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 <marklgoldb...@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 <j...@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 > >> > >> > _______________________________________________ > 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. > _______________________________________________ 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.