[sdiy] Dual/quad opamp bleed-through

[George Hearn]

I agree with JH to not share IC's between VCO's in an ideal world but I
would say that it is most likely possible to overcome these problems without
a major rethink of the design using separate IC's.  You're problem is most
likely to do with coupling via the power rails or parasitic capacitive
coupling between circuits.  I would advocate using resistances in series
with power supply pins of the sensitive IC's that have a reasonable degree
of PSRR (power supply rejection ratio) like opamps and OTA's before the
decoupling caps.  Also try to not use adjacent tracks of the solderless
breadboard for sensitive tracks and ground the track in-between to avoid
capacitive coupling between these tracks.  I doubt that the frequency pots
would benefit from limiting resistors on the rails and I think that you're
cross-dependence problem is due to the very slight alteration of the "end of
ramp" condition where you're triangle core alternates to ramping the other
way.  Capacitive coupling effects are always going to be more prominent at
higher frequencies and this seems to be what you are observing.  I'd be
interested to see how you get on.  Many thanks, George 

-----Original Message-----
From: synth-diy-bounces at dropmix.xs4all.nl
[mailto:synth-diy-bounces at dropmix.xs4all.nl] On Behalf Of JH.
Sent: 31 January 2009 22:06
To: David G. Dixon; 'sdiy'
Subject: Re: [sdiy] Dual/quad opamp bleed-through

Never share ICs between VCOs.
Don't even share copper traces.

JH.


----- Original Message ----- 
From: "David G. Dixon" <dixon at interchange.ubc.ca>
To: "'sdiy'" <synth-diy at dropmix.xs4all.nl>
Sent: Saturday, January 31, 2009 10:37 PM
Subject: [sdiy] Dual/quad opamp bleed-through


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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