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

[Bob Camp]

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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