[time-nuts] New to list and GPSDO questions

Attila Kinali attila at kinali.ch
Thu Jun 13 16:39:01 EDT 2013

On Thu, 13 Jun 2013 12:17:53 -0700 (PDT)
Bob Stewart <bob at evoria.net> wrote:

> I want this to be as low budget as is reasonably possible, barring wastage,
> so I think a 1PPS source and Bert's FLL are the right choice.  Is there
> actually a PLL circuit that works with a 1PPS reference?  I only found a
> very few GPSDO projects during my search, and Bert's was the only one using
> the 1PPS signal.  And, I just like the way he did it, right down to giving
> the possibility of powering the output op-amp from the VREF output of the
> 34310-T.  I'm assuming that won't overload it, of course, so any comments on
> that would be appreciated.

[1] is probably the most cited GPSDO design. It's simple, it works and
you can build it quite cheaply

[2] is similar to what Shera did, with slight modifications on how to
measure the "phase".

[3] Describes how to use a clock synchronizer to build a GPSDO. Probably
not the easiest and not the cheapest way, but definitly one with a very
low parts count.

I think the cheapest way how to do a GPSDO that is fairly well accurate
is to use a uC with a high clock rate and a 32bit capture/compare unit
(eg STM32F4 which can do 78MHz on their c/c unit, thus giving ~14ns
resolution for ~15USD for the naked chip or an eval board).
Feed the 10MHz from your OCXO to the uC's clock input. Use the internal
PLL to upconvert it to what you need (i.e. as high a frequency as possible).
Feed the 1PPS to one of the c/c pins and measure when it enters. 

If your OCXO is fairly stable, you can integrate over a minute or two
(or even longer) and get down to uncertainties below 10^-9.

If you tweak your control loop such, that the mean of your PPS pulse
is exactly between two bins of your c/c, then you can get even better
than the resolution of your c/c, if your GPS generates a symetrical
distribution of the PPS pulse.


			Attila Kinali

[1] A GPS Based Frequency Standard, by Shera, 1998

[2] Building a GPSDO controller, by Pedersen, 2011

[3] The AD9548 as a GPS Disciplined Stratum 2 Clock, by Gentile, 2009

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