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

Charles Steinmetz csteinmetz at yandex.com
Mon Sep 12 16:27:28 EDT 2016

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.

Best regards,


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