[time-nuts] TAPR "PulsePuppy" Pot Selection

[Dana Whitlow]

I should comment that my bad experiences were in trimming out opamp DC
offsets.
As I recall, the amount of backlash was equivalent to a fair fraction of
one turn
of the shaft.

I never fully trusted the philosophy of sneaking up on the sweet spot and
then
walking away- I felt it was necessary to tap the board a few times to
verify that
the thing was going to stay trimmed during normal handling, temperature
excursions,
etc.

These trimmers were the 3/4" Cermet variety, made by Bournes and such.

Dana


On Sun, Dec 24, 2017 at 11:26 AM, Mark Goldberg <marklgoldberg at gmail.com>
wrote:

> I am using a Bournes 3224 and I do see backlash issues. I do come from one
> direction to set it and if I overshoot, I go way past and come back from
> the other direction. I also see non-monotonic sections. If one of those is
> where you want to set the frequency, it is pretty hard to do. I chose one
> with significantly lower impedance than the input impedance of the TCXO
> control port. The 3296 datasheet has Adjustability specs and the 3224 does
> not. The 3269 is only 12 turns but does have an Adjustability spec on the
> datasheet. Maybe I will consider that. I would have to change my board to
> use a throughole part.
>
> Thanks for the info.
>
> Mark
>
>
> On Sun, Dec 24, 2017 at 8:58 AM, John Ackermann N8UR <jra at febo.com> wrote:
>
> > I'm glad that the PulsePuppy post spawned some good discussion!
> >
> > The pot I'm using is a Bournes 3296W-1-103LF which is a 25-turn, 10K,
> > cermet pot, spec'd at 100ppm/degree, so it's not anything super fancy.
> The
> > number of turns provides decent setability, and it seems to be a good
> match
> > for the class of oscillator the PulsePuppy is designed for -- I found
> that
> > I could trim the Isotemp oscillator without problems.
> >
> > I'll admit up front that the PulsePuppy wasn't designed as a host for
> > ultra-stable oscillators.  I tried to keep the circuit board size and
> cost
> > down, as well as the number of components that users would have to
> > install.  And since the EFC trimmer is one of those user-installed parts,
> > it's possible to substitute as nice a pot as you'd like.  I have one unit
> > that's hooked up to an external precision pot with a turn-counter dial
> > (just because it was there).
> >
> > John
> > ----
> >
> > On 12/24/2017 03:57 AM, Mark Goldberg 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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