[time-nuts] Oscillator Buddy

Murray Greenman Murray.Greenman at rakon.com
Thu Aug 11 21:23:40 UTC 2011


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




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