[time-nuts] TAPR "PulsePuppy" Pot Selection

[Dana Whitlow]

Mark is correct, but with a caveat:  Unless the pot slider sees a load
impedance that
is much much larger than the pot's end-to-end resistance, contact resistance
variations can also play a big role, especially when the pot gets old.  For
this reason
alone I favor sticking with the 3-terminal "ratiometric" configuration, AND
using a
high-Z buffer amplifier between the pot slider and the load whenever
appropriate.

The downsides are noise and drift contributions from the amplifier, but
these can often
be held to acceptable levels with careful selection of the amplifier type
and model.

It all depends on the specific situation.  But in any event, I cringe when
I see a design
using a pot as a 2-terminal variable resistor, especially in situations
when an open
condition could cause damage.

Dana


On Sun, Dec 24, 2017 at 2:57 AM, Mark Goldberg <marklgoldberg at gmail.com>
wrote:

> Can you specify what pot you have used? I am using some for my TCXO boards
> and am not quite happy with the settability or mechanical stability.
> resulting in noise and higher Allan Deviation.
>
> A low noise regulator driving it also helped.
>
> I subscribe to the opinion to not use any extra resistors. When the pot is
> used as a voltage divider, theoretically it should have the same TC
> throughout, so temperature effects should not affect the divide ratio or
> the output. Only the input impedance of the control voltage input to the
> oscillator relative to the effective resistance of the pot will provide
> some effect with temperature.
>
> Mark
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