[time-nuts] Digital Pots

[Dana Whitlow]

Don't "good" OCXOs float the oven stuff and use a separate return pin
instead of a
GND pin common with the OSC GND pin?

Dana


On Tue, Feb 4, 2020 at 7:42 AM Bob kb8tq <kb8tq at n1k.org> wrote:

> 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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