Karen wrote:
I raised this question as a ADEV measurement result of my GPSDO/NEO-7M is about 1E-9 for tau = 1sec and it isn't meet my expectation. In my understanding (before I published my question here) the reason for this is not a good enough short range stability of my frequency counter's reference OCXO that's why I had an idea to use external rubidium source.
The Pendulum is a well-respected instrument for this sort of measurement, as you have now discovered by reading the datasheet. Much better than the ADEV you are getting. (In fact, it is probably better than the datasheet specification.)
Now I see that even my internal reference OCXO provides a good ADEV value 1E-10 for tau =1 sec (according Pendulum CNT-91 with opt.19 datasheet).
If so what can be a reason of not a good ADEV result of GPSDO/NEO-7M?
(1) poor performance of the particular OCXO you have, or (2) poor GPSDO design or construction.
To check (1), remove the OCXO from the GPSDO and measure its ADEV by itself. A good OCXO should be better that 1e-11 at 1 second, certainly better than 1e-10. I believe you said you are using a Morion MV-89. Many of the surplus MV-89s available in the US are broken and have poor ADEV or lots of spurs. The situation may be better where you are.
If the ADEV of the oscillator by itself is only 1e-9, then you need [at least] a different oscillator. If the oscillator by itself is OK, then the GPSDO is not doing a good job of disciplining it.
The purpose of a GPSDO is to let the oscillator run without interference at tau where the oscillator stability is better than the GPS timing signal, then to cross over at longer tau and let the GPS control the oscillator at averaging times where the GPS stability is better than the oscillator. Usually, this will be somewhere between 100 and 1000 seconds. So the control loop must be VERY slow, with a time constant in the hundreds or thousands of seconds -- it should be doing essentially nothing at tau = 1 second. The ADEV of the GPSDO at 1 second should be the same as the ADEV of the OCXO at 1 second, or very close to it.
Best regards, Charles _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
