EFC fine tuning circuit

[tkamiya]

HP11081 has both physical tuning and EFC.  On EFC, I can apply +/-5V to swing as much as 10Hz on some models.  However, that means tuning rate is VERY fast even with 10 turn pots.  I do need an ability to swing to both extreme and have fine tuning capability using EFC only.

So I'm trying to come up with a setup where there is a coarse and fine tuning of EFC.  Cascading pots didn't work very well.  Fine tuning rate isn't constant depending on where main VR is positioned.  Does someone have a creative solution?

[fourfathom]

You might try something like this.  The resistance values can be change as appropriate for your EFC input impedance, what I am showing is the topology where one pot has a 10x (approximately) influence over the control voltage.  The "vernier" pot can have a higher resistance than the "coarse" pot.

[tkamiya]

Wouldn't that circuit form a voltage divider at the additive network (registers) though?  The way I'm reading it, 1K VR forms variable voltage from single source, the difference between the two will become the input to 10:1 register divider.  The maximum voltage I can get out of that will be 0.5V.  Or, am I reading that wrong? 

[5065AGuru]

Taka,

The +- 10Hz range on a 10811 is via the coarse tuning capacitor thru the hole in the case.

The EFC only ranges a 1Hz total range over +- 5V.

I normally just uses a 10K pot in series with a 500 Ohm pot in the ground leg.

I drive them with an LM399AH unless the oscillator provides it own low TC reference voltage out.

Cheers,

Corby

[tkamiya]

Corby

That's true for most models of 10811.  However, there are models that will do much more.  Types used in HP's GPSDO and Cs will do half of mechanical range in EFC.  The one I have, and I don't recall where it came from actually does full swing.  Your method makes sense though.  I'm using a 3 terminal regulator as HP usually does.  Perhaps that will need to be improved for my purpose.

[fourfathom]

Wouldn't that circuit form a voltage divider at the additive network (registers) though?  The way I'm reading it, 1K VR forms variable voltage from single source, the difference between the two will become the input to 10:1 register divider.  The maximum voltage I can get out of that will be 0.5V.  Or, am I reading that wrong?

I should have put reference #s for the resistors and pots!

The circuit is a voltage divider of sorts, but you can still get the full 0-5V range from it.  With the left-hand pot (course) set to 50%, adjusting the right-hand pot (fine) will give an output range of 2.5V +/- 0.227V.  With the coarse pot set at max (5V) the output voltage will be between 4.545V and 5V.  You can change the ratio of the fixed resistors to give you a larger or smaller fine-tuning range.