[SI-LIST] Re: Remove Ground underneath Differential signal is deserved or...

In a message dated 7/29/2002 5:08:51 PM Pacific Standard Time, 
leeritchey@xxxxxxxxxxxxx writes:


> What I show in my class is that guard traces are always LC networks that
> resonate at some frequency and can and do turn into bandpass filters at 
> some
> frequencies.  I demonstrate this will actual failed circuits.

Lee:

It should be clear to most designers that providing either ungrounded guard 
traces or traces grounded at both ends will create a resonant structure that 
will be stimulated by transient (fast rise time) signals. These resonant 
structures allow reinforcement of signal strength (at their resonant 
frequency) and will couple this energy into other nearby traces. These 
resonant structures are commonly used to match impedances and therefore 
transfer maximum power into patch antennas. That is why one must use vias to 
ground at spacings on the order of 1/20 wavelength of the highest frequency 
of concern on the guard traces for them to be effective as a shielding 
mechanism. 

When properly implemented, guard traces are very effective at improving 
isolation between signal traces. For a 50 ps test signal rise time, I have 
achieved isolation improvements of 20-40 dB for stripline and 12-14 dB for 
microstrip using via-grounded guard traces with the same center-to-center 
signal trace spacing as non-guarded traces.

Going on travel for a couple of days now, be back soon.

Mike

Michael L. Conn
Owner/Principal Consultant
Mikon Consulting

                   *** Serving Your Needs with Technical Excellence ***


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