[sdiy] Sine VCO

[Gene Stopp]

I was on jury duty yesterday sitting in the selection waiting room, and for
reading material I brought the good ol' Electronotes PCC, so this stuff is
fresh in my mind. There's two circuits that come to mind for a sine VCO:

1. Triangle-based VCO with 3080 integrator and 3080 schmidt trigger, with
another 3080 after the triangle to round it to a sine. Nice low parts count,
and if you use a MAT-03 or similar matched PNP pair and a tempco, you'll
have a temperature-compensated fully tracking 1v/oct wide-range sine VCO
suitable for keyboard use. The sine will be approximate but audibly pretty
darn good.

2. 4-pole lowpass VCF with resonance set to oscillation. Four 3080-based
cascaded integrators, and similarly if you match the trannies and use a
tempco, also fit for precision tracking. The sine wave should be very pure
if resonance is set properly - the first integrator will be clipping
slightly, but it is followed by three tracking integrators. One possible
problem would be resonance staying at the same level across the entire audio
range - my experience with 4-pole LPF's is that sometimes the oscillation
dies out at lower frequencies. Maybe an AGC on the feedback?

As a side-thought, you could go the first option and follow it with a couple
of tracking integrators - kind of a hybrid between the two.

Both designs are very 3080-intensive (be it ever so humble).

- Gene



-----Original Message-----
From: "Rönnberg, Niklas" [mailto:nikro at itn.liu.se]
Sent: Friday, March 28, 2003 12:08 AM
To: synth-diy at dropmix.xs4all.nl
Subject: [sdiy] Sine VCO


I'm looking for a sine VCO. I don't want (or need) any other waveform from
this VCO, but I guess that that is unavoidable. I also want the VCO to have
an input for FM...
I think that I saw a schematic once that was based on a LM13600. This VCO
made a triangle wave, so maybe this would be a good start for a sine
VCO(?).

Can anyone help me pointing out a schematic or giving a tip on designing
this circuit?

Regards,

Niklas