[SI-LIST] Re: Guard Traces - Use 'Em, or Not?

  • From: "Lambert Simonovich" <bertsimonovich@xxxxxxxxxx>
  • To: "'Oh, Dan'" <doh@xxxxxxxxxx>, <ken@xxxxxxxxxxxxxxxx>, <EMC-PSTC@xxxxxxxxxxxxxxxxx>, "'SI-LIST'" <si-list@xxxxxxxxxxxxx>
  • Date: Fri, 25 May 2012 14:59:07 -0400

Hi Dan,

Thanks for the comments. I'm glad you found the data useful. I was trying to
keep impedance the same between stripline and ustrip for the same line width
and spacing to compare apples to apples, as well as get close to a
real-world practical example stackup. If we are concerned about microstrip
only, then you are right varying the height will show different results. The
key will be what knobs need to change to keep the same impedance. To
maintain 50 ohms, when the dielectric is increased, you will have to change
the track width to be wider, which means you also have to increase the
spacing between the aggressor and victim to fit the same width guard trace
in. If you keep the same geometry and merely increase the dielectric, then
the impedance will be higher, which will result in an overshoot to the
original aggressor step waveform for 2TD for 50 ohms. This overshoot will
also increase the NEXT and FEXT.

I agree it's an experiment to try and quantify. I have tried to show the
methodology, so designers can experiment on their own for their particular
case, and decide for themselves. If you add solder mask then that too will
affect the result. The scenarios are endless. It's fine to do the Physics
experiment, but, as you say, this might not be a typical conventional
microstrip configuration in modern designs.

I will try and do a follow-up for microstrip only as you suggest. Increasing
the dielectric to 15 mils and keeping the same 5-5-5 geometry.

BR/

Bert Simonovich, Consultant & Founder
LAMSIM Enterprises Inc.
"Innovative Signal Integrity and Backplane Solutions"
Email: lsimonovich_at_lamsimenterprises.com
Blog: http://blog.lamsimenterprises.com/
LinkedIN: http://www.linkedin.com/in/bertsimonovich
Web Site: http://lamsimenterprises.com/

========================    

-----Original Message-----
From: Oh, Dan [mailto:doh@xxxxxxxxxx] 
Sent: May-25-12 12:35 PM
To: bertsimonovich@xxxxxxxxxx; ken@xxxxxxxxxxxxxxxx;
EMC-PSTC@xxxxxxxxxxxxxxxxx; 'SI-LIST'
Subject: RE: [SI-LIST] Re: Guard Traces - Use 'Em, or Not?

Hi Bert,

Thanks for great data and paper. We can finally discuss this issue over real
data!
As I have mentioned in the earlier posting, the guard trace effectiveness
strongly depends on the trace height and spacing. The capacitive
(electrical) coupling saving by placing the guard trace is pretty minimal as
others pointed out. However, I believe the main advantage of the guard trace
is reducing the inductive (magnetic) coupling which decays rather slowly
than the capacitive coupling.

The example you used only had 3mils height and 5mils spacing which makes the
most of signal return through the ground plane rather than the guard trace.
It is not surprising that you would not see much improvement in this case.
If you increase the height, you will definitely see the improvement (say
15mils). Of course, this may not be a typical conventional microstrip
configuration.

The point I am trying to make is that let's not make a myth out of this
guard trace. It is a simple physics. If the trace ground plane further away,
use the guard trace (of course with proper stitching and impedance
matching). Otherwise, don't. It would be nice if you can re-simulation your
case with various heights.

Best,
-Dan Oh
___________________________________________
Dan (KyungSuk) Oh, Ph.D.
Technical Director of Signal and Power Integrity Rambus Inc.
(B) 408-462-8363


-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Lambert Simonovich
Sent: Thursday, May 24, 2012 5:15 PM
To: ken@xxxxxxxxxxxxxxxx; EMC-PSTC@xxxxxxxxxxxxxxxxx; 'SI-LIST'
Subject: [SI-LIST] Re: Guard Traces - Use 'Em, or Not?

Ken et al,

Good blog post. I have been doing a little research myself; hopefully to put
the subject to bed. I was putting the finishing touches on a white paper
when you posted your link.

Abstract:
To guard or not to guard? That is the question often asked by digital
hardware design engineers. As bit rates continue to climb, there is
increased debate on whether to use guard traces to control crosstalk in
high-speed digital signaling.  By doing so, it is believed the guard trace
will act as a shield between the aggressor and victim traces.  On the other
hand, the argument is that merely separating the victim trace to at least
three times the line width from the aggressor is good enough.  This paper
studies the application of guard traces and quantifies the results against
non guarded scenarios.

Conclusions:
This study has shown that adding a guard trace, at 3 times the line width,
terminated at both ends with 50 Ohms, does little to improve crosstalk on
the victim. In fact, in most cases, it was worse. The same was true when the
guard trace was grounded only at each end. But adding a ground-stitched
guard trace, with the same spacing, showed it was the best solution for
microstrip, and had little to no benefit for stripline. However, by
increasing the spacing to 5 times the line width in microstrip, the
crosstalk, for all intensive purposes, was the same. In stripline, there was
no benefit in adding a guard trace for digital signaling.

You can download a copy from my website. Hope everyone finds it useful. Here
is the link:

http://bit.ly/KSM5BZ

Best regards,

Bert Simonovich, Consultant & Founder
LAMSIM Enterprises Inc.
"Innovative Signal Integrity and Backplane Solutions"
Email: lsimonovich_at_lamsimenterprises.com
Blog: http://blog.lamsimenterprises.com/
LinkedIN: http://www.linkedin.com/in/bertsimonovich
Web Site: http://lamsimenterprises.com/

=======================================================

-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Ken Wyatt
Sent: May-24-12 2:43 PM
To: EMC-PSTC@xxxxxxxxxxxxxxxxx; SI-LIST
Subject: [SI-LIST] Guard Traces - Use 'Em, or Not?

Hi Group,
After noticing the continued banter and discussions regarding guard traces
in both the EMC and SI groups over the past months, I decided to consult a
couple experts on the subject - Howard Johnson and Eric Bogatin. I
summarized their thoughts in my latest blog posting on the Test &
Measurement World web site:
http://www.tmworld.com/blog/The_EMC_Blog/41806-Guard_Traces_Use_Em_or_Not_.p
hp. I invited both to add any additional comment, if they wished. Feel free
to add to the discussion.

Cheers, Ken
_______________________
Kenneth Wyatt
Wyatt Technical Services LLC
Woodland Park, CO
Email Me! | Web Site | Blog
Subscribe to Newsletter
Connect with me on LinkedIn


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