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.
As drawn, the EFC supply starts out with a bridge so you can steal just about any center-tapped AC in the instrument. I generally use 4700 uF filter capacitors. If you get to the point where the voltage you stole is so high that dissipation in the 5 V regulators is a problem, add some dropping resistors between the bridge and the filter caps. Some instruments will have +/- DC supplies you can steal. Not shown are the regulator output capacitors -- I use a tantalum and a multilayer ceramic to ground on each regulator output.
The LEDs are standard red or UV parts, and the diodes in series with the adjustment pot are 1N4148 or similar (any silicon signal diode, really). Together, these provide first-order temperature compensation for the EFC voltage. The LEDs and diodes should be reasonably isothermal, but there is no need to go nuts here. I typically choose the resistors at the regulator outputs for 10-15 mA through the LEDs, and the potentiometer string for around 1 mA with about +/- 0.2 to 0.5 V across the pot. The output filter should have as low a frequency corner as you can arrange. I usually use 100 k and a 5 uF film capacitor for a corner around 0.35 Hz. If you offset the frequency with the internal trimmer, you will know which polarity of EFC you need and therefore can use a larger, polar cap for a lower noise corner -- but I have never done that. A 10-turn pot has more than enough resolution, but I usually use a 20 or 25-turn anyway.
Note that pin 6 is grounded external to the 10811 in instruments that do not use the EFC, generally on the instrument's motherboard (or an intermediate daughter board, depending on the particular instrument). You will have to determine how it is done in your instrument and undo it for the EFC to work.
Finally, install the EFC fine frequency control where you can adjust it from the top, so you don't have to turn the instrument on its side or top to make the adjustment.
Best regards, Charles
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