Home-made Modular Synth teething trouble II

[Martin Czech]

> I mailed this list a week or two ago about problems with my home-made
> analogue synth concerning the unwanted interaction of modules.
> Most of the advice I have received suggests that putting capacitors over
> the supply rails is the way to go. However, my power supply is regulated
> so I cannot see why this should be.
> Can anyone explain briefly why I need these capacitors, what they are
> doing and why I may need more than one capacitor over one supply rail.
> 
> Are these capacitors acting as filters, and therefore different
> capacitor sizes filter different (unwanted) supply frequencies?
> 
It was capacitors and star layout/wiring for all power rails and GND.
Regulators have a limited bandwidth, at higher freq.  they are less able
to supress unwanted ac stuff on your power rails.
Further ac suppression will only be possible using caps, RC lowpass
filters for sensitive circuits, or even LRC filters.

The reason for using different caps in parallel is that you want
sufficient capacitance: You take an electrolytic , say 10uF.  But:
these things have low resonance, ie. they turn into an inductor for say
f>100kHz or so. Thus you take a better hf cap in parallel (ceramic),
to short the higher frequencys. Say 100nF. These caps have a resonance
(from my measurements) of about 1Mhz. If you need suppression abovemthat,
you need another cap, say 10nF or 1nF. This way you short a broad band
of frequencys to ground. What ground ? These shorting currents will
spoil your ground, so you maybe need different grounds, signal ground
and dirty ground.

It is very interesting to measure passive components on an hf impedance
meter, I'm in the lucky position to have some crazy expensive HP in
our lab.  If someone is interested, I could print and scan some examples.



m.c.