[sdiy] Dual/quad opamp bleed-through

[Bruce]

The first prototype of my dual VCO module had this same problem.
I then split the two VCOs onto separate boards with shared panel controls.
This design still had sync problems. It was only after I placed the 
boards at least 3/4" apart using standoffs that the syncing was
eliminated. This was with good de-coupling and hefty gauge star wiring etc.
No shared ICs or even on the same board.

Bruce

At 04:37 PM 31/01/2009, you wrote:
>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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