[time-nuts] Thunderbolt? (re simple gpsdo.)

Azelio Boriani azelio.boriani at screen.it
Fri Dec 30 12:40:48 EST 2011


Yes, it can be done based on a PPS only timing. You must design a PI (maybe
PID) regulator: the EFC must stay steady when the phase difference between
the two PPSes is zero (integral action). Then you must move the EFC (when
there is a difference) proportionally with the difference itself and only a
small part of the difference drives the integrator to increment/decrement
the new steady EFC level. When the difference is again zero the proportinal
part is gone and remains the small amount of correction for the integral
part. There is a gain for the integral action (samll) and a gain for the
porportional action (should be large, but to be evaluated).

On Fri, Dec 30, 2011 at 6:26 PM, Chris Albertson
<albertson.chris at gmail.com>wrote:

> On Thu, Dec 29, 2011 at 8:24 PM, Mark Spencer <mspencer12345 at yahoo.ca>
> wrote:
> > One of the designs using the 10 kHz output from a Jupiter gps engine and
> a simple PLL to discipline an ocxo might be good starting point if suitable
> gps engines are still available.  There won't be much to tweak but the
> performance could be quite good.
>
> I just finished reading about that one.  It requires no longer
> available GPS reciever.   Maybe I should r-phrase the question:
>
> "What is the simplest design for a GPSDO that uses only the PPS signal
> from a modern GPS?"
>
> Half of the reason for the question is academic.  Then if a simple
> enough design presented itself it would be fun to try it.
>
> The simplest design I can think of now is based on a flip-flop.  The
> PPS "sets' the FF and the next raising edge of the local oscillator
> "resets" it.   (The local oscillator might need to be divided down or
> a slower 1Mhz oscillator used so the FF remains "on" for a reasonable
> time.)
>
> Next the FF gates a current source to a capacitor.   The voltage in
> the cap is amplified and controls the local oscillator frequency via a
> low pas filter.  Likely the "low pass filter" would be an active
> device that we call an "integrator"
>
> You need to discharge the cap for the next cycle.  One could rig a
> one-shoot timer to discharge the cap.  Actually there are about four
> states that need to be cycled every second.
>
> Chris Albertson
> Redondo Beach, California
>
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