[time-nuts] Digital Pots

[Bob kb8tq]

Hi

There are a lot of different designs out there. What you find on eBay and in most
OEM systems these days has a common ground pin for everything. The typical 
logic on the system designer’s end is “we only have one ground on the PC board, 
it’s all going there immediately”.  Out of a thousand designs, you might see one or 
two with something fancy done on the EFC return. 

Bob

> On Feb 4, 2020, at 9:26 AM, Dana Whitlow <k8yumdoober at gmail.com> wrote:
> 
> 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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>> 
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