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

Graham / KE9H ke9h.graham at gmail.com
Tue May 24 16:53:48 UTC 2016


If you want an "existence proof" for a simple, mostly analog, a few digital
counters, no software or microprocessor GPSDO, look at the "Miller" GPSDO.
He designed it for his own use, then put it into production because of
demand for a simple, cheap GPSDO.  It has been characterized, and works
well for the simple circuit.

The schematics are published, since it started as a hobby project.

http://www.jrmiller.demon.co.uk/projects/ministd/frqstd0.htm

http://www.jrmiller.demon.co.uk/projects/ministd/frqstd.htm

You can buy his, or build your own with his design as a starting point.

He does use a GPS with 10 kHz output to simplify some of the timing and
integration issues Bob referred to.

--- Graham

==



On Mon, May 23, 2016 at 8:21 PM, Bob Camp <kb8tq at n1k.org> wrote:

> Hi
>
> Ok, so how would you do a pure analog GPSDO?
>
> The GPS receiver and that side of it are what they are. I’ll assume that
> you have a 1 pps out of a module.
>
> Your OCXO needs to get to 1 Hz via dividers. You can do that with digital
> dividers or with a chain of regenerative
> dividers. One is a bit more analog, the other may be “ok” under the “don’t
> go to crazy” ground rule.
>
> You now have a PPS that is off somewhere relative to the GPS. A push
> button will get them into rough alignment.
> Your OCXO is quite likely a bit high or low. A multi turn pot on the EFC
> will let you get it within 1x10^-9 without a
> lot of crazy work. A reasonable counter tied to a reference will let you
> do this.
>
> Net result: The pps signals are roughly aligned and drifting < 1 ns / s.
> Considering the delta between them is
> bopping around by 10 ns, that’s quite good.
>
> Run a very normal bipolar charge pump off of the delta between the two pps
> signals. Fire a sample and hold when
> the transition is over. You now have a (maybe) +/- 60V signal that
> corresponds to the phase error. Since you are using
> film capacitors, the 60V comes along for free. Taking it to the maximum is
> just a way to save money on caps.
>
> Next up, do a fairly simple 20 second time constant R/C filter. That will
> take out a lot of the hopping around and make
> the rest of the system a bit easier to quiet down. You now have a somewhat
> linear +/- 60V signal that tells you how
> far off phase the setup is. After the RC you have a high input impedance /
> low drift buffer amplifier. Yes that’s a little
> tricky.
>
> Next you need a P and an I term. Both need to be variable as the system
> calms down. A rotary switch will do fine for
> this. Relays might also do the job. The P is a bank of resistors, each one
> to scale the buffered R/C to your control amp.
> The I goes off to a similar set of resistors driving an integrator. Net
> time constant there will be in the 200 to 2,000 second range.
> That’s were the ovenized caps come in. You also need a really good amp as
> part of the integrator to buffer out the signal.
>
> The nice thing about doing it this way is that you can *see* it all
> happening. There is a nice *clunk* noise as the filter
> steps off. Each number in the filter has a (likely large value) resistor
> that sets it up. To change the filter characteristics,
> you swap out resistors or twiddle pots.
>
> If you do the math, even with 60 V on the system, you probably don’t want
> anything over 1 meg ohm involved. At 2K seconds
> that gets you to a pretty big film capacitor bank. Even the 20 second
> lowpass isn’t exactly small by the standards of fancy
> capacitors.
>
> There are a few interesting tidbits like wire wound / high value / low
> temp co resistors that would help things a bit. Swapping
> those in and out as you change filter settings experimentally could get a
> bit crazy.
>
> The net result should be a good starting point for a GPSDO. You still
> would need to spend all of the time working out values
> and matching it up to your OCXO. The need for a good local reference and
> good measurement gear while doing this still is
> a limit, just like the pure digital approach.
>
> Bob
>
>
>
> > On May 23, 2016, at 12:46 PM, Nick Sayer via time-nuts <
> time-nuts at febo.com> wrote:
> >
> >
> >>
> >>> 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.
> >
> > It’d be kind of an awkward fit. For the OCXO/TCXO, you’d need to pull
> the oscillator as well as the GPS (I believe you said you had an oscillator
> already), and your EFC would be 1.65 volts wide centered on 1.65 volts.
> That’s unlikely to be absolutely correct for your oscillator. You could
> change around the Vref for the DAC, but at that point I’d consider
> redesigning the board for your purposes instead.
> >
> > That said, I think it’d be easy to adapt the circuit and code for a more
> arbitrary setup. And I believe my system is good down to the ADEV 10E-11
> level at tau 1s or so. I don’t know how much better it can do, as I’ve
> simply not tried to go below that (and I likely couldn’t properly measure
> the results anyway).
> >
> > There’s also the FE-5680 board, but it has an RS-232 level shifter in
> place of the DAC. On the other hand, it does have a very nice 2A @ 15V
> power supply, which likely is very close to what you’d need for a really
> good OCXO. A mash-up of that with the DAC put back in might be closer. But
> either way, you’re designing a new board, I think.
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