[sdiy] quadrature VCO - a different approach

[jhaible at debitel.net]

> So you want to control the amount of frequency shift by moving a slider 
> potentiometer up and down? 


Almost. As long as my manual movements are "fast" enough (i.e. coming
back to the starting point within a minute or less), I expect to have 
direct control over the phase of the modulator, even if I'm far from 
making movements with any consistent frequency. The filter network
should take care of the quadrature relationship for any kind of
movement. Real DC components are a problem, though, so 1st thing I
did was adding an AC coupling with 16mHz corner frequency at the input.


> How about an endless rotary knob with 
> something like in those optical volume pedals? If you turn this knob 
> the aperture would open and close periodically. With two LED/LDR 
> combos, 90 degrees apart, you'd have sine and cosine outputs. I think 
> it's easier to turn a knob evenly than moving a slider up and down.

Or I could simply use a joystick and make circular movements.

But for now I just want to try this filter, because it will also
work on ordinary VCOs / LFOs with (hopefully) better precision
than a direct analogue quadrature oscillator solution.

At the moment I have too much HF noise building up over the 12
stages, and the usual compensation caps won't work because they f*ck
up the precise phase difference. I'll try different opamps
now (ordered a set of TL064s and TL054s to replace the TL074s).

JH.

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