[time-nuts] Simple solution for disciplining OCXO with 1 PPS

Attila Kinali attila at kinali.ch
Sun May 22 18:52:17 EDT 2016


Hoi Bernd,

Is there any special reason for this endavour or are you just courious
what can be done?

On Sun, 22 May 2016 19:41:25 +0200
"Bernd Neubig" <BNeubig at t-online.de> wrote:


> I am well aware, that such a solution would have a lot of disadvantages,
> as it cannot effectively compensate for short-term variations. However I
> would be happy if such a KISS solution could achieve  a stability (STS) of
> better 1E-10 over an hour. I know this is a damned long integration time for
> an analogue integrator...

With a pure analog solution, anything with a tau more than a couple of
seconds will depend on the GPS receivers performance.

Judging from the ADEV plots of the M12 receiver from TvB[1], which hit
10^-10 (w/ sawtooth) around 200s, I'd say that 10^-10 @1000s should be
easy.

> If that sounds too weird, I am open to receive advises for a microcontroller based solution.

If you want to go that way, probably the simplest solution would be to
take one of Nick Sayers boards, pull out the GPS receiver and feed the
PPS input from your GPS receiver.

> Thanks a lot for your comments to come.
> BTW: you need not to teach me about basics of short-term stability.
> I just want to evaluate the limits of a possible analogue solution. For
> sure, that's not real disciplining, but more like a long-tau integration PLL

The simplest solution that comes to my mind would be some form of time
to amplitude conversion, using the PPS from the GPS as start and some
OCXO derived (aka divided down) signal. It can be a full fledged TAC
like the ones used for TICs (see Bruce Griffith's design[2]) or something
as simple as the 4046 based one in Nick Sayers circuit.

Then, use a sample and hold circuit to replicate and stabilize this voltage
and feed it to a discretely build PI controller.

The big problem of this circuit will be the long time constant you will
need in the PI controller and the droop of the capacitors in it. You
will need a clean board and some guard rings at the critical points
to keep the leakage currents low.

			Attila Kinali

[1] http://www.leapsecond.com/pages/m12-adev/
[2] https://www.febo.com/pipermail/time-nuts/2012-December/073224.html
    (in case you have trouble viewing it, let me know, i have a pdf version)
-- 
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		-- unknown


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