PLAN B analog blog

Bakis,

As always., great to hear from a fellow Greek!

I have noticed this anomaly as well...when my M15s were in mounted
close to my one remiaing Blacet VCO.  Please try this, as it fixed the
problem for me (admittedly a bandaid of sorts):

Move the PWM pot of the module you suspect to be bleeding over to te
M15'a to full CWQ or CCW - enough for that squarewave to be locked out
of oscillation (held full pos or neg).  This takes care of it, atteast
on my system. ,  

As far as what's causing it, is it terrible to say I'm not 100% sure?
 How's that for honest?  One thing I do know, M15's do not cause other
M15's to do this, and I've made special precautions to limit the
amplitude of the square outs well short of the power rail of the op
amps driving them. My suspicions are there are a few square wave
vco/lfo's out there that are not attenuated - that run from rail to
rail. This causes havoc on any power system, much like when LEDs are
not properly current limited and I believe my VCO is especially prone
to picking up on that.  

Most PWM circuits run on the same prinicple as a manual one shot
comparator.  The PWM pot sets the threshold, and the incoming waveform
(non square wave) goes into the other side of the op amp.  This config
allows anything in the incoming wave which is greater in amplitude
than the pot setting to pass to the output as a high.  Problem is, 
unless you design it differently, that high will be at the rail - the
same amplitude as the voltage powering the op amp.  this fluctuation
in current then ricochets through the entire power system.  SOme
istruments are sensitiive to this, some aren't.

The Model 1 power buss I think may take care of this problem.  The
first run of these (50) are arriving in about a week's time form our
PCB vendor.  I'll give you all an update at that time.

When tim allows, I will do some experiements with increasing the value
of the two main bypass caps on the VCO engine bard - currently 10mFs.

- P