[time-nuts] How to measure Allan Deviation?

Didier Juges didier at cox.net
Sun Oct 22 23:50:45 EDT 2006

Dr Bruce Griffiths wrote:
> Didier Juges wrote:
>> Dr Bruce Griffiths wrote:
>>> 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.
>> 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
> Bruce
> ________________
The trigger levels on the 5370 are strange. The normal inputs have 
trigger levels that can be adjusted between -1.5 and +0.6 V or something 
like that.
I am not sure where that is coming from.
For the ARM input, if someone is going to drive it with long coax 
cables, the best would be a 50 ohm termination right at the instrument's 
The schematic did not make it, but I believe I understand what you mean: 
3 gates with common input, each output has a 150 ohm series resistor and 
the outputs (far end of the resistors) are tied together, to provide a 
good 50 ohm drive to the cable. A series capacitor would probably be 
recommended to center the signal around ground (assuming it's 50% duty 


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