Re: [time-nuts] TAPR "PulsePuppy" Pot Selection

[Mark Goldberg]

Absolutely I see what you describe below. Bournes actually has an old paper
that describes all of these issues, but they do not seem to address them in
selection guides these days. I have no way to find out which are good until
I try 50 of them. I get a few that are just awful, basically a random
connection between the screw and the pot position.

www.bourns.com/docs/default-document-library/bourns_trimmer_primer.pdf

I have tried all kinds of tricks, going back and forth, sneaking up on it,
yes, tapping it, sweeping the wiper back and forth around the desired
setting to hopefully clean off crud from the element in that area, etc.
Some just don't want to be adjusted to a specific setting that just happens
to be on frequency. Most are fine. Some are a real pain to set where you
want.


Mark


On Sun, Dec 24, 2017 at 3:26 PM, Dana Whitlow  wrote:

> I think I need to clarify what I mean by "backlash".  It is not simple free
> play in
> the adjustment mechanism- it is something much more irritating, as follows:
>
> I sneak up on the desired result, but manage to overshoot slightly.  So I
> back
> off on the screw, and find that at first the result continues to change in
> the
> *original* direction (making the overshoot even worse) for a bit before
> finally
> reversing as I wanted it to.  This behavior is not conducive to having a
> good
> time making critical adjustments, nor does it lend any confidence in the
> stability
> of the adjustment in the face of handling.
>
> Dana
>
>
> On Sun, Dec 24, 2017 at 4:06 PM, Charles Steinmetz 
> wrote:
>
> > John 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.
> >>
> >
> > The hot tip is not to just "sneak[] up on the sweet spot and then walk[]
> > away," as Dana put it.
> >
> > Anytime you have an adjustment with some hysteresis (classic example is
> > setting a d'Arsonville movement to zero), you want to sneak up to the
> > perfect setting and then run the adjuster *back* the way you came just a
> > touch, to leave the adjusted part on its own without any mechanical
> > connection to the adjustor mechanism.  Such contact is almost always the
> > culprit if the adjustment drifts after you set it.
> >
> > This takes some "feel" for the motion of the adjuster mechanism, but it
> is
> > well worth investing the time to learn it by repeated trials of the
> > adjuster before you leave it alone.
> >
> > Dana is spot on with his advice to tap the board (or whatever
> mechanically
> > supports the adjusted part) to make sure it doesn't drift.  If it does,
> you
> > either failed to pull the adjuster out of contact with the moving
> adjusting
> > part, or the adjusted part just can't hold its setting.  In either case,
> > better to know that now than after you button the instrument back up.
> >
> > Best regards,
> >
> > Charles
> >
> >
> >
> > ___
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> > ailman/listinfo/time-nuts
> > and follow the instructions there.
> >
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[time-nuts] TAPR "PulsePuppy" Pot Selection

[Mark Sims]

Another very irritating "feature" of pots is that over time the wiper can 
deform the resistance element.  When you go to adjust a pot (like when making a 
calibration adjustment on an old piece of equipment) it can be very difficult 
to get (and keep) the setting where you want it to be if the new setting is 
close to where the old, well aged, setting was.  The wiper will wind up on the 
deformed "slope" of the resistance element which is no longer a stable place.  
Also once the wiper is moved off the deformed point the wiper may relax over 
time and the adjustment will drift.  You may have to replace the pot to be able 
to set and keep it where it needs to be.
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Re: [time-nuts] TAPR "PulsePuppy" Pot Selection

[Angus]

Hi,

Normally it's best to adjust a multi-turn trimmer in just one
direction, so if you do overshoot, go back 2 or 3 turns and then on
again to the place you want to be. The imperfections in the mechanism
will drive you nuts (in a bad way) if you try to do fine adjustments
back and forth.

It's even better if you can monitor the wiper voltage when adjusting
it.

Angus.

On Sun, 24 Dec 2017 16:26:43 -0600, you wrote:

>I think I need to clarify what I mean by "backlash".  It is not simple free 
>play in
>the adjustment mechanism- it is something much more irritating, as follows:
>
>I sneak up on the desired result, but manage to overshoot slightly.  So I back
>off on the screw, and find that at first the result continues to change in the
>*original* direction (making the overshoot even worse) for a bit before finally
>reversing as I wanted it to.  This behavior is not conducive to having a good
>time making critical adjustments, nor does it lend any confidence in the 
>stability
>of the adjustment in the face of handling.
>
>Dana
>
>
>On Sun, Dec 24, 2017 at 4:06 PM, Charles Steinmetz 
>wrote:
>
>> John 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.
>>>
>>
>> The hot tip is not to just "sneak[] up on the sweet spot and then walk[]
>> away," as Dana put it.
>>
>> Anytime you have an adjustment with some hysteresis (classic example is
>> setting a d'Arsonville movement to zero), you want to sneak up to the
>> perfect setting and then run the adjuster *back* the way you came just a
>> touch, to leave the adjusted part on its own without any mechanical
>> connection to the adjustor mechanism.  Such contact is almost always the
>> culprit if the adjustment drifts after you set it.
>>
>> This takes some "feel" for the motion of the adjuster mechanism, but it is
>> well worth investing the time to learn it by repeated trials of the
>> adjuster before you leave it alone.
>>
>> Dana is spot on with his advice to tap the board (or whatever mechanically
>> supports the adjusted part) to make sure it doesn't drift.  If it does, you
>> either failed to pull the adjuster out of contact with the moving adjusting
>> part, or the adjusted part just can't hold its setting.  In either case,
>> better to know that now than after you button the instrument back up.
>>
>> Best regards,
>>
>> Charles
>>
>>
>>
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>> ailman/listinfo/time-nuts
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>>
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Re: [time-nuts] TAPR "PulsePuppy" Pot Selection

[Poul-Henning Kamp]

In message , "Brian, WA1ZMS" 
writes:

>I have seen similar issues to Dana's and have told myself it must
>be torque left in the gear-train within the pot. Maybe all in my
>mind as well, but it seems real to me on some equipment.

Or simply that you are too impatient and your previous correction has
not fully been effected yet.

-- 
Poul-Henning Kamp   | UNIX since Zilog Zeus 3.20
p...@freebsd.org | TCP/IP since RFC 956
FreeBSD committer   | BSD since 4.3-tahoe
Never attribute to malice what can adequately be explained by incompetence.
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Re: [time-nuts] TAPR "PulsePuppy" Pot Selection

[Brian, WA1ZMS]

I have seen similar issues to Dana's and have told myself it must be torque 
left in the gear-train within the pot. Maybe all in my mind as well, but it 
seems real to me on some equipment.

-Brian, WA1ZMS

> On Dec 24, 2017, at 5:26 PM, Dana Whitlow  wrote:
> 
> I think I need to clarify what I mean by "backlash".  It is not simple free
> play in
> the adjustment mechanism- it is something much more irritating, as follows:
> 
> I sneak up on the desired result, but manage to overshoot slightly.  So I
> back
> off on the screw, and find that at first the result continues to change in
> the
> *original* direction (making the overshoot even worse) for a bit before
> finally
> reversing as I wanted it to.  This behavior is not conducive to having a
> good
> time making critical adjustments, nor does it lend any confidence in the
> stability
> of the adjustment in the face of handling.
> 
> Dana
> 
> 
> On Sun, Dec 24, 2017 at 4:06 PM, Charles Steinmetz 
> wrote:
> 
>> John 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.
>>> 
>> 
>> The hot tip is not to just "sneak[] up on the sweet spot and then walk[]
>> away," as Dana put it.
>> 
>> Anytime you have an adjustment with some hysteresis (classic example is
>> setting a d'Arsonville movement to zero), you want to sneak up to the
>> perfect setting and then run the adjuster *back* the way you came just a
>> touch, to leave the adjusted part on its own without any mechanical
>> connection to the adjustor mechanism.  Such contact is almost always the
>> culprit if the adjustment drifts after you set it.
>> 
>> This takes some "feel" for the motion of the adjuster mechanism, but it is

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Re: [time-nuts] TAPR "PulsePuppy" Pot Selection

[Dana Whitlow]

I think I need to clarify what I mean by "backlash".  It is not simple free
play in
the adjustment mechanism- it is something much more irritating, as follows:

I sneak up on the desired result, but manage to overshoot slightly.  So I
back
off on the screw, and find that at first the result continues to change in
the
*original* direction (making the overshoot even worse) for a bit before
finally
reversing as I wanted it to.  This behavior is not conducive to having a
good
time making critical adjustments, nor does it lend any confidence in the
stability
of the adjustment in the face of handling.

Dana


On Sun, Dec 24, 2017 at 4:06 PM, Charles Steinmetz 
wrote:

> John 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.
>>
>
> The hot tip is not to just "sneak[] up on the sweet spot and then walk[]
> away," as Dana put it.
>
> Anytime you have an adjustment with some hysteresis (classic example is
> setting a d'Arsonville movement to zero), you want to sneak up to the
> perfect setting and then run the adjuster *back* the way you came just a
> touch, to leave the adjusted part on its own without any mechanical
> connection to the adjustor mechanism.  Such contact is almost always the
> culprit if the adjustment drifts after you set it.
>
> This takes some "feel" for the motion of the adjuster mechanism, but it is
> well worth investing the time to learn it by repeated trials of the
> adjuster before you leave it alone.
>
> Dana is spot on with his advice to tap the board (or whatever mechanically
> supports the adjusted part) to make sure it doesn't drift.  If it does, you
> either failed to pull the adjuster out of contact with the moving adjusting
> part, or the adjusted part just can't hold its setting.  In either case,
> better to know that now than after you button the instrument back up.
>
> Best regards,
>
> Charles
>
>
>
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> ailman/listinfo/time-nuts
> and follow the instructions there.
>
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Re: [time-nuts] TAPR "PulsePuppy" Pot Selection

[Charles Steinmetz]

John 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.


The hot tip is not to just "sneak[] up on the sweet spot and then walk[] 
away," as Dana put it.


Anytime you have an adjustment with some hysteresis (classic example is 
setting a d'Arsonville movement to zero), you want to sneak up to the 
perfect setting and then run the adjuster *back* the way you came just a 
touch, to leave the adjusted part on its own without any mechanical 
connection to the adjustor mechanism.  Such contact is almost always the 
culprit if the adjustment drifts after you set it.


This takes some "feel" for the motion of the adjuster mechanism, but it 
is well worth investing the time to learn it by repeated trials of the 
adjuster before you leave it alone.


Dana is spot on with his advice to tap the board (or whatever 
mechanically supports the adjusted part) to make sure it doesn't drift. 
 If it does, you either failed to pull the adjuster out of contact with 
the moving adjusting part, or the adjusted part just can't hold its 
setting.  In either case, better to know that now than after you button 
the instrument back up.


Best regards,

Charles


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Re: [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 
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  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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Re: [time-nuts] TAPR "PulsePuppy" Pot Selection

[Mark Goldberg]

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  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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Re: [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  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  
> 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 
>> 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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 ___
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Re: [time-nuts] TAPR "PulsePuppy" Pot Selection

[John Ackermann N8UR]

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  
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 
>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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>>> ailman/listinfo/time-nuts
>>> and follow the instructions there.
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Re: [time-nuts] TAPR "PulsePuppy" Pot Selection

[Dana Whitlow]

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  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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Re: [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  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
>> ___
>> time-nuts mailing list -- time-nuts@febo.com
>> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
>> and follow the instructions there.
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Re: [time-nuts] TAPR "PulsePuppy" Pot Selection

[John Ackermann N8UR]

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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Re: [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  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 
> 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
>> ___
>> time-nuts mailing list -- time-nuts@febo.com
>> To unsubscribe, go to https://www.febo.com/cgi-bin/
>> mailman/listinfo/time-nuts
>> and follow the instructions there.
>> 
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Re: [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 
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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> mailman/listinfo/time-nuts
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Re: [time-nuts] TAPR "PulsePuppy" Pot Selection

[Poul-Henning Kamp]

In message 
, Mark 
Goldberg writes:

>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.

I researched this a bit a couple of years ago in relation to HP5065 C-field 
tempco

The major tempco in pots are mechanical in nature.

Plastics have *horrid* tempcos, most in the hundreds of
PPM and the best (nylon) barely making it under 20 PPM.

Mind you, that is usually measured on relatively large linear
extrusions, not small bits of geometrically complicated plastic,
like you would use to encapsulate a trimpot.

There are trimpots on the market which claim 5PPM ratio stability,
but the conditions under which that is measured are not very easy
to implement in practice.

If you want anything close to 1PPM trimpots, hunt eBay for "ESI dekapot"


-- 
Poul-Henning Kamp   | UNIX since Zilog Zeus 3.20
p...@freebsd.org | TCP/IP since RFC 956
FreeBSD committer   | BSD since 4.3-tahoe
Never attribute to malice what can adequately be explained by incompetence.
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[time-nuts] TAPR "PulsePuppy" Pot Selection

[Mark Goldberg]

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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