[time-nuts] 10811 EFC control

Charles P. Steinmetz charles_steinmetz at lavabit.com
Thu Oct 28 17:27:32 UTC 2010

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,


-------------- next part --------------
A non-text attachment was scrubbed...
Name: 14cb6c7.jpg
Type: image/jpeg
Size: 48318 bytes
Desc: not available
URL: <http://www.febo.com/pipermail/time-nuts/attachments/20101028/12a832cf/attachment-0001.jpg>

More information about the time-nuts mailing list