[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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Didier
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.
Bruce
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