[time-nuts] A new take on the all-hardware GPSDO concept

Nick Sayer nsayer at kfu.com
Mon Sep 12 16:47:47 EDT 2016

> On Sep 12, 2016, at 1:27 PM, Charles Steinmetz <csteinmetz at yandex.com> wrote:
> Tim wrote:
>> You know Nick, the loop time constant typically used with the HMC1031 loop
>> filter is typically 5 milliseconds. I'm sure some bigger R's and C's can
>> used for a longer time constant, and I'm sure that'll help clean up the
>> awful 10MHz output of the Venus838LPx-T. But it is hardly what I'd call a
>> "GPSDO".
> One point that may not be obvious to people thinking of designing GPSDOs is this:  The requirement for a very long time constant control loop (hundreds to thousands of seconds) has nothing to do with the frequency of the reference signal from the GPS (that is, it is not due to the reference being a 1PPS signal).  Rather, it is determined by the tau at which the stability of the GPS signal becomes better than the stability of the local oscillator (generally an OCXO).  Unless the designer chooses a very bad local oscillator, this will be in the region of tau = 100-10kS (maybe only 20-50S if using a TCXO instead of an OCXO).
> The stability of any higher-frequency signal from a GPS that does not have a disciplined, high-stability LO (e.g., 10kHz in the case of the obsolete Navman Jupiter GPS receivers, or 10MHz in the case of the Venus) will be no better than the stability of the PPS, so the high-stability local oscillator you add will still need to be disciplined with the same slow loop you would use with PPS discipline if you want the sort of results time-nuts expect from a GPSDO.

Ok. Is it the case that the loop filter bandwidth is related to the time constant? Are there methods for minimizing the loop filter bandwidth that might be useful here? I would hazard a guess that obtaining >100s of TC equivalent in pure hardware would be at best difficult. At the same time, it seems like having a microcontroller act as an averager in software between an ADC and a DAC would be just silly.

Jim Miller's 10 kHz GPSDO that’s been referenced here has either solved this problem, or the 10 kHz output of the Jupiter is substantially better than the Venus’ 10 MHz output, or the design doesn’t give the results time-nuts expect from a GPSDO. Which of those applies?

> Best regards,
> Charles
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