Warren, I agree with other responders that a closed loop approach to OCXO minding is feasible using a simple micro and D-A.
I designed a system which has as its main focus low ADev and reasonably good frequency accuracy, without necessarily requiring the tight timing of a commercial GPSDO or the use of high resolution D-A. You might consider my system 'loose' control, as it relies on the OCXO being pretty good in the first place. It just corrects for long term effects such as ageing. What I came up with may be helpful. Essentially it phase locks the OCXO to GPS 1pps, but does so with extremely long integration times (hours), and with modest gain, so provided the OCXO is not subject to sudden temperature extremes or has an excessive ageing rate, will provide good control. As others have suggested, if you limit the control range (in my case I have a manual coarse setting) you can achieve very good closed-loop control with limited D-A resolution. I used 15 bit PWM, using a 12-bit timer and 3 bits of dither, providing a range of under 1ppm and a step size of about 3e-11. The system has no predictive capability, but does achieve (with a good OCXO) quite good holdover performance. By studying the control voltage over the last year, I've determined by extrapolation that manual intervention (coarse adjustment) will be unlikely within 10 years. I used a small AVR processor, did all the counting and maths in the processor, and operated the analog circuitry from the internal reference in the OCXO, resulting in very good thermal performance. The design is described in some detail at http://www.qsl.net/zl1bpu/MICRO/SIMPLE/SimpleGPS.htm. Regards, Murray Greenman _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.