In message <[EMAIL PROTECTED]>, David Forbes writes: >At 6:43 AM -0800 1/18/06, James Maynard wrote:
>I suppose it would make a lot more sense to run the GPS 1PPS into the >Rb source so that it will discipline itself, and the phase error be >monitored to make sure it stays at zero. > >Am I barking up the right tree here? Sorry if this is a bit longwinded: You would want to be quite carefull in your choice of timeconstant on the PRS10, if you set it too short you will ruin your short term stability and if you set it too long you will get worse performance than you have today. I've been playing a lot with the PRS10 and one of the things I found is that a raw PPS signal from a GPS is badly suited as input for the PRS10 because of the short term jitter (+/- N nanoseconds for some N > 20 typically, dependent on your GPS receiver. The PRS10 can enable a a 256 sample running average on the PPS input, but that is not nearly long enough to filter out the sampling artifacts in the GPS/PPS (The ones Tom calls "Hanging Bridges"). In an experiment I slaved an OCXO oscillator to the PPS from the GPS, and divided that down to 1Hz as input to the PRS10. That allowed me to use about four times longer timeconstant in the PRS10 and disable the "256 filter" in the PRS10 and the stability was better overall. An even better result was obtained by moving the PLL from the PRS10 onto a computer where the "negative-sawtooth" information from the GPS could be taken into account. I fed the GPS/PPS into the PRS10, used the TT command to read the measured phase as input to the PLL and the SF command to set the frequency. The best results where had using my HP5370B for the timestamping. My conclusion was that even though the timestamping circuitry in the PRS10 is good, it is not good enough to do the Rb part justice and leaving the negative sawtooth correction out of the PLL is ruinous for the Rb parts good performance. So if I were you, I would use the SR620 to measure between the GPS and the PRS10 and implement the PLL in a small computer which talks to both, or alternatively, use the PRS10 for the timestamping, but have the external computer do the PLL. Ideally, it should be possible to have a microcontroller read the GPS data stream, extract the negative sawtooth and send TO commands to the PRS10, but at least in my firmware version that is not possible. SRS told me that they have allowed that in more recent firmware. Poul-Henning PS: If you read the PRS10 docs, it's pretty simple to replicate their PLL parameters. Just remember that they had to do it on a small microcontroller and you'll see how simple it is. -- Poul-Henning Kamp | UNIX since Zilog Zeus 3.20 [EMAIL PROTECTED] | TCP/IP since RFC 956 FreeBSD committer | BSD since 4.3-tahoe Never attribute to malice what can adequately be explained by incompetence. _______________________________________________ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts