[sdiy] Ferrite beads on module power inputs

[Neil Johnson]

Hi all,

A private discussion today on a module design raised the issue of the
use of ferrite beads on module power inputs.  Some designers use them,
while some don't, either opting for low-value resistors (10-22 ohms
typically), or just a direct connection from the input to the main
smoothing electrolytic (perhaps via polyfuse and reverse Schottky
diode :-)

Looking at the datasheets for the sorts of ferrite beads used in
modules it is clear that at audio frequencies they have zero effect -
their impedance only becomes useful (10s to 100s of ohms) when you're
in the MHz range and up. In effect you'd get just as much noise
filtering from a short length of tinned copper wire.

So I'm curious to hear from those who do use them as to why, and any
evidence or calculations which show that they do have some measurable
and useful effect in synth module supply circuits.

Or is it simply that EMC (radiated and received) really is that much
of a problem?

Cheers,
Neil
-- 
http://www.njohnson.co.uk