[time-nuts] TAPR "PulsePuppy" Pot Selection

[Bob kb8tq]

Hi

The nice thing is that the footprint on the Pulse Puppy is pretty generic.
There are a lot of OCXO’s on eBay that will fit the pattern on the board. 
When you get into the more exotic parts, the footprints (and functions) 
don’t tend to be as standardized. 

The small package OCXO’s will always have an issue with drafts and 
temperature stability. There just isn’t much room in there for a fancy 
stuff in there. Putting a bunch of baffling around them is problematic. 
You can quickly bump up the thermal gain and likely destabilize the control
loop ….

Lots of fun …

Bob

> On Dec 24, 2017, at 10:58 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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