RE: [SI-LIST] : Attenuation on a PCB trace

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From: Jian X. Zheng (jian@zeland.com)
Date: Tue Apr 24 2001 - 10:55:39 PDT


Hi, Michael:

More comments on microstrip and stripline:

From the fundamental theory, stripline should be much more superior to
microstrip as long as rise time (or frequency dispersion) is concerned.
However, there are some other considerations we need to worry:

In practical design, we will have vias on the circuits, and holes on the
ground. The vias and holes are the so-called discontinuities. They are
certainly not good to the performance of circuits. They are especially bad
to stripline circuits. For a normal strip line circuit, the potentials on
the two ground planes are identical and they are negative (-) and the
potential on the trace is positive (+). Unfortunately, when the wave hits a
via on the circuit, part of the energy will be converted into parallel plate
mode with a voltage difference between the top ground and the bottom ground.
The parallel plate mode will not propagate along the trace any more. It will
radiate out in the radial direction. This radiation is in fact very serious
at high frequency. One single via may radiate out more than 10% of the
power. The radiation does not only cause loss of propagation power, it also
creates coupling and cross-talk to adjacent traces. In order to suppress the
radiation, microwave people use vias to short the two ground planes at where
the signal vias are. It will reduce the radiation significantly. However, in
a multiple layer PCB environment, one of the grounds may be a true ground
and the other may be a VDD ground. They are actually of differnet potentials
at DC. We can not use vias to connect the two grounds together to suppress
the parallel plate modes. Until someone can find a simple way to suppress
the radiation from the vias and holes on striplines, it is hard to say that
stripline is superior to microstrip. Thanks!

best regards,

-----------------------------------------------------------------------
Jian-X. Zheng, Ph.D
Zeland Software, Inc., 48890 Milmont Drive, 105D, Fremont, CA 94538, U.S.A.
Tel: 510-623-7162, Fax: 510-623-7135, Web: http://www.zeland.com
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