[time-nuts] How to measure Allan Deviation?

Dr Bruce Griffiths bruce.griffiths at xtra.co.nz
Mon Oct 23 00:38:26 EDT 2006

Didier Juges wrote:
> Bruce,
> Thanks for the reminder. That was my intention. I was planning to use a 
> 74HC74, and whatever dividers I can get my hands on. I am not looking 
> forward to daisy chain seven 7490s, so I will probably try something 
> else. With the D flip-flop, the dividers don't really matter, as long as 
> the delay is below 100nS.
> I need to find the best way to go from the 10 MHz sinewave to the 
> divider, probably through an LM119 comparator with modest hysteresis.
> The need for a low jitter divider is the same for the GPS disciplined 
> oscillator, so I should be able to reuse the divider for my frequency 
> standard.
> Thanks
> Didier
> Dr Bruce Griffiths wrote:
>> Didier
>> If you are going to use a PPS divider to divide the oscillator frequency 
>> down to 1Hz, you will need to measure the inherent jitter of the divider 
>> to ensure that it doesn't degrade the measurement resolution. It may be 
>> necessary to resynchronise the divided output using a fast D flipflop to 
>> reduce the inherent divider jitter to less than the 20ps resolution of 
>> the 5370.
>> Bruce
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If you use a comparator, the LM119 is a bit on the slow side, a more 
modern lower power comparator such as an AD8561 or similar would be 
Failing this, an overdriven longtailed pair using 2N3904's or equivalent 
devices is usually perfectly satisfactory when the input amplitude is 
sufficiently large (>= 1V pp).

To discipline the oscillator using GPS you dont actually need to divide 
its frequency all the way down to 1Hz. As long as the output frequency 
of the divider is a multiple of 1Hz and its period exceeds the 
combination of the GPS PPS jitter and the oscillator timing drift/wander 
over the loop response time any convenient output frequency can be used.

With a 10MHz oscillator, dividing its output by 16 allows a timing 
wander jitter tolerance of 1.6us this is probably a little tight in most 
cases, dividing the 10MHz by 256 gives you a jitter/wander budget of 
25.6us which should be more than adequate for a good oscillator and a 
loop response time of 1000 sec or so.


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