[time-nuts] 10811 EFC control

Rick Karlquist richard at karlquist.com
Thu Oct 28 18:12:58 UTC 2010

You need to understand that there is a +6.2V Zener diode reference
connected to the cathode of the varactor.  If you really want to
minimize any perturbations of the oscillator by the tuning diode,
you should operate up near +5V, so that the voltage across
the varactor is minimized.  Also, ideally, you should bring out
the 6.2V node as the true reference for the EFC (as opposed to ground).

Rick Karlquist N6RK

Charles P. Steinmetz wrote:
> OK, maybe if I just attach the file rather than trying to embed it?
> A while back, someone asked how to use the EFC on a 10811 to allow
> fine frequency adjustment without spoiling the inherent stability of
> the oscillator.  I finally found a copy of the schematic for the
> circuit I have used with success, which I'm pasting below in the hope
> it will make it through the list server.
> Pin 6 of a 10811A accepts an EFC voltage of +/- 5 V relative to
> oscillator ground (pins 2, 4, and 5), for a total frequency
> adjustment range of around 1 Hz.  I believe the D version works the
> same.  Since I can generally get within a few mHz using the existing
> trimmer and some patience, I find no need for anywhere near this much
> range in the EFC.  Accordingly, I shoot for +/- 0.2 to 0.5 V across
> the trimmer, so that the tempco of the circuit has that much less
> effect on stability and the adjustment has more resolution.

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