[sdiy] Patching thru-zero oscillators from regular VCOs

[Donald Tillman]

   > From: Grant Richter <grichter at asapnet.net>
   > Date: Tue, 8 Jan 2008 22:05:50 -0600
   > 
   > Ooooops!
   > It was Don Tillman who did the very cool thru-zero trapezoid core.

(Or Laurie Anderson.  :-) )

I presented a circuit for a new thru-zero VCO with a trapezoid core in
2003.  The articles are here:

  http://till.com/articles/QuadTrapVCO/index.html

(Okay, yeah, I have some updating to do.)

   > Also, it was pointed out to me that most West Coast VCOs do not have  
   > sine wave outputs.

'Sounds like the Beach Boys: "The west coast has the sunshine and the
girls all get so tan..."

I think the Trapezoid VCO is interesting because it's very much in the
tradition of the classic VCO designs but it addresses the problems of
sawtooth cores (reset spike, reset time, waveshaping) and triangle
cores (waveshaping) by completely rethinking the core architecture and
using a new core waveform.  With the trapezoid you can do thru-zero
easily, you get quadrature for free, and while the design is a little
more complex, it's still very straightforward.

Typically you'd use an undamped 2-pole filter for a quadrature
oscillator, but those lose their stablity at low frequencies.

The QTVCO is very well behaved.  It's a lot of fun to run it down to
0.1 Hz, stop it dead, watch it sit there for a while, bring it through
0 Hz down to minus 0.1 Hz, then up to several thousand Hz, down to
minus several thousand Hz.  No problem at all, no glitches, no
misbehavior through zero Hz.

  -- Don

-- 
Don Tillman
Palo Alto, California
don at till.com
http://www.till.com