[time-nuts] TAPR "PulsePuppy" Pot Selection

[Bob kb8tq]

Hi

With these little OCXO’s you likely will move them 0.1 ppb simply walking 
past the bench. If they have a total of 0.25 ppm of trim, a 25 turn pot will get 
you 1 ppb per turn. Setting these pots to 1/10 turn is not all that crazy. 

Yes, there are a bunch of assumptions made there and your setup may or
may not behave quite like the one I have described. 

Bob

> On Dec 24, 2017, at 11:32 AM, John Ackermann N8UR <jra at febo.com> wrote:
> 
> I didn't really notice much backlash, though when setting oscillators I try to approach (slowly) from one direction until it's "good enough" and then stop, to avoid that problem.
> 
> On Dec 24, 2017, 11:28 AM, at 11:28 AM, Dana Whitlow <k8yumdoober at gmail.com> wrote:
>> John,
>> 
>> Do you notice a backlash effect when homing in on the desired setting
>> with
>> those
>> tripots?  I last used such things back in the 1980's, and remember
>> often
>> having
>> enough backlash to make close trimming rather difficult.
>> 
>> I wonder if they have gotten better in that regard.
>> 
>> Dana
>> 
>> On Sun, Dec 24, 2017 at 9:58 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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