[time-nuts] How to measure Allan Deviation?
Dr Bruce Griffiths
bruce.griffiths at xtra.co.nz
Sun Oct 22 21:08:13 EDT 2006
Tim Shoppa wrote:
> Dr Bruce Griffiths <bruce.griffiths at xtra.co.nz> wrote:
>> Most GPS receivers with higher frequency outputs than 1Hz, phase
>> modulate the high frequency output in this way and the datasheets
>> explicitly indicate this.
>> Thus there would appear to be little advantage in phase locking to the
>> 10KHz signal with a short loop time constant.
>> To be absolutely sure you will need to use an oscilloscope to observe
>> the synchronous jitter in the 10KHz waveform.
> The jitter from the "phase jerked" 10kHz would be in the several to
> tens of ns range, once a second, right?
> Period of 10kHz is 1E5 ns.
> I don't think I can trust my old analog scopes to do this, but a
> fancy coincident trigger (require coincidence with PPS)
> on a digital storage scope with a crystal timebase might see this.
> Sounds more like a job for histogramming interval measuremnts.
> (In my lab days we did this with a TAC followed by a PHA for
> much larger scale jitter measurements).
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Yes the phase jerks on the 10KHz output should be virtually identical to
the phase jitter on the PPS output.
A digital scope is probably necessary unless one can digitally delay the
PPS signal (without affecting the timing of the 10KHz signal phase jerks
) using the receiver clock that is used to position the PPS pulse.
If the 10KHz output were delayed by 100usec (jitter less than that of
the PPS pulse) and the PPS signal was used to trigger the scope then it
may be possible to use an analog scope together with an oscilloscope
camera to capture the waveform.
Unfortunately this is now probably impractical although a few decades
ago it would have been relatively simple but time consuming.
In principle this measurement could be made with a time interval counter:
PPS -> START
delayed 10KHz -> STOP
Vary the delay and watch the jitter jump when the leading edge of the
PPS signal occurs during the 10KHz burst which was phase coherent with
the previous PPS pulse.
The only problem is finding a suitable variable delay device with
sufficiently low (<=1ns??) jitter.
Time stamping the 10KHz and PPS pulse transitions with adequate
resolution and sufficiently low jitter would probably be the most effective.
The time stamp records can be accumulated for a few minutes and then
analysed in software.
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