Hi The TBolt is a GPSDO rather than a GPS module. It’s a very different beast than a ZED-F9P. ZED-F9T, or the M-12 the OP is using.
Bob > On Aug 13, 2020, at 12:09 PM, David C. Partridge > <[email protected]> wrote: > > IIRC the Thunderbolt DOES lock its internal clock to the GPS > > David > -----Original Message----- > From: time-nuts [mailto:[email protected]] On Behalf Of Bob > kb8tq > Sent: 13 August 2020 14:39 > To: Discussion of precise time and frequency measurement > Subject: Re: [time-nuts] ! PPS Source > > Hi > > Here’s the “whole story”, sorry if it repeats things you already know … > > All GPS modules that I have ever seen use a free running clock. The internal > oscillator is *not* locked to GPS. When they want to generate a 1 pps output > they drop / add cycles from the the internal oscillator to get it “as close > as > possible”. That means that you will always have an error in the PPS. > > Since they *know* this is going on, many devices report this error on a > second by second basis. Since the error looks like a sawtooth if you graph > it, this is often called “sawtooth correction”. This correction also takes > care > of “hanging bridges” where the sawtooth stays to one side or the other of > “correct” for a long time. > > Normally when feeding a PRS-10, the sawtooth correction is not used. That > results in a degraded pps accuracy. The best GPS module to use in this > case is one with a very small sawtooth “window” ( = a fast internal clock). > Right now, the Furuno parts are winning this particular race. > > If you *do* use the sawtooth correction (possibly by feeding a variable > delay line chip), then indeed the F9P and F9T will do a much better job. > > Some numbers: > > Sawtooth on some older modules can be out around +/- 20 ns On newer > parts it might be down around +/-10 ns. On the F9 parts it is +/-4 ns. The > Furuno parts run half that. > > Corrected, on a modern part, and looking at second to second variation, > you can get below 1 ns with various modules. On the F9’s you can get well > below 1 ns. > > ===== > > All of that is looking at short term variation. Your Rb does not move much > short term (unless the temperature changes …). Its stability and aging likely > are quite good. > > GPS (as received / uncorrected ) swings around a bit during a normal day. > Swings of 10 to 20 ns are pretty normal. > 50 ns is possible under odd > conditions. That’s more than your Rb is likely to move around over a 4 to 12 > hour period. > > If you “follow” GPS with your Rb through a conventional loop, you likely > degrade the stability of the Rb. It takes a fairly fancy loop to do a good job > on an Rb. > > Bob > >> On Aug 12, 2020, at 11:44 PM, Joe Hobart <[email protected]> wrote: >> >> I have been using 1 PPS from a Motorola M-12 timing module to steer a SRS >> PRS-10. I recently heard that a U-Blox ZED F9P module receives both L1 and >> L2 >> and can provide much improved positional accuracy. >> >> Would better positions translate into a smoother 1 PPS? Does anyone have >> experience with this U-Blox module? Can this be set up with a fixed >> position as >> a timing module? >> >> Is there a better source of 1 PPS at a reasonable cost? The U-Blox is about >> $200. >> >> Thanks, >> Joe, W7LUX >> >> --- >> This email has been checked for viruses by AVG. >> https://www.avg.com >> >> >> _______________________________________________ >> time-nuts mailing list -- [email protected] >> To unsubscribe, go to >> http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com >> and follow the instructions there. > > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there. > > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
