[sdiy] What is a quadrature oscillator good for?

[jhaible]

>    > One question: You mentioned a jitter near 0Hz if the two halves
>    > of the 3280 don't zero-cross precisely. Can you get this adjusted
>    > so precisely that you don't mess up the tuning- and waveform-purity
>    > when the VCO "just touches" the 0Hz region ?
>
> Yes; I've run the VCO around 0.05 Hz without problems.

Maybe I wasn't asking precisely enough: I don't mean the VCO running
permantently at a low frequency. I mean something like this:

Unmodulated VCO running at 1000 Hz. Then add audio rate linear FM
from a second VCO, running at 500.1 Hz. (The .1 difference to f/2
shall provide some nice beating - just an example.)
Then increase the modulation index such that the _momentary_ frequency
of your modulated VCO changes from 1 Hz ... 1999 Hz. No problem,
but rather boring. Now increase the modulation index such that
the momentary frequency goes from -1 Hz ... 2001 Hz.
During this transition, are there no artefacts produced? No wavering pitch,
no waveform distortion?

JH.