[sdiy] Suggestions for oscillator coarse control methods?

[Mike Beauchamp]

On 1/22/20 9:03 AM, Spiros Makris wrote:
> Hello,
> I am looking for ideas on how to implement a coarse tune control on an 
> oscillator. I am trying to get the most stable performance, based on my 
> understanding of the topology (let's see how well I do), so the control 
> method has to be drift resistant or, at least, not be susceptible to it. 
> PCB surface is important, monetary cost less so (let's keep it under 5 
> euros for single quantities?). SMD when possible is preferred.


Interesting concern about potentiometer temperature dependency. Is it 
the same across all types (Carbon, conductive plastic, etc?) and I guess 
more importantly does a temperature dependent change in resistance even 
matter if the potentiometer is set-up as a voltage divider? (maybe only 
if it's feeding a relatively low-impedance input? if so then could you 
buffer the pots output before hitting the summing op-amp on a standard 
vco setup?)

You mentioned wanting the coarse tune to be stable.. so if you're doing 
it as a pot, are you using reference voltage sources to feed the pot 
instead of the rails?

My go-to setup is a 6 position rotary switch as an octave switch, but 
that's because I like larger chonky controls. For tuning I use a pot 
with a +/- ~7 semitone range so all frequencies are obtainable. The need 
for a further "FINE TUNE" knob isn't required if the diameter of this 
knob is large enough. The ability to finely tune a control depends on 
it's range but just as importantly, the diameter of the knob which 
dictates how many useable finite values you can actually get out of it.

Once I collaborated with someone whose eurorack setup included off the 
shelf oscillators that had a single tune control that covered the entire 
audio range and was just a 1/4" plastic shaft. He spent 10 minutes 
getting it "close" until everyone else decided to just tune to him.