Hi Depending on how much you spend on a mechanical piston trimmer, the innards will be coaxial to some tolerance. To the extent they rotate or “swing” as one piece moves in and out of the other, the capacitance will be more linear or less linear vs rotation of the trimmer.
What you want - a straight line capacitance vs screw turns. What you get - a bit of a wiggly line of capacitance vs screw turns. On one side of the wiggle, the adjustment moves a bit fast. On the other side of the wiggle, the adjustment moves a bit slow. Next up is backlash. This a common issue in many mechanical systems. It’s most apparent in trimmers where a screw drives a moving part rather than the whole moving end being threaded. As the threads wear, they get a little slop in them Turn the screw clockwise all the time and everything is linear. Stop with clockwise and go counterclockwise and the threads have to mate no the other side of the screw. You don’t have anything happening until they do. If you read up on running a mechanical milling machine, you will see a lot of talk about this in terms of precision milling. Then of course you have broken trimmers. Ceramic trimmers can have the metabolized portions “stick” to each other. When you force them to move, you tear the metal off of the ceramic. Now you have a broken trimmer that really does odd things. Piston trimmers can get crud in them (either from outside or from their own moving parts). It does not take a very big chunk of stuff to short out the trimmer (if it’s conductive) or to mess up the tuning (if it’s not). Trim pots have their issues as well. The wipers can build up a bit of resistance from sitting in one place for a while. Move the trimmer and you clean up the contact. Depending on the circuit, this may or may not have much effect on the EFC to the varicap. Since trimmers can get a bit of force exerted on them, all the usual broken solder joint and ripped off the board sort of stuff applies to them as well. Lots of fun !! Bob > On Oct 21, 2014, at 2:50 PM, John Miles <[email protected]> wrote: > >> Great insight thanks. You nailed it: out with the old oscillator and in with >> one >> that doesn't have that problem. >> >> Btw the mechanical tuning issue you mentioned is essentially the same exact >> problem: even the slightest turn will make the frequency jump too high or too >> low. It can drive you (and the loop) crazy trying to get it on-frequency. > > Whenever I've seen this behavior, it has always been caused by uncertainty or > "quantization" on the part of the trimpot's wiper, rather than anything that > could be blamed on the varactor. What would be a good example of a TCXO or > OCXO model that exhibits EFC hysteresis? I don't immediately understand what > could cause this phenomenon, and I'd like to reproduce it here to see what's > happening. > > -- john, KE5FX > Miles Design LLC > > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
