[time-nuts] Digital Pots

[Bob kb8tq]

Hi

The great thing about a wire wound pot is that as a voltage divider, it has
a very low temperature coefficient. The problem with adding resistors is
that their temperature coefficient will not match that of the winding material
on the pot. The “value” of the wire wound is lost in this case. You might as well
just run a multi turn cermet pot. 

One alternative are the big multi dial voltage dividers you see from time to time
on eBay. Prices (as with anything on eBay) are all over the map. Score some 
cheap and you can get a lot of resolution and a lot of stability. 

Another alternative would be to grab some “zero drift” op amps. Buffer up a pair of 
pots and then drive a thin film network with them. (or spend around $20 each on some
shiny new low TC resistors). Then buffer the sum junction with another op-amp. 

===========

If the oscillators are OCXO’s, ground current is going to make problems pretty 
quickly. With a common ground pin and voltage changes measured in millivolts, 
it limits how well a normal OCXO can be set. 

Bob

> On Feb 4, 2020, at 2:18 AM, Perry Sandeen via time-nuts <time-nuts at lists.febo.com> wrote:
> 
> Learned Gentlemen,
> Thanks for the two references for affordable 10 turn precision pots.
> This will allow me to go back to my original and simpler circuit of a series string of resistors and in that string connecting the 10K10 turn pot in parallel with one of the resistors, probably a 1K, which will be set by the upper slightly above and the lower just below the *sweet spot* needed. It will take a bit of juggling for each oscillator but that will be far simpler than using 3 digital pots.
> Regards,
> Perrier
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