[time-nuts] Phase meter for synchronizing osc's to GPSDO

[Attila Kinali]

On Tue, 31 May 2016 13:06:52 -0500
"Bud Patten" <bud.patten at frontiernet.net> wrote:

> Back in the spring of 2012 an article was published in VHF Communications
> entitled "A Phase Meter: a help to synchronize oscillator's to GPSDO".  In
> it the authors indicated that they hoped to add a "rotation counter" perhaps
> using Arduino.    Does anyone know whether further work was done on this?

Thanks to a friendly time-nut I got a copy of the article to read.

All the article describes is an RS-flipflop based phase detector,
with a following low-pass stage and an opamp as buffer/amplifier.
Nothing fancy.

As for the "extensions". They "promised" to do a digital version
with a phase counter that does more than 360°.

If you want to go digital, i would recomment something like the
Shera GPSDO's phase detector[1]. It's in my opinion better suited
to digtal readout than using the design by Darett&Canale together
with an ADC. The only thing you have to take care with the Shera
design is that the counter can go metastable, as the clock is
being gated. For a GPSDO that doesn't matter much as the outlier
will be filtered by the following low-pass filter. If you want to
count cycles (aka use it for frequency measurement) you need to be
able to detect those errors and filter them.

An alternative would be to use the capture/compare (aka timer) unit
of an microcontroller directly. With a modern microcontroller that
should give you higher resolution as with the Shera design, as
you can clock the timer with up to 70-100Mhz. These should also have
protection against metastable upsets, making their probability much
lower (albeit not zero).

No matter which way you choose, the software itself is pretty
easy to do. It just takes a little bit of time to design properly.

			Attila Kinali



[1] "A GPS-Based Frequency Standard," Brooks Shera, 1998
http://www.rt66.com/~shera/QST_GPS.pdf


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