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Re: [RFI] Computer cases and RFI

To: "Jim Brown" <jim@audiosystemsgroup.com>,"RFI List" <rfi@contesting.com>
Subject: Re: [RFI] Computer cases and RFI
From: "Jim P" <jvpoll@dallas.net>
Date: Mon, 2 Jan 2006 18:39:03 -0600
List-post: <mailto:rfi@contesting.com>
Many of those comments I think were applicable 
several computer generations ago, back in the era of 
the TTL VAX 11/780 and the first few Intel into the 
Wintel era PCs when very active board 'traces' may 
have been on the top and/or bottom layers and used 
grossly sized through-hole 'DIP' parts when Ball Grid 
Array (BGA) and CS (Chip Scale) components used 
today were still a gleam in the industry's eyes.

Today's machines with 533 MHz 'front side bus'
technology has to take a completely different view 
of the signal 'world' at board level to 'make that stuff 
work well'. I'm speaking as somebody who has 
worked on fast TTL machines and was amazed, 
at time with all the 'ringing' that we saw with a 
scope, that that stuff worked at all, and some of 
that stuff was built using wire-wrap technology
(probably THE WORST when it comes to maintaining
any signal integrity at all).

I have a 33 MHz 386 that has certain apps on it 
that I can't migrate easily to a Pentium class
machine, and whenever I have to 'run' that machine
I am reminded how poor the inherent 'shielding' is
by the 'jamming' that machine does to CH 11 TV
and even an 800 MHz frequency that the local trunked 
system uses. The layout and inter-chip wiring in those 
days was gross, coupled with physically huge leaded 
parts - compared to today's PCs with much improved 
board layouts, BGA technlogy, judicious use of interlay 
ground planes and apparently much more knowledgeable 
use of multilayer boards which provide an environment 
suitable for the stable, clean, non-glitched operation 
of a 533 MHz front side bus.

Jim P   // WB5WPA  //

----- Original Message ----- 
From: "Jim Brown" <jim@audiosystemsgroup.com>
To: "RFI List" <rfi@contesting.com>
Sent: Monday, January 02, 2006 5:22 PM
Subject: Re: [RFI] Computer cases and RFI

> On Mon, 2 Jan 2006 17:36:24 -0500, Martin, AA6E wrote:
> >When I have troubles, it's always with the external
> >cabling.  (Without chokes, external wiring can be an efficient
> >radiator - comparable in length to a wavelength.)
> Oh -- it's the fault of the cable that the computer puts the RF 
> current on the cable?  :)  
> Let's be clear. ALL of that trash is coming from the electronics 
> -- either the computer, the monitor, the network hubs/switches, 
> etc. These electronic devices may radiate the trash directly if 
> they are unshielded, and they can shove it out on the cables if 
> they don't have sufficient common mode filtering (or if they have 
> pin 1 problems on those cables). And the trash radiated directly 
> by the wiring/circuit board of an unshielded box can vary widely 
> depending on how well (or how poorly) the designer minimized the 
> size of the current loops carrying RF currents, how well (or how 
> poorly) that trash was filtered and suppressed internally. Henry 
> Ott, Ralph Morrison, Howard Johnson, and Clayton Paul all cover 
> these issues quite nicely in their books and EMC lectures. 
> Those mechanisms are additive. When we stick ferrites on the 
> cables, we are minimizing (or attempting to minimize) the common 
> mode RF current that the electronics is trying to put on those 
> cables. The trash we hear will be directly related to the 
> magnitude of that current. But even if we manage to reduce that 
> current by 30 dB with really effective ferrite chokes, we will 
> still hear whatever the various electronic devices are radiating.
> Jim Brown K9YC
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