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.
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.
You can buy his, or build your own with his design as a starting point.
integration issues Bob referred to.
Post by Bob CampHi
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 <
Post by Attila KinaliPost by Bernd NeubigIf that sounds too weird, I am open to receive advises for a
microcontroller based solution.
Post by Attila KinaliIf 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
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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