Electronics > RF, Microwave, Ham Radio

GPSDO phase between PPS and clock

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Matth32:
Hello,

I'm interesting by GPSDO design and after reading a lot of documentation about that, I asking myself about one thing but I'm not able to answer myself.

On a GPSDO using a 10MHz clock, is the delay between the rising edge of the PPS and the rising edge of the 10MHz important? Let says, if I have a fixed delay (20 ns for example) between the two edge, is it OK?
Is the goal to have the minimum delay possible or any value at +/- 50 ns (because of the 10 Mhz) is OK (even if it's sliding)?

Could anyone help me?

capt bullshot:
With a GPSDO it's about a stable frequency, not a defined phase from GPS PPS to 10MHz clock.
Indeed, the phase will vary over time for some +/- tens to hundreds of ns, depending on the particular GPS receive. GPS provides good long term stability, but rather poor short term, so the main purpose of the oscillator part of a GPSDO is to provide a short term stable frequency, that is long term locked to the GPS PPS reference.
Figure the 10MHz divided down to a PPS - ideally this PPS should have a phase of +/- single digit ns to the GPS reference PPS. You'd achieve this by once synchronizing your PPS to the GPS, e.g. by resetting the divider chain, and then running some PLL to keep your oscillator in track to the GPS PPS. There's no need for a particular phase relationship of the one relevant raw 10MHz edge to the GPS PPS - it depends on your particular method to generate the local PPS.

Matth32:
Thanks for your answer.
My first idea was indeed to use a VC-TCXO of 10 MHz and use an MCU with the PPS connected in input. The MCU count the number of 10MHz pulse between two (or more to average) PPS and so adjust the frequency.

Using a PLL to track the PPS, I need one with a very low BW but perhaps, performance will be better?

capt bullshot:
A common way is to implement the PLL or some other kind of loop that controls the frequency on the microcontroller as software. No need for a hardware PLL.

David Hess:
For a GPSDO, the frequency is locked to the pulse per second output, and the phase does matter.  The problem is that errors in the the GPS timing solution vary over a 12 or 24 hour period, so when a stable oscillator is used, the phase of the PPS output will shift in comparison to the stable oscillator.

This means that oscillator stability should be considered over at least a 12 to 24 hour period so that the integrated phase comparison between the GPS and oscillator is valid over that same period.

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