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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