[SI-LIST] Re: Guard Traces

  • From: Eric Steimle <eric.steimle@xxxxxxxxxx>
  • To: "leeritchey@xxxxxxxxxxxxx" <leeritchey@xxxxxxxxxxxxx>, "aaditya.kandibanda@xxxxxxxxx" <aaditya.kandibanda@xxxxxxxxx>, "si-list@xxxxxxxxxxxxx" <si-list@xxxxxxxxxxxxx>
  • Date: Wed, 7 Mar 2012 19:48:09 +0000

So when I was a younger engineer someone (Johnson maybe) put me on to this 
simple experiment I did in the lab when I was trying to convince my bosses at 
the time that coupling in our system was really mostly inductive and not 
capacitive.    With a little modification you can also use it to get a feel for 
how well or not well a guard trace works.    All you need is a Pulse generator, 
or some kind of square wave in the Mhz range, some wire, a piece of copper clad 
PCB and some resistors.   Oh and motivation to go into the lab.
Anyway take your piece of copper clad and put these two wires over the top like 
this, and make sure to GND the top of the copper with the GND cable from the 
coax. 

Aggressor (tape this down)
Pulse Generator----50 Ohm coax (BNC cable) --- 3” of Regular wire ---- R1 50 
Ohm Resistor to GND

Victim
GND -------------------------------------3” of Regular wire ---- R2 50 Ohm To 
GND
                                                                

It’s kind of cool, if you move the victim 3” of wire closer or further from the 
aggressor you’ll see the cross talk voltage go up and down when measured at R2. 
   Then if you want, get yourself another wire to use as a guard trace in 
between the Aggressor and Victim.   Just ground       it at both ends and 
you’ll see the difference it makes (which should be pretty tiny), or go crazy 
and ground it everywhere.   Just remember to keep the spacing between the 
Aggressor and Victim the same in this part of the experiment or you’ll end up 
fooling yourself ;)   

This isn't going to tell you what to do in your system but it's a neat way to 
get your head around the problem.

Good Luck,
-Eric                           

-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On 
Behalf Of Lee Ritchey
Sent: Wednesday, March 07, 2012 1:48 PM
To: aaditya.kandibanda@xxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Guard Traces


Short answer- Don't use guard traces!  Use separation to isolate signals from 
each other.

-----Original Message-----
>From: Aaditya Kandibanda <aaditya.kandibanda@xxxxxxxxx>
>Sent: Mar 7, 2012 10:31 AM
>To: si-list@xxxxxxxxxxxxx
>Subject: [SI-LIST] Guard Traces
>
>Hello Everyone,
>I have few questions about the guard traces.
>
>1. How do I terminate the guard traces?
>2. While using the guard traces for differential pair, how shall I 
>place them between the differential pair?
>3. How should I place the vias for guard traces?
>4. What are disadvantages of Guard traces?
>
>Thanks and regards
>Aaditya
>
>
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I just used the energy it took to get mad and wrote some blues.  Count Basie

Lee W. Ritchey
Speeding Edge
P.O. Box 2194
Glen Ellen, CA
707-568-3983
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