[time-nuts] GPSDO Alternatives

Hal Murray hmurray at megapathdsl.net
Fri Dec 7 07:12:01 UTC 2012

> The output from a OCXO is divided down and then the phase of the divided
> down 10MHz RF is compared to the PPS and you don't need to even know the how
> far apart they are.  All you need to know is "led or lag"  just a one bit
> answer.   An XOR gate or a flip flop can tell you that. 

Does a 1 bit A/D work?  Is there a good web page discussing this aspect?  Am 
I just confused?What question should I be asking?   ...  Can somebody give me 
a circuit or (pseudo) code so I can simulate things?

I'm far from a PLL wizard.  I think the catch in this case is that the EFC 
controls the frequency and what you are measuring is the phase, the integral 
of the frequency.

Suppose you just implement a simple bang-bang control.

Suppose the EFC is 1 volt and the frequency is correct but the GPSDO phase is a bit early relative to the GPS PPS.  So the FF says early and the software says go-faster.  That keeps happening for a while, the frequency keeps getting faster and faster.  Finally, the GPSDO PPS catches up with the GPS PPS, but now it's frequency is way fast.  The FF says go slower, so the control software starts dropping the EFC.  But the frequency is still way too high so the error is still increasing.  After a while the frequency gets low enough so the PPS/phase error starts catching up.  Eventually the PPS error crosses over, but by then the frequency offset is way way low.  ...  Isn't that cyclic pattern stable?

Is there a simple tweak to break that loop?  Do you first have to recognize that you are in that mode?  If so, how?  ...

I might be able to do fix that in software by looking at the times when things change state.  Suppose it's 193 seconds between the first early and the last early and that the EFC went from X to Y.  I think that's enough info to work out the crossover point and work back to the desired EFC.

But that all sounds too complicated.  What would hardware-only guys do with a 1 bit A/D?

These are my opinions.  I hate spam.

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