TAPR "PulsePuppy" Pot Selection
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@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@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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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
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 csteinmetz@yandex.com
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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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 k8yumdoober@gmail.com 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 csteinmetz@yandex.com
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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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 k8yumdoober@gmail.com 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 csteinmetz@yandex.com
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
In message D9BB2FE5-C3FA-4FE0-9F32-676D7A006E7E@att.net, "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
phk@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.
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 csteinmetz@yandex.com
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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