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RE: [RFI] shielding answer

To: "Rfi" <rfi@contesting.com>
Subject: RE: [RFI] shielding answer
From: "David Robbins K1TTT" <k1ttt@arrl.net>
Date: Wed, 24 Dec 2003 20:10:51 -0000
List-post: <mailto:rfi@contesting.com>
> Was your shielded speaker cable also twisted pair, or was it parallel
> conductors?  It is well known that cable shields provide relatively
> little magnetic shielding (Ott says almost none except in narrow
> frequency ranges), but does provide shielding to the electric field.
> Most LF interference is magnetically coupled, while the E-field
> more important as frequency increases. On the other hand, twisting
> provides considerable rejection of magnetic fields.

the original cable was cheap zip cord that came with the dvd player.
The new stuff is 16ga shielded twisted pair.
> Another question. Lots of electronic gear has a pin 1 problem. Any
> that isn't bonded to the chassis is probably a pin 1 problem. No, I
> don't mean continuity to the chassis through the pc board, I mean the
> shell must be in physical contact with the chassis. Pin 1 problems are
> a VERY common coupling mechanism for RFI, and it gets worse at higher
> frequencies. Did your RCA connectors appear to be bonded to the
> chassis, or were they insulated from it?

well, on the pcm-optical box the rca connector must have been grounded
to the chassis, what there may be of it.  The thing is just a small
plastic case so there probably isn't much shielding inside.

On the tv I can't tell what the rca connector might be connected to, but
it was the only metal on the front of the set that I could find.
Connecting it to the aluminum shield really helped so it must be
connected to something decent inside... or maybe not, and my added
ground fixed a design problem.

The dtv box is mostly metal and the connectors on the back are mounted
on the metal case so I assume they are fairly well grounded internally.
There was noticeable improvement when connecting the front rca jack to
the aluminum shield so either something wasn't grounded inside or it
just helped reduce the differential pickup between the components.  I
would have used a case screw like I did on the dvd player but they are
tiny torx head things and it was just easier to grab the rca jack.

David Robbins K1TTT
e-mail: mailto:k1ttt@arrl.net
web: http://www.k1ttt.net
AR-Cluster node: 145.69MHz or telnet://dxc.k1ttt.net

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