[sdiy] Dual/quad opamp bleed-through

[David G. Dixon]

Here's another one for the gurus:

I've just finished breadboarding the triangle-core part of my "Everything
Dual VCO" (which is essentially two of Thomas Henry's "VCO-1" designs with a
bunch of added bells and whistles), and it works perfectly, except for one
aspect:

One VCO affects the other if its frequency is turned up.

I've laid out the circuit to give two identical VCO circuits on either side
of the breadboard, with the opamps on opposite sides of quad and dual TL072s
(and a LM13700 dual OTA amplifying the expo currents for the triangle
integrators) serving identical functions for the two VCOs.  In other words,
the two VCOs are mirror images of each other, sharing dual and quad opamp
and OTA chips.

So, is my problem:

a) bleedthrough between separate opamps on the same chip at different
frequencies,

b) noisy power rails (I've used eight 0.1-uF ceramic caps and two 10-uF
electrolytic caps on the rails, so this shouldn't be the problem),

c) an artifact of using the solderless breadboard, or

d) something else that I haven't thought of?

Also, if a), do you think it's one of the TL07x's (which I can deal with
easily) or the LM13700 (which I can't deal with, since this is the entire
basis for building a dual VCO in the first place!)?

Any help would be most appreciated!

David G. Dixon
Professor
Department of Materials Engineering
University of British Columbia
309-6350 Stores Road
Vancouver, B.C. V6T 1Z4
Canada
 
Tel 1-604-822-3679
Fax 1-604-822-3619
 
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