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 <[email protected]> > 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. > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
