Re: shield layer connection

Dr. Edward P. Sayre (esayre@nesa.com)
Fri, 12 Jan 1996 16:03:02 -0500

At 12:09 PM 1/9/96 -0600, you wrote:
>Dear Dr. Sayre:
>
>People often confuse EMI shields with the grounds used in stripline
>construction, and may mistakenly conclude from your remarks that
>striplines are of little value against EMI. Is it worth elaborating
>a bit on this point?
>
>Regards,
>Richard Schumacher
>
>----------------------------------------------------------------------
Richard:

Thanks for your observation. The planes I was discussing are PCB planes
added to a design and meant specifically to be connected to chassis ground.
They are in no way
involved with the power and ground planes associated with stripline or
microstrip.

The use of conducting planes to achieve stripline, buried microstrip and/or
surface microstrip are absolutely necessary to achieve controlled impedance
transmission
systems in PBCs. Such T-lines which have VERY low emission levels since they
are predominately TEM transmission systems at the risetimes and clock
frequencies
currently in used digital designs. The presence of planes in PCB controlled
impedance
transmission lines is to provide the return current path for the transmission
line signals.

It is worth while to mention that excessive perforation of power/ground planes,
especially when the anti-pads that provide clearance for vias and plated
through holes overlap, not only causes EMI radiation but also results in
signal integrity problems
due to the inductive nature of such discontinuities. Hence the quality of the
power and ground planes is something to be kept in mind.

Again, thanks for the observation

Edward P. Sayre, Ph. D., P. E.
North East Systems Associates