# Re: [SI-LIST] : Guard trace question

S. Weir ([email protected])
Sat, 25 Sep 1999 00:00:51 -0700

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Ron,

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.

Regards,

Steve.
At 04:01 PM 9/24/1999 -0700, you wrote:
>Gary
>
>I approach this from a slightly different angle but it may be useful to
>you.
>
>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:
>>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
>>trace.
>>
>>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.
>>--
>>Regards,
>>Gary L. Sanders, Staff Analog Engineer, [email protected]
>>L3 Communications, Inc. Celerity Systems www.csidaq.com
>>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."
>>
>>**** To unsubscribe from si-list: send e-mail to
>>[email protected] In the BODY of message put: UNSUBSCRIBE
>>si-list, for more help, put HELP. si-list archives are accessible at
>><http://www.qsl.net/wb6tpu/si-list>http://www.qsl.net/wb6tpu/si-list ****
>
>--
>Ronald B. Miller _\\|//_ Signal Integrity Engineer
>(408)487-8017 (' 0-0 ') fax(408)487-8017
> ==========0000-(_)0000===========
>[email protected], [email protected]

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Ron,

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.

Regards,

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

Gary

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

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:
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
trace.

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.
--
Regards,
Gary L. Sanders, Staff Analog Engineer, [email protected]
L3 Communications, Inc.   Celerity Systems   www.csidaq.com
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."

**** To unsubscribe from si-list: send e-mail to [email protected] In the BODY of message put: UNSUBSCRIBE si-list, for more help, put HELP.  si-list archives are accessible at http://www.qsl.net/wb6tpu/si-list ****

```--
Ronald B. Miller  _\\|//_  Signal Integrity Engineer
(408)487-8017    (' 0-0 ') fax(408)487-8017
==========0000-(_)0000===========