[SI-LIST] : Signal Ground at the connector

ycchien (ycchien@alpha1.srrc.gov.tw)
Thu, 17 Dec 1998 16:50:55 +0800

Hi,
I'm currently designing a circuit board with ribbon cable running
outward to another daughter board. The signals' tr/tf time riding on the
cable are approximately 2ns. To eliminate the RF energy enemating from
the signal line, EMI filter is considered to be inserted along the
signal line near the connector before it leaves the board.
After reading some articles on filtering the signal line, I'm still
confused at how to ground the filter. Most of the articles suggest
grounding the filter to the chassis ground not the signal ground and
then capacitively couple these two grounds. Several measures can be
adopted, they are

A. Introduce a chassis plane to the layer stack, the filter ground is
connected through the via to the this chassis plane. But, where should
the signal return grounds of the cable be connected to? The chassis
plane or the signal ground plane?

B. Moat the signal ground around the connector, leaving a wide piece of
copper to serve as bridge accross the moat. Reserve srew holes on the
ground plane of the connector part to provide a shorted path to the
external chassis plate, when low frequency isolation is not a concern.
However if low frequency isolation do matter, how to capacitively couple
this ground plate to external chassis plate?

C. Moat the signal ground along the board circumference, with the
segmented ground plate along the circumference to serve as chassis
plane. The filter and signal return ground are connected to this plane.
The same question is encounterd as in item B.

Any comments and advices are greatly appreciated?

Eric Chien
Assistant Researcher
Synchrotron Radiation Research Center
Taiwan, R.O.C.
ycchien@alpha1.srrc.gov.tw

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