[time-nuts] How to measure Allan Deviation?

Dr Bruce Griffiths bruce.griffiths at xtra.co.nz
Sun Oct 22 23:27:11 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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Since the HP5370 arm input is high impedance(1 Megohm) and its input 
range lies betwen -2V and +2V its desirable to drive it from a back 
terminated source with an amplitude of 2V or less to avoid overdriving 
(the comparators are better behaved if the input signal remains within 
the specified range) the input and ensure that the pulse at the ARM is 
relatively clean and reflection free. A suitable buffer can be built 
using 3 74AC04 inverters as depicted below: The 74AC04 inverters load 
currents are well within the specified limits even if the buffer output 
is shorted to ground. Thus the driver should have a long service life.
ARM driver


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