Tuning a Baxandall inverter.

On Monday, August 2, 2021 at 12:54:23 AM UTC+10, Piotr Wyderski wrote:
> whit3rd wrote: 
> 
> > How about using a little DC to start saturating an inductor?
> Hey, that's my line! ;-)
> > Also not a great idea for power, but the other idea is to servo a ferrite slug in a 
> > coil, which is fairly easy at a few hundred kHz. A suitable motor/advance mechanism can 
> > be found in any old floppy drive...
> If a mechanical solution is not a problem, then there was that airborne 
> military transceiver that used a motor for winding a coil with silver 
> tape as a way for tuning the LC tank. Something along these lines, but 
> not the exact model I have in mind: 

<snipped the links>

Sort of cute. The simplest and most popular way of mechanically tuning an inductor is to buy a gapped pot core (or RM core) with cylindrical hole down the middle.

They can be supplied with an adjusting slug which is a close fit in the central hole and can be screwed up and down with a screw-driver to bridge the gap to a  greater or lesser extent. Here's a current RM8 core data-sheet that shows the option.

https://www.farnell.com/datasheets/2138487.pdf

The actual adjusting slug is on page 9.

I've used them and they work fine. Putting a stepper motor to screw it up and down might get a bit messy. Warming up memory metal might be a bit more elegant - physicists have been known to think so - but getting it to work reliably in production could be tricky.

-- 
Bill Sloman,  Sydney