[time-nuts] TAPR "PulsePuppy" Pot Selection

[Bob kb8tq]

Hi

The other issue with putting in an amp is noise. You really do not want to feed
a bunch of added “stuff” into the FM modulation port on the oscillator. You get
some thermal noise from the pot, so it is never zero. How close you need to get
to zero - who knows? When the part was tested to see if it met spec, it probably 
didn’t have an amp in there …..

This of course all assumes a stand alone device like the Pulse Puppy. Once you
go to a structure like a GPSDO, things inevitably get a bit more complicated. That
is one of the many reasons testing this and that on a servo’d system is part of the
normal design process. 

Bob

> On Dec 24, 2017, at 6:53 AM, Dana Whitlow <k8yumdoober at gmail.com> wrote:
> 
> 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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