AW: [sdiy] Seperate Grounds

[Magnus Danielson]

From: harry <harrybissell at prodigy.net>
Subject: Re: AW: [sdiy] Seperate Grounds
Date: Sat, 21 Jul 2001 12:29:57 -0400

Harry and all,

I've just back from a week at Cote d'Azzur (working :-P) and you seem
to enjoy yourself in noise and EMC country again... I'll read the full
thread (and comment!).

> > There's another demand, by the EMC people: the ground of a jack has to
> > be connected directly to the chassis where the jack is, no "long" (i.e.
> > few inches) wire to a ground reference point. Now things really get
> > complicated ;-)

Indeed it must! And your chassi better be properly conducting and not
have a load of isolation points and just a random or single earthing
connection.

I've done EMC ;)

> Bingo... that's parasitics again.  I spent a week trying to get RF out of a
> commercial
> mixer for this reason... the RF was riding the 'ground' shield inside the
> chassis... through about 3" of cable before it touched the real ground. The
> chassis was a poor ground and the signal got inside the case... where it could
> radiate to the sensitive preamps.
> 
> Imagine my surprise when I plugged a shorted audio cable into the mixer and the
> noise got worse. How can that be ??? Easy (once you understand)... the shield of
> the cable was not really "ground" so the whole thing was adding an antenna to the
> radio receiver.
> 
> The fix involved making the 3" wires inside the box very unattractive to RF...
> Ferrite
> beads on all.  Now why didn't the manufacturer do this ??? Either they did not
> notice the problem (not near a transmitter...) or they didn't care. It might have
> been good enough for them.

Toss a couple of capacitors at the contact directly to chassi ground
and your ferrit beads can do a even better job. If you also have a
capacitor setup on your board to ground, you've got a complete
PI-filter, a classic in EMC world.

> OTOH this is why a lot of us have day jobs... I hope that ground problems don't go
> away anytime soon...  (they won't)

Actually, things will just be worse with all the digitalheads... but
the problems will be more magic!

So, Harry and friends, prepare yourself to become magicians! ;O)

Cheers,
Magnus - performs miracles while you wait