[SI-LIST] : RE: Vias, radiation from board edge

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From: Dan Swanson (dswanson@bartleyrf.com)
Date: Thu May 11 2000 - 04:53:31 PDT


The main points I remember:

1) Time-varying currents on vias can inject radial TEM-mode waves into the
space between planes.

2) The energy thus injected bounces around the cavity volume between the
planes. The board edge is a discontinuity in the medium and so results in
partial reflection of the propagating wave and partial transmission, i.e.
radiation from the board edge.

All true, but why not kill it at the source????

You can place 4 ground vias around the signal via and greatly reduce
the launching of energy into other layers, basically short out the parallel plate
modes at the source. There is also a simple formula for the impedance of the
resulting five wire transmission line, so the via can be a controlled impedance
continuation of your signal trace. For really high frequency performance, you
also have to "tune" the via by removing pads and adjusting the diameter of anti-pads.
Anti-pad dia wants to be smaller near microstrip layers and larger near stripline
layers.

If you look at a simple case of microstrip to stripline on the right field-solver, you
can actually plot the mode conversion from microstrip mode to parallel plate mode
with and without the vias present.

The dimensions work out well for every case I have looked at so far, resonable
spacing between vias given the via diameter that the fabricator demands.
I realize this may not be possible in a very high signal density environment,
but it has worked very well where there is room. Have used this in digital and RF
designs.

Dan

Dan Swanson EMAIL: d.swanson@ieee.org
Bartley R.F. Systems TEL: (978) 241-1091
38 Water Street FAX: (978) 388-7077
Amesbury, MA 01913

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