Home-made Modular Synth teething trouble II

[Mark Amundson]

Phil Callaghan (Core Design Ltd.) wrote:
> 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?
> Regards, Phil Callaghan

I want to add to the other responses posted so far.

The best way to run your supply rails is to "star" them like your
grounding. Your rails have real milliohm resistances plus small
inductances. I suspect your modules are "railed" like train tracks so
that your modules are "stops" upon the rails. Each module's current
demands induce minor voltage drops along the rails back toward the power
supply regulators. This little voltage disturbance affects all modules
along the path.

The recommended filter capacitors help reduce such small inter-module
voltages on the rails by providing low ohmic AC paths to ground along
the rails. Alot of ICs recommend these bypass caps to be co-located near
the chips to reject supply ripples. The regulators will take of much of
the ripple below 40kHz but need help at the far flung regions of the
circuit(s). The more cautious will combine larger capacitance
(electrolytics, tantalums) with smaller values (ceramics, micas) for
broadband ripple rejection. The big caps can be thought of as local
reserviors of energy to stabilize sudden changes in current demand (fast
risetime signals).

Mark Amundson,