[sdiy] quadrature VCO - a different approach

[JH.]

Still working on my FS-2 project (it goes very slow - most of my spare time
is consumed by that big project of building a house now), I wanted to
make a better QVCO than the first time. Something that tracks as good as
a normal VCO, and which has lower THD than the first version.

I made a real big circuit, 3 times as many components as the first version,
but I wasn't really satisfied. It's really hard to have exact 90 degree
phase difference from 1/60 Hz  to 15 kHz, and also keep the triangle
levels constant (for precise sine conversion with a diode network),
and also keep dc offset low (to avoid carrier bleedthru in the FS),
over the whole wide range.

So I thought about a different approach: Using an ordinary VCO
and another dome filter. (Like some commercial FS with input for
an external oscillator). The catch is than now I need a dome filter
that goes down to 1/60Hz, in order to make slow modulation
with a period of 1 minute. 

A dome filter with 2 x 6 stages can create a phase difference
of 90 deg (+/- 0.3 deg) from 15Hz to 15kHz.

I made some calculations and found that I only need 2 x 12 stages
to do the same thing for a range from 15mHz to 30KHz.
This was less than I had expected, and so I've started to build
it. It will use no caps larger than 4.7 uF (no electrolytics needed)
and no resistors larger than 10 MegOhm (no special care for isolation
needed). The largest time constant is 30 seconds, so the filter should 
be able to work nicely a few minutes after power-on. With
careful component selection, I expect a phase error < 0.5 deg
in the audio range, and < 2 deg in the LFO range (where you
don't hear the carrier anyway).

I've only started to build this - I'll let you know about the progress.
Schematics will be published when I know that it really works.

JH.