[sdiy] (no subject)

[Harry Bissell]

Hi Ian

I'm simulating what you described... it seems like it should work but right now
my schmidt trigger is failing to work after a through-zero event. Strange.
I'll keep you posted. Of course by now you may have used a solderless slab-o'-trouble...

H^) harry



----- Original Message -----
From: Ian Fritz <ijfritz at comcast.net>
To: synth-diy at dropmix.xs4all.nl
Sent: Sun, 27 Feb 2011 16:38:36 -0500 (EST)
Subject: [sdiy] (no subject)

Hello --

My latest chaos system uses a thru-zero FM VCO to generate traveling wave 
signals. (Posted previously. Also see the thread on e-m.)  So I'm thinking 
about how to make a simple sort of auxilliary TZ oscillator for this 
function.  Here's an idea I've come up with, and looking around I don't see 
anything similar already done.

The oscillator consists of a signal polarity switcher, a bipolar integrator 
and a Schmidt trigger all connected in a loop.

The switcher is the familiar circuit with an opamp, three R's and an 
FET.  It's often used in VCO waveshapers.  The input signal feeds this 
circuit.  The switcher drives a multiplying integrator, just an OTA driving 
a cap to ground. The switcher goes to the OTA's bipolar input and the other 
input just needs a DC  bias current for my application. (In general, it 
could have the usual exponential current control, or a linear, unipolar 
modulation source.) The opamp Schmidt trigger reverses the polarity of the 
input whenever the signal reaches +/- 5V, causing the ramp to reverse 
direction.  So the whole thing would be just two opamps and an OTA.

Anyone see a reason why this wouldn't work?  I'm a bit worried about the 
signal escaping the +/-5V range, but a clamp at slightly above this level 
should allow the signal to get back to normal reasonably quickly.  Has 
anyone seen a TZ FM VCO done this way?  Seems pretty simple, at least 
conceptually.

Ian 

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-- 
Harry Bissell & Nora Abdullah 4eva