----- Original Message ---- From: Bruce Griffiths <[EMAIL PROTECTED]> To: Discussion of precise time and frequency measurement <time-nuts@febo.com> Sent: Thursday, May 22, 2008 10:01:27 PM Subject: Re: [time-nuts] Comparing Reference Accuracy
Stanley Reynolds wrote: > Stanley Reynolds wrote: > >> Disabling all the GPS receivers at the same time would allow a comparison of >> the hold over of each. As we are also logging the temp and power some >> changes in these would also help determine how "good" each DO is as far as >> stability. >> Stanley >> >> >> > You still need a frequency comparison scheme with sufficient resolution > and low intrinsic noise (perhaps 1E-12 or better at 1sec). > Since the long term average frequencies of the various oscillators are > not necessarily identical in hold mode, a phase comparison scheme with a > relatively wide dynamic range is required to avoid frequent phase wrapping. > > Bruce > > With the default settings the reduced VE2ZAZ controler would count all > 160,000,000 pluses in a 16s period, phase is not measured so it has no wrap > just the +-1 count. > Looking at the orginal Brooks Shera design you would not need the DAC chip > and any neg power requlator to use it as a measuring device. And the clock U7 > ECS300S-24 could be shared between the multi sets of boards. I'm not sure if > it could be adapted futher for this meassuring task. > > If you can live with the resultant degraded performance. The point is that the resolution is at best 100ns single shot with a 10MHz clock. This can be improved by using a higher frequency clock. However using say a 1GHz clock introduces significant engineering/design problems if 1ns resolution is to be achieved. If you persist in not using a synchroniser, as they do, then calculated averages are biased. If the system noise isn't around 1 clock period other "interesting" artifacts occur. The performance is inadequate for characterising even a moderately good oscillator with a 16s gate time. When using an interpolator the required system noise level for unbiased averaging is significantly decreased to a few interpolator LSBs. Merely counting the number of pulses in a fixed gate time is inferior to high resolution timestamping every Nth zero crossing of the frequency to be measured. Bruce This counter may be close : http://www.psplc.com/datasheets/vigo/VOITC-200.pdf or http://ztc.wel.wat.edu.pl/kalisz/T-2300R.pdf Stanley _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
