[time-nuts] GPS Disciplined Oscillators

Mike Feher mfeher at eozinc.com
Wed Apr 25 12:46:29 EDT 2007

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

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