John Ackermann N8UR wrote: > Dr Bruce Griffiths said the following on 10/22/2006 07:33 PM: > >> 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. >> > > 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. > > John > > _______________________________________________ > time-nuts mailing list > time-nuts@febo.com > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > > 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. Bruce _______________________________________________ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts