Re: Shielding

apanella@molex.com
2 Jul 97 08:41 CDT

Guard traces can work (i.e. coplanar waveguides).. the trouble in
typical multilayer PCB structures is that you really need to "pin"
them down (plated thru holes to the gnd plane) very often. The higher
the highest frequency component, the more often you need to tie down
the guard traces...

Advantages:
* Better isolation that non-gurded traces

Disadvantages:
* Plated thru holes can (will?) effect the characteristic Impedance
* Plated thru holes can cause other routing issues
* Plated thru holes are not free
* Sometimes plated through holes can not be placed due to space
restrictions
* Sometimes it is difficult to find all of the "stubs" in the guard
traces

Other:
* Guard traces will change the charecteristic impedance of the
transmission line. This might be important to the overall system
design


It is might be better to adjust the ratio of the

"distance between signal conductors on the same plane"
TO
"distance between signal conductor and plane "

ratio.


( "signal conductor to plane" should be closer togather than the
"signal conductors that are on the same plane")


Thoughts....
Could it be that the customer is making avery special system that
might require much less coupling than standard PCB's or even possibly
a board that will be used for system tests where they customer would
want to remove any "extra" errors that could be induced due to
fixturing (PCB)?


_gus panella


apanella@molex.com




______________________________ Reply Separator _________________________________
Subject: Shielding
Author: SMURRAY@SLIP.NET at internet
Date: 97/07/01 3:29 PM

I'm a little confused on the subject of shielding (guard traces). I'm
currently designing a board for a customer, in which the analog & digital
traces are on seperate layers. Both layers are shielded by their respective
planes (i.e. Digital by Dgnd; Analog by Agnd). My question is; do the analog
traces still need to have guard traces to protect against any noise that
might be coming from other analog traces on that layer? (and likewise for
Digital). The customer has stated that he wants the traces shielded, but
when asked why he could not give me a scientific answer. I was under the
understanding that you only needed to shield the traces if they were very
high speed and were not utilizing proper ground shielding (planes)
techniques.

Any help?

Regards,
Sean Murray
M&M Specialties, Inc.