[SI-LIST] : Looking for good resources on SSTL transmission and termination..

Greim, Michael ([email protected])
Tue, 5 Oct 1999 09:54:22 -0400

This bounced the first time. Sorry if you receive a second copy.

Best Regards,


The bounds of Time, Space or Mechanics should never stand
in the way of a perfectly good idea.......

The time is gone, The email's over, thought I'd
something more to say.........

Michael C. Greim Consulting Engineer
Mercury Computer Systems, Inc email: [email protected]
199 Riverneck Road V: 978-256-0052/x1607
Chelmsford, MA 01824-2820 F: 978-256-4778

> -----Original Message-----
> From: Eric Bogatin [SMTP:[email protected]]
> Sent: Saturday, September 25, 1999 3:35 PM
> To: [email protected]; [email protected]
> Cc: eric
> Subject: RE: [SI-LIST] : Guard trace question
> Gary-
> 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.
> 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.
> 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)
> 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.
> 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.
> 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.
> --eric
> Eric Bogatin
> Training for Signal Integrity and Interconnect Design
> 26235 W. 110th Terr.
> Olathe, KS 66061
> v: 913-393-1305
> f: 913-393-1306
> pager: 888-775-1138
> e: [email protected]
> web: www.bogatinenterprises.com
> -----Original Message-----
> From: [email protected]
> [mailto:[email protected]]On Behalf Of S. Weir
> Sent: Saturday, September 25, 1999 2:01 AM
> To: [email protected]
> Subject: Re: [SI-LIST] : Guard trace question
> 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 <http://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."
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> --
> Ronald B. Miller _\\|//_ Signal Integrity Engineer
> (408)487-8017 (' 0-0 ') fax(408)487-8017
> ==========0000-(_)0000===========
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> Jose, CA 95131
> [email protected], [email protected]
> << File: guard.PDF >>

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