[time-nuts] GPSDO control system

Charles Steinmetz csteinmetz at yandex.com
Sun Mar 23 19:08:16 EDT 2014

Chris wrote:

>4) why use a PPS as the communications link from GPS to GPSDO?

Generally, because that is the only precision timing output you get 
from a GPS unit.  The models with a 10kHz output were prized by the 
"simple GPSDO" crowd precisely for the fact that the phase-locked 
10kHz signal made the control loop much easier to design (or, made an 
analog loop *possible* to design).

>Can I find a way to move the OXCO into the guts of a GPS receiver.

I don't know.  Can you?  That is essentially what the Thunderbolt 
does, and it brings with it the tremendous advantage that it removes 
the sawtooth error -- which is why some of us think so highly of that 
design.  But now you are really down and dirty, designing your own 
GPS receiver.  Are you competent (and, if so, willing) to do 
that?  (You are very, very far from a "simple" GPSDO if you are 
considering this.)

Honestly, I never understood why all timing receivers aren't designed 
that way.  It seems to be such a superior way to do things that I'd 
have thought the other approaches would have died out once everybody 
knew about it.  Even if Trimble has a patent, I can't imagine it 
would be that expensive to license.  But even Trimble doesn't use it 
on all of their timing receivers.

Anybody know why this scheme hasn't been used more widely?  Is there 
something I'm missing?

Best regards,


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