[sdiy] New VCO: SAW w/TZFM

[Ian Fritz]

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