Date: Mon May 08 2000 - 16:51:46 PDT
One exemplary case I recently came across had a series-terminated clock line
that was immediately submerged to a stripline which went to a connector on
the edge of a PCB. The optional card that would use the connector was NOT
installed. The connector receptacle pin rose above the PCB about 3/16 inch.
This small 3/16-inch stub coupled harmonic energy to the SURFACE of the PCB
(a ground plane) at the SURFACE resonant frequency of the PCB. Surface
currents could be measured all over the surface ground plane (with a small
Fischer Communications probe). NASTY radiation resulted.
Simple removal of the series termination resistor at the clock driver end
reduced system radiation over 30 dB.
The 20H rule had nothing to do with suppressing this radiation. The moral?
Avoid inducing surface currents any way you can.
Michael L. Conn
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