Sorry for the cryptic title and ignorant question. I'm trying to build a simple PPS generator that outputs a pulse every second in sync with the UTC second tick but without the jitter of a gps module PPS output. It's done using a cheap gps module, no sawtooth correction, no multi band receiver. The output of the VC-TCXO is divided into a PPS. The time between this PPS and the PPS from the gps module is used to control the frequency of the VC-TCXO. It all seems to work. But now I'm trying to understand how stable in time versus UTC this PPS can be in practice. To measure the variation over time on one location one could use two or three of these pulse generator and measure the time between their PPS. What stability could be expected. Is 100ns maximum PPS time difference good? Or should/could this be below 10 ns?

Hi You are building a basic GPSDO. It may not sound like that, but that’s what you are doing. What you get depends on the control loop you set up. Fast control loop lets through a lot of noise from the GPS. It also tracks tightly. Slow control loop takes noise but does not track as tight. Without sawtooth correction, hanging bridges will always be a “question”. That gets you into the grubby details of how your GPS module generates it’s pps signal. Bob > On Dec 14, 2024, at 11:39 AM, Erik Kaashoek via time-nuts <time-nuts@lists.febo.com> wrote: > > Sorry for the cryptic title and ignorant question. > I'm trying to build a simple PPS generator that outputs a pulse every > second in sync with the UTC second tick but without the jitter of a gps > module PPS output. > It's done using a cheap gps module, no sawtooth correction, no multi band > receiver. The output of the VC-TCXO is divided into a PPS. The time > between this PPS and the PPS from the gps module is used to control the > frequency of the VC-TCXO. > It all seems to work. But now I'm trying to understand how stable in time > versus UTC this PPS can be in practice. To measure the variation over time > on one location one could use two or three of these pulse generator and > measure the time between their PPS. > What stability could be expected. Is 100ns maximum PPS time difference > good? Or should/could this be below 10 ns? > _______________________________________________ > time-nuts mailing list -- time-nuts@lists.febo.com > To unsubscribe send an email to time-nuts-leave@lists.febo.com

Hi Erik, I assume you already seen Bob Camps's post. On 2024-12-14 17:39, Erik Kaashoek via time-nuts wrote: > Sorry for the cryptic title and ignorant question. > I'm trying to build a simple PPS generator that outputs a pulse every > second in sync with the UTC second tick but without the jitter of a gps > module PPS output. > It's done using a cheap gps module, no sawtooth correction, no multi band > receiver. The output of the VC-TCXO is divided into a PPS. The time > between this PPS and the PPS from the gps module is used to control the > frequency of the VC-TCXO. > It all seems to work. But now I'm trying to understand how stable in time > versus UTC this PPS can be in practice. To measure the variation over time > on one location one could use two or three of these pulse generator and > measure the time between their PPS. > What stability could be expected. Is 100ns maximum PPS time difference > good? Or should/could this be below 10 ns? For shorter time-intervals your noise will be dominated by the systematic noise of the PPS cycle-assignment of the GPS/GNSS receiver. It will create the "hanging bridges" as the VC-TCXO is close to some even multiple of 1 Hz of the PPS. All classical. You can reduce this noise through the sawtooth corrections, and I strongly suggest you do this. I think you know all that already. However, further out on the time eventually the floor of the GPS/GNSS will dominate. Here you have an option, if you have a good enough steered oscillator you can filter out some of this noise, and the more stable oscillator you have, the further out you can go. With a rubidium you can go to time-constants of about a day or so. Depending on your rubidium, you might want a jitter-cleanup before going out thought. You can get a rough estimate of RMS due to GPS/GNSS by scaling the ADEV with tau/sqrt(3), thus T_(ADEV)(tau) = tau / sqrt(3) * ADEV(tau) The tau-scaling is needed for conversion from frequency stability to phase stability and the 1/sqrt(3) factor is for renormalizing from white frequency modulation to white phase modulation. You then add any sawtooth error on top of this, and to combine them for 95% confidence interval you do T_err = T_saw + 2*T_(ADEV)(tau) This is similar to how TDEV and MTIE is combined in telecom. TDEV is the MDEV based measure similar to the above. There is no standardized time deviation based on ADEV. Cheers, Magnus