.
At 1 second, ADEV shows you the noise aka jitter performance of the OCXO which is mostly independent of GPS. The frequency situation may not be as well determined, but in any practical GPSDO, the OCXO isn't going to be bouncing off the walls. The locked status prevents a frequency offset as averaged over the length of the discipling loop's time constant. But OCXO frequency changes over periods less than the time constant would still show up as poor ADEV. If there are no frequency changes, there can't be a frequency offset.
A simple experiment:
Yank the antenna on the GPSDO and look at the ADEV. The numbers inside 10 seconds look no different than the numbers with the antenna connected. The OCXO *could* be off anywhere.
Okay, using 10 seconds as an example of the loop time constant. Can I revise your statement to say "If you freeze the disciplining loop, the numbers inside 10 seconds look no different than the numbers with the loop active."? I'm sure you know that some smart GPSDOs can compensate for OCXO aging during holdover conditions. I'm playing to the audience here.
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Yes, the OCXO *could* be off anywhere, but if it was doing that, you would have been warned by the poor short-term ADEV while it was in the locked condition. So, in practice, that won't occur.
Leave the antenna off and watch the ADEV plot of the OCXO. As the unit stabilizes (over days / weeks), the curve past 10 seconds flattens out. The longer it is on, (in a stable environment) the flatter it is likely to get. That may continue out past 1,000 seconds or it may start to turn up a bit. The flatter that curve is, the better the short term frequency of the OCXO will be compared to the long term.
Bob, I totally agree with you on this point. This is standard OCXO behaviour. They like to run continuously and if you let them they will continue to improve - typically beyond their factory specifications. Sometimes, with top-grade OCXOs, they can approach Rb standards for long-term aging. So, as your OCXO improves, you'd like to increase your time constant to make the best use of the OCXO's low noise performance while still using GPS to cancel out the aging. This is why the OP might want to run his GPSDO continuously - so the OCXO reaches its maximum potential. Whether that matters is up to him.
Yes, it would be nice if the ADEV curve dropped off as 1/sqrt(tau). That's not what happens in an OCXO. The physics don't work that way.
Bob, you totally lost me on that point. Did I imply something about 1/sqrt(tau) ?
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Ed