[sdiy] VA vs. RA: Square wave

[Amos]

Hi Paul,

Thanks for your clear explanation!  I appreciate your insight.

In other news, have you had a chance to ship Matthew's x0xb0x?

Many thanks,

Amos

On 4/4/07, Paul Schreiber <synth1 at airmail.net> wrote:
> > I have noticed in some analogue oscillators squarewave output, that
> > there is an unchanging time constant governing the loss of voltage
> > from the "high" state of the oscillator, such that at low frequencies
> > you actually see a gentle ramp downward in voltage from the start of
> > the cycle until the next reset, instead of a perfect square.  Since
> > this ramp downwards is not frequency-dependent (or not so much), at
> > high frequencies the same oscillator puts out a more perfect square
> > wave.
>
> The VCOs are not doing this. It's a function of inter-stage AC coupling.
>
> The caps (and the associated input impedance) create a quasi-BP filter (mostly
> HP). In order not to see this on a scope, the cutoff needs to be about 5 times
> below the lowest frequency. This can lead to rather large caps (over 100uf)
> which are physically large and expensive. So, a good compromise is to use 10uf
> caps (they need to be non-polar). Assuming a 33K input impedance for an audio
> summer, that gives a ~0.5HZ cutoff so anything above say 30Hz would not exhibit
> much "tilt".
>
> Where you really start to notice it is when you use a AC coupling cap of say
> 0.47uf or below.
>
> For a VA, this can be accounted for by either tilting the VCO output itself
> (*all* waveforms are effected, it's just easier to see it with a square) or a
> simple IIR filter to emulate the coupling effects.
>
> Paul S.
>
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