[time-nuts] How to measure Allan Deviation?
Dr Bruce Griffiths
bruce.griffiths at xtra.co.nz
Sun Oct 22 21:37:22 EDT 2006
John Ackermann N8UR wrote:
> Dr Bruce Griffiths said the following on 10/22/2006 07:33 PM:
>> 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.
> The few experiments I've done indicate that there isn't that much
> sample-to-sample jitter in a divider built up out of a reasonable-length
> string of 74HC390 dividers, but the temperature stability is horrible.
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> time-nuts at febo.com
This propagation delay drift may degrade the measurement accuracy of
long term trends in the time difference between the divider PPS output
and another PPS source such as a GPS receiver. Resynchronising the
divider output to the divider clock will drastically reduce this
particuarly if a fast (ACMOS?) D flipflop is used. However using a
ripple counter divider chain means that for a PPS output,
synchronisation to the divider input frequency may require cascaded
synchronisers. First synchronising to a frequency generated by the first
divider stage to ensure that the divider PPS output transitions do
always occur well away from the synchronising clock transitions despite
worst case propagation delay variations. Finally the output of this
synchronisers is itself synchronised to the divider input clock using
another D flipflop.
The temperature dependent variation of the GPS receiver delay and (for
long cable lengths) the antenna cable delay may also be problematic
particularly if high accuracy is required. This is why some geodetic GPS
receiver installations regulate the temperature of the antenna, preamp,
receiver and associated cabling.
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