[time-nuts] How to measure Allan Deviation?

Dr Bruce Griffiths bruce.griffiths at xtra.co.nz
Sun Oct 22 22:08:24 EDT 2006

Didier Juges wrote:
> Dr Bruce Griffiths wrote:
>> In principle this measurement could be made with a time interval counter:
>> 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.
> The delay device can be triggered by the 1 PPS, then will drive the ARM 
> input of the counter, so as long as the delay device's jitter  is less 
> than the 10 kHz period, if we adjust the delay to 0.99985 second 
> (between the last 2 periods of 10 kHz before the 1 PPS), then the TI 
> counter will START on the last 10 kHz pulse before the pps, and STOP on 
> the 1 PPS.
> Tek has some time delay generators in the TM-500 and 7000 plug-in 
> series. I knew one day I would need one of those, I now know why :-)
> Didier
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You could always use an HP5359A.
These are still available on the surplus market.

However if a jitter of say 100ns is OK you can easily construct a 
suitable digital delay device using a 10MHz clock (slower if more jitter 
is OK).
Although it could be built using a collection of CMOS parts it may be 
much simpler and cleaner to use a programmable gate array or its equivalent.
Loading the required delay via a synchronous serial interface will 
reduce the board layout complexity.
It is essential to use a synchroniser (shift register) to synchronise 
the input PPS pulse to the delay generator clock to minimise the 
probability of metastability induced problems.


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