RE: [SI-LIST] : How to construct guard bands in Ansoft 2D

Eric Bogatin (bogatin@ansoft.com)
Fri, 29 May 1998 13:59:13 -0500

Gee, the SI list gets 2 answers for the price of one from Ansoft.

I neglected to mention in my description of what Charles is doing that he has ground vias placed every inch between the "guard traces" and the two ground planes, in which case, for his rise time, the "guard traces" are effectively tied tightly to ground. Being positioned closer than 1/3 the effective spatial extent of the leading edge will short out any induced voltages on the guard lines.

However, for those that want to evaluate the induced noise when the guard traces are only connected at their ends to ground, the approach that Eric Bracken suggested is correct- treat the guard traces as signal lines and use SPICE to extract the induced noise on both signal and guard lines.

--eric bogatin

At 01:50 PM 5/29/98 -0400, Michael Tsuk wrote:

>Eric,

>

>> In the problem set up, each of the traces, signal and ground traces

>are drawn, in

>> addition to the planes. In Maxwell 2D Extractor, only one conductor

>can carry the return

>> path (ie, be labeled ideal ground), so all the conductors that are to

>be grounded; the

>> "guard traces" and the planes, are grouped under the same name-

>labeled "ret_path",

>> for example.

>

>I'm not sure this is the best way to do this. Charles Grasso was

>particularly interested

>in the effects of the vias that periodically connect the guard traces to

>ground. I think

>you would do better to leave the guard traces as "signals" and connect

>their ends to

>ground in your circuit simulator. That way, you allow for the effect of

>standing waves

>on the guard traces, which can happen if you don't have enough vias

>connecting them

>to ground.

>

>--

>Michael Tsuk

>Michael.Tsuk@digital.com

>

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>

**********************************************************************

Eric Bogatin

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Ansoft Corporation

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