[time-nuts] How to measure Allan Deviation?

Dr Bruce Griffiths bruce.griffiths at xtra.co.nz
Mon Oct 23 00:24:41 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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You can use a set of cascaded 74HC4017 Johnson counters to divide 10MHz 
down to 1 Hz as long as you use a 2 stage (74AC74??) resynchroniser to 
reduce the PPS jitter and reduce the effective propagation delay. With 
an extra gate or 2 this will even work for a 5MHz input. It is not 
necessary to have a 50% duty cycle signal to drive the HP5370 allowing 
more flexibility in the divider design. I have designed a circuit using 
74HC4017's to do this with a 5MHz input. If you want I can send it to you.

Satisfactory drivers for the HP5370 START and STOP inputs can also be 
built by using a 270 ohm resistor in series with an ACMOS gate or 
flipflop output. This produces a swing of around 800mV into a 50 ohm 
load which is more than sufficient. The load current ratings of the 
driver device are not exceeded even if the load is shorted.


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