[sdiy] New VCO: SAW w/TZFM

[JH.]

Exciting!

JH.


----- Original Message ----- 
From: "Ian Fritz" <ijfritz at comcast.net>
To: <synth-diy at dropmix.xs4all.nl>
Sent: Thursday, October 02, 2008 4:18 PM
Subject: [sdiy] New VCO: SAW w/TZFM


Here's another approach to through-zero FM.  It uses a (modified) SAW core
driven by a bipolar modulated current source.  The output waveform stops
and reverses direction as the FM signal passes through zero, just as it
should.  A scope shot and a couple of quickie demos are up here:
http://electro-music.com/forum/topic-29149.html

More details:

The current source is a standard expo converter driving the control pin of
an OTA.  The voltage input to the  OTA produces a linear bipolar modulation
of the output current.

The core is an integrator-with-reset circuit.  The integrator ramps either
up or down depending on the sign of the control current.  A pair of
comparators reset the integrator when the ramp reaches either positive or
negative five volts.

Finally, the levels of the positive and negative segments are level-shifted
so they match up.  This gives the correct phase reversal of the output
waveform.

This method seems to me easier and more straightforward that the other
approaches I have looked at.  The prototype is running nicely, with
tracking to within 0.2% up to 20kHz.

Ian

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