FW: [SI-LIST] : Trace Impedance Selection

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From: Tom Dagostino (tom_dagostino@mentorg.com)
Date: Thu May 04 2000 - 10:46:46 PDT


Tom Dagostino
ICX Modeling Group
tom_dagostino@mentor.com
503-685-1613

-----Original Message-----
From: Tom Dagostino [mailto:tom_dagostino@mentorg.com]
Sent: Thursday, May 04, 2000 9:48 AM
To: si-list@silab.eng.sun.com
Subject: RE: [SI-LIST] : Trace Impedance Selection

For a trace on a board to be useful it has to transfer the signal from the
driver to the receiver such that the receiver can act on the signal. If the
Zo of the terminated trace is so low that the driver cannot place a valid
logic level, with some margin, at the receiver then the trace impedaance is
too low. This puts a lower limit on Zo.

Tom Dagostino
ICX Modeling Group
tom_dagostino@mentor.com
503-685-1613

-----Original Message-----
From: owner-si-list@silab.eng.sun.com
[mailto:owner-si-list@silab.eng.sun.com]On Behalf Of MikonCons@aol.com
Sent: Wednesday, May 03, 2000 8:01 PM
To: si-list@silab.eng.sun.com
Subject: Re: [SI-LIST] : Trace Impedance Selection

Interesting question, Doug.

Here's my initial reaction, but I may have to add more caveats after I think
about it some more....

Given that the trace is on the surface of the board and that you have a
specified (and finite) amount of energy to deliver via the trace to a load,
you should (within whatever practical realization constraints exist)
minimize
the impedance of the trace with respect to the circuit reference. The lower
the impedance the poorer the match with the free space impedance of 377
ohms;
therefore, you would realize a lousy antenna and minimize radiated
emissions.
 This approach breaks down for low frequencies and very low impedances
(hence, very high currents) as the local magnetic fields generated may be
more of an EMI problem. Therefore, the "right" answer may have to be
correlated to the surrounding environment. A good example would be the
unwanted creation of magnetic fields in the vicinity of a magnetometer on a
space craft that is trying to measure field variations in space surrounding
the earth.

One might simply say (for medium to high frequencies) that the lower
radiation is a result of a smaller loop when the trace is spaced closer to
the reference (which happens to achieve lower impedance), but in reality
they
go hand in hand.

Good luck on this one.

Mike

Michael L. Conn
Owner/Principal Consultant
Mikon Consulting

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