Hi Lachlan, Having others to compare against is not so much an issue on this list. Many of us have Cs, or more.
For GPS 1PPS measurements, for sawtooth removal, for GPSDO, for L1 common view -- usually 1 ns resolution is adequate, because that's the numerical granularity of the GPS receiver firmware. I recall your older postings too. I'm aware of several common view efforts and it would be good for you to have a realistic measure of the expected performance, and over what tau, and how real-time. Note some people ignore real-time and just use GPS post-processing. I assume this is related to your PhD project? You can contact me off-list with more info. About your 1PPS offset -- this is just a constant, easily removed with math, and has no bearing on GPSDO phase or frequency locking, or common view applications. Some people like the GPS 1PPS to lead, some to lag, some don't care. If you have a good TIC, fixed time offsets don't matter. This is especially true for timestamping counters. /tvb ----- Original Message ----- From: "Lachlan Gunn" <[email protected]> To: "'Discussion of precise time and frequency measurement'" <[email protected]> Sent: Monday, September 02, 2013 4:12 PM Subject: Re: [time-nuts] Trimble Resolution T - PPS offset and stability > It sounds like an experiment is in order, then. > > To give some background, around six months ago I posted on here about the > idea of running an amateur common-view GPS network over the internet. This > is all well and good, but if I'm to have anyone else to compare with then I > will need to bring the hardware down to $20 from Farnell/Digikey/etc. and > five minutes of work. > > At least initially, this will mean a microcontroller rather than an FPGA to > make the comparison. I've been using a $15 STM32F4 devboard as a DSP > platform, but it has timers that can run at 168MHz, and can use them for > event timestamping. Not as fast as I've done with FPGAs, but it could > certainly be worse. Using something like this means that the timebase > stability will be awful, and so I'd rather use the PPS offset to avoid > degradation as a result if possible. > > Thanks, > Lachlan > > -----Original Message----- > From: [email protected] [mailto:[email protected]] On > Behalf Of Bob Camp > Sent: Tuesday, 3 September 2013 7:54 AM > To: Discussion of precise time and frequency measurement > Subject: Re: [time-nuts] Trimble Resolution T - PPS offset and stability > > Hi > > On all the GPS's I have tried it on, shifting the PPS has no real impact on > stability. A few things to consider: > > Normally the shift is a few hundred ns either way The shift process is > always in steps of the main clock (100 ns for 10 MHz) GPS by it's self > bounces around a bit. > > If you are talking about a shift of a big fraction of a second (and it > sounds like you are) then the stability of the GPS's local clock could come > into play. On something like a TBolt that's not going to matter. On a TCXO > based gizmo that is only corrected to 1.0x10^-7 you could get an extra 50 ns > of error at a half second offset. Weather you see that on this or that GPS > depends a lot on who wrote the firmware and what they worried about when > they did. > > The better alternative is to use a counter with a reasonable time base to > look at the difference between pps signals. If the counter has an OCXO time > base and it's properly calibrated you are about 10 to 100X better off than > the 50 ns in the example above. > > Bob > > On Sep 2, 2013, at 6:05 PM, Lachlan Gunn <[email protected]> wrote: > >> Hello. >> >> >> >> Has anyone here tried varying the PPS offset on a ResT (or UT+ or any >> other GPS receiver for that matter) and measuring the resulting stability? >> >> >> >> I ask because my Rb has only a 1PPS output, and while I have been able >> to get at one of its internal HF signals, would like to see what I can >> do with just 1PPS. The obvious problem with doing this is that I will >> need to align the PPS outputs together to get reasonable accuracy, but >> I worry that a large offset in the GPS receiver will degrade stability >> as the pulse moves away from the relevant packet in the GPS signal. >> >> >> >> Am I being over-cautious? >> >> >> >> Thanks, >> >> Lachlan >> >> _______________________________________________ >> time-nuts mailing list -- [email protected] To unsubscribe, go to >> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >> and follow the instructions there. > > _______________________________________________ > time-nuts mailing list -- [email protected] To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
