[sdiy] TB-303 smps interference

[cheater00 .]

On Sat, Nov 23, 2013 at 5:39 PM, cheater00 . <cheater00 at gmail.com> wrote:
> Changing the VCO pitch doesn't change anything. However, if I start
> the sequencer, the waveforms change in frequency a lot! You can see
> some slides. I'm thinking this must be related to the oscillator
> pitch.

This idea was incorrect. Using a highly precise measurement instrument:

http://imgur.com/a/idtGJ#46

I have been able to recover the volume envelope from the output:

http://imgur.com/a/idtGJ#54

Then, using the external trigger output in my timebase, I have made it
trigger (that is, display one "screenful" of the waveform) only on the
highest peaks of that recovered envelope. (to do this I have changed
the triggering mode to normal on the 7B53A plugin)

By using the variable storage mode of my oscilloscope mainframe I have
overlaid all of those triggered waveforms on top of eachother. The
frequency was stable. There were no slides present. Note that the
fuzziness is just a general property of the storage mode on very dense
graphs:

http://imgur.com/a/idtGJ#58

(if the waveforms didn't align /perfectly/, the graph would be very messy)

This shows that the frequency of the ripple I am experiencing is
related to load (I don't think it's wrong to assume that higher volume
output = higher load for the power supply).

The next questions to answer are: I wonder what this frequency
bleeding into the output actually is? Can this be the switching
frequency of the smps? If so, why would it change frequency according
to current draw? Can I improve ripple rejection by swapping out any
decoupling capacitors on the power rails?

Cheers,
D.