(2) dirty/clean ground again [sdiy]

[Neil Johnson]

> I know that decoupling with a series resistor is a popular technique,
> ... but it has one major drawback ... the series resistance effective
> increases the output impedance of the power supply regulator to the
> value of the resistor. So instead of your carefully designed and
> constructed low-impedance power supply, you now have a 100Ohm (or
> whatever) power source.

True...and very helpful if you're welding steel!

Seriously, though, I think what we're all converging on is that the way
forward is to manage the power distribution around the circuit, isolating
to some degree sections from each other (noisy oscillator from quiet expo,
for example), and trying keep down the amount of rubbish we spill out onto
the reference ground line.

> I've had good luck decoupling with ferrite beads.

Both myself and Tony Allgood use ferrite beads on our PCBs.  If you look
at the photo of my VCO you'll see then either side of the power connector.

> Odd that no one has mentioned this possibility in this thread.

Doh!  Just done it...!!

> In my last VCO project I put beads on the supply lines where they enter
> the board and also from that point to several different sections of the
> circuit. This is the first VCO I have made where the switching
> transients don't couple back into the expo converter section.

Mmmm...good idea.  If I get stuck I'll try that too, although I am rather
constrained by space!

> If ferrite beads don't do the job, then probably it would be a good idea to
> go to local regulation (raw dc to the board plus an on-board regulator) or
> to re-regulate to a lower voltage using, say, a precision zener.

IIRC, the Oberheim OBX boards had local supply regulators on them (100mA
TO-92-sized 78xx/79xx ones).  The central PSU generated +/- 19V, which was
then locally regulated down on each voice card to +/- 15V (or 12V, can't
quite remember).  A very good way to prevent voice cards sync'ing to each
other.

Neil

--
Neil Johnson :: Computer Laboratory :: University of Cambridge ::
http://www.njohnson.co.uk          http://www.cl.cam.ac.uk/~nej22
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