[time-nuts] GPS Disciplined Oscillators

[Bill Hawkins]

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: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] 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
 
 


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