Mike, If you have three different oscillators that are locked to GPS, then any difference seen when you read your clock once a day will be caused by the short term noise. When you take the noise difference counts as a percentage of counts in a day, the result is a smaller number than if the oscillators were not locked to GPS.
Some people on this list are worried down to the phase noise level because they are correlating events that have been timed by clocks whose only link is GPS or a portable secondary time standard. Primary standards tend not to be portable, but quantum physics could change that. Locking means that there is only a phase difference between GPS and the oscillator, not a frequency difference. For 10 MHz, that's 100 nanoseconds or (much) better while locked. Said another way, if GPS lock is maintained for some length of time, the maximum difference at the end of that length of time is 100 nanoseconds, plus the drift of GPS, if any. Ageing is compensated by a control servo while the frequencies are locked. The full name for "lock" is "phase lock" which means that any phase error is restored to nominal by the control servo. The servo is constrained by a long time constant filter so that it won't dither around zero error, trying to follow every noise spike and always being too late. Losing lock means that the control servo has saturated, and can no longer move in the direction required to maintain lock. At that point, there is no controlled relationship between GPS and the oscillator. The performance of the oscillator is then the performance with no GPS available. You can measure ageing by measuring the output of the controller. If the loss of lock was due to some hiccup, then lock could be restored but the clock counter will be wrong by the number of counts added or dropped while lock was lost. Hope that helps. Bill Hawkins -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Mike Feher Sent: Wednesday, April 25, 2007 11:46 AM To: 'Discussion of precise time and frequency measurement' Subject: [time-nuts] GPS Disciplined Oscillators I have to show my ignorance here, because this has been bothering me for a while, and, I wonder if there is a relatively simple answer. This question has to do with frequency accuracy and stability only. Also, let's talk of long term like 24 hours or more, so let's ignore phase noise and just concern ourselves with long term accuracy/stability. If I have three separate oscillators, let's say a Rubidium, an OCXO and a TCXO all with EFC's capable of closing the loop to lock to GPS, what kind of absolute frequency difference should I see amongst the three at any given time, random times, or, over the entire test period. Let's also make it simple and say all three are at 10 MHz nominal, unlocked to GPS. When locked, and properly designed with a narrow loop filter, I would expect the long term accuracy to be very close amongst all 3 oscillators. Certainly better than a few parts in 10^-11. First, am I wrong in this assumption? In either case, crystals, and even Rubidium cells do age, while at different rates, so, it is possible, that if lock with GPS is lost for some reason, because the oscillator may have drifted/aged out of loop range, it cannot be disciplined again. I, for the time being, also assume that the EFC on all 3 oscillators has a range wide enough to keep the oscillator locked even as it ages. Are the narrow loop bandwidths and wide EFC ranges contradictory? So, to reiterate the question, if I was clear enough, what kind of frequency excursions should I anticipate to see amongst my three disciplined oscillators in lets say 24 hours, or in a month. Assume GPS disciplining was working all of that time (can I even assume that with aging?). BTW, how is my assumption regarding the oscillators aging? If the oscillator basic frequency determining element drifts out of lock range, during lock, will it stay in lock? - Thanks in advance for any enlightenment - Mike Mike B. Feher, N4FS 89 Arnold Blvd. Howell, NJ, 07731 732-886-5960 _______________________________________________ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts _______________________________________________ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
