RE: [SI-LIST] : Guard trace question

Shimon Amir ([email protected])
Mon, 27 Sep 1999 09:35:48 -0400


what was the guard trace connected to?



At 02:34 PM 9/25/99 -0500, Eric Bogatin wrote:




<fontfamily><param>Arial</param><smaller>I think your original problem
you are trying to solve is related to achieving high isolation between
two signal lines in a stripline configuration, with the use of a guard
trace. Your question was related to how to estimate the isolation using
some approximations and rules of thumb, based on a reasonable starting
place. I am a strong advocate of using both of these types of analysis,
and applaud your attempts. In addition, there is also a third type of
analysis you can use that, in this case, will save you a lot of time and
which should be a tool in your portfolio of skills: numerical analysis.


<fontfamily><param>Arial</param><smaller>There are a slew of 2D field
solvers on the market that are accurate and easy to use, from which you
can get an answer to your problem, accurate to probably better than 5%,
in minutes. In one of the classes I teach, I review the use of 2D field
solvers for calculating the cross talk coefficients in various structures
and how to use these tools to help establish design rules that minimize
coupling. For mixed signal applications, there are often cases where you
need 100 dB isolation or more, as when you have a sensitive receiver line
near a digital line. I have appended a copy of an example I use that
illustrates the extra isolation gained by a guard trace between two
stripline signals. This particular example was done with the Ansoft
Maxwell 2D field solver. In addition, the Hyperlynx tool can also analyze
these types of problems.


<fontfamily><param>Arial</param><smaller>The specifics for this case are:
2, 50 Ohms lines, 5 mil line width, about 12 mil thick dielectric between
the planes, FR4 dielectric. In the blue curve, we look at the near end
cross talk coefficient of just the two traces, as we grow the separation
(edge to edge) between them. You can get 100 dB of isolation without the
guard trace if you increase the separation to about 50 mils. (These
results are a little different from Ron's below. He offered 40 dB at 30
mil separation and 100 dB at 75 mil separation. I calculated 40 dB at 15
mils and 100 dB at 50 mils. However, I'm not sure the geometry in my
example is the same as Ron's)


<fontfamily><param>Arial</param><smaller>The purple trace is the near end
cross talk coefficient when a guard trace is added to the space between
the lines, with the condition that it is wide enough to fill the gap,
leaving a 5 mil space to the nearest adjacent trace, and it starts out 5
mil wide. With this condition, typical of what a fab house would require,
the spacing between the signal lines has to be at least 15 mils before a
guard trace will fit. You can see that the presence of a guard trace does
have a dramatic effect on lowering the coupling. At 15 mil spacing, there
is about -42 dB coupling in the stripline, all by itself, but -60 dB with
the guard trace.


<fontfamily><param>Arial</param><smaller>This illustrates that guard
traces may not have a common role in straight digital applications (as
distinct from mix signal), as when do you need more than 40 dB isolation,
which you can get without a guard trace? But, when you do need more than
40 dB isolation, the guard trace can add almost 20 dB isolation. By 30
mil separation, the stripline is at -70 dB and the guard trace is better
than -100 dB.


<fontfamily><param>Arial</param><smaller>I can already hear folks saying-
why does the isolation of the guard trace start to flatten out at about
30 mil separation- why doesn't it get any better? I'm going to leave this
an open question to the forum to see what folks post as possible
explanations, then I'll comment.






<fontfamily><param>Arial</param><smaller>Training for Signal Integrity
and Interconnect Design</smaller></fontfamily>

<fontfamily><param>Arial</param><smaller>26235 W. 110th

<fontfamily><param>Arial</param><smaller>Olathe, KS




<fontfamily><param>Arial</param><color><param>0000,0000,ffff</param><smaller>[email protected]</smaller></color></fontfamily>

<fontfamily><param>Arial</param><smaller>web:</smaller></fontfamily> <fontfamily><param>Arial</param><color><param>0000,0000,ffff</param><smaller></smaller></color></fontfamily>

<excerpt> <fontfamily><param>Times New Roman</param><smaller>-----Original Message-----

<bold>From:</bold> [email protected] [mailto:[email protected]]<bold>On Behalf Of</bold> S. Weir

<bold>Sent:</bold> Saturday, September 25, 1999 2:01 AM

<bold>To:</bold> [email protected]

<bold>Subject:</bold> Re: [SI-LIST] : Guard trace question


Either you have a different formula, or I believe that you slipped a decimal point:

The isolation required: -100db watts/watt = 1*10^-5 volts/volt

The high frequency coupling limit = 1/(1 + D/H^2)

Solving, I get:

D/H = sqrt( 1/99,999 ) = 1/316.

Which is a bit much to ask for.

I do not have a formula which is both simple and accurate for the case where a via fence is used but for this high of an isolation, it is likely a practical solution. There was a paper referred to about a week ago which I have not got around to reading yet which might provide a decent guestimate.



At 04:01 PM 9/24/1999 -0700, you wrote:

<excerpt> Gary

I approach this from a slightly different angle but it may be useful to


1. For 40 db isolation = 1 % voltage coupling at the worst coupling

frequency quarter wave coupled length the spacing should be 3 X the

total height or 2 X 5mil material = 30 mil spacing. for 100 db it should

be about 2.5 X 30 mils = 75 mils with no guard trace at all.

2. Another rule of thumb you should consider is that you can get

coaxial, near complete isolation if the spacing between the stitching

is much shorter than a quarter wavelength at the highest frequency

of concern. Figure about 170 ps per inch or about 6 mils per ps in FR4.

These are rules of thumb I use and I am willing to listen to other

rules if anyone wishes. However, I will not defend the accuracy.

They are quick and dirty.

Ron Miller

Gary Sanders wrote:

<excerpt> I want to use guard traces on a multilayer PCB to provide almost

complete isolation, simulating the results as if coax were used instead.

I want to bury two signal layers between ground plane layers.

It seems to me that superposition must apply to EM fields in this

situation. I want to find the spacing and trace width of the guard

traces to provide about 100 dB of total isolation. I know that the

additional isolation provided by a guard trace is 6-12dB (depending on

whether ground stitching is provided). This assumes that the guard trace

is the same width as the signal trace, and is centered between the

aggressor and the victim traces, spaced by the width of the signal


Here's the specific question- is the isolation provided by a very wide

guard trace the superposition of the isolation of several normal width

guard traces placed side-by-side? Example--- assume 5 mil signal traces,

a 5 mil layer thickness of FR4, and a spacing between them of 20 mils

(between centers). The isolation calculated by formula without any guard

trace is 24.6dB. A stitched-ground guard trace would increase this

isolation to 36.6dB.

NOW- if we instead used a 25 mil wide guard trace, centered such that it

is 25 mils from the other traces, what is the isolation? I calculate

48dB without the guard trace. If we assume superposition, then the 25

mil guard trace is like having five 5 mil guard traces, each providing

12dB of isolation; so we have a total of 60dB isolation. This added to

that of the total spacing is 48dB + 60dB = 108dB of isolation. SO- is

this correct? Can I assume that the wide guard trace is the

superposition of several narrow guard traces? Note that this is a big

improvement on using a normal width guard trace, since the spacing

between the aggressor and victim would have to be 1" to get 104dB of

isolation (with a 5mil guard trace); and the wide guard trace approach

gives the same isolation with a spacing of 80 mils. I appreciate

anyone's thoughts.



Gary L. Sanders, Staff Analog Engineer, [email protected]

L3 Communications, Inc. Celerity Systems <<>

Cupertino, CA dir 408-861-7325 fax 408-873-1397

Ultra Fast Acquisition & Data Generation Systems

"The fog is wine. The sun is my gold."

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Attachment Converted: "c:\eudora\attach\guard.pdf"



Shimon G. Amir Avici Systems, Inc.

[email protected] 101 Billerica Ave

Signal Integrity N. Billerica, MA 01862-1256



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