Re: [SI-LIST] : Trace Impedance Selection

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From: [email protected]
Date: Wed May 03 2000 - 20:00:58 PDT

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.


Michael L. Conn
Owner/Principal Consultant
Mikon Consulting

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