RE: [SI-LIST] : May 9th Presentation: "Radiation from Edge Effec ts in Printe...

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From: Nadolny, Jim (nadolnj@bergelect.com)
Date: Tue May 09 2000 - 05:54:02 PDT


Mike,

I agree with you and offer a similar experience.

In doing research on a backplane connectors influence on radiated emission
we needed a "low noise" PCB structure. We were feeding a "perfect
connector" with a stripline trace and wanted to be sure we measured the
connector and not the PCB. For the stripline structure of the PCB, we had
to copper tape the edges of the PCB. For the instrumentation feed we had to
use a special coax to stripline feed with the shield of the coax soldered
360 degrees to the surface ground plane of the PCB. With this type of
arrangement and a "perfect" backplane connector (semi-rigid with outer
shield soldered to the ground plane) we could get down to the
instrumentation noise floor. Open edges of the PCB or a typical thru hole
SMA feed would excite the structure which we detected as common mode
current.

An extreme example perhaps, but it pointed out that a stripline is only as
quiet as the quality of the implementation.

Jim Nadolny
FCI
-----Original Message-----
From: MikonCons@aol.com [mailto:MikonCons@aol.com]
Sent: Monday, May 08, 2000 7:52 PM
To: si-list@silab.eng.sun.com
Subject: Re: [SI-LIST] : May 9th Presentation: "Radiation from Edge
Effects in Printe...

Chris R.:

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.

Mike

Michael L. Conn
Owner/Principal Consultant
Mikon Consulting

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