[SI-LIST] Re: Question about pcb grounding
- From: <art_porter@xxxxxxxxxxx>
- To: <joel@xxxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Tue, 6 Nov 2007 07:22:32 -0700
You're on the right track. I believe the impetus for this guideline was =
the thought that two ground return conductors will form a loop, which =
can resonate. Keeping the round trip distance around the loop short =
keeps the resonant frequency high. But the suggested limit of 1/20 of a =
wavelength does seem a bit conservative. If you had such 2 conductors, =
and they were directly connected to one another at each end, the total =
distance around the loop would then be 1/10 of a wavelength. =20
Art Porter
Agilent Technologies =20
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] =
On Behalf Of Joel Brown
Sent: Monday, November 05, 2007 4:30 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Question about pcb grounding
I have been reading some material regarding multipoint grounding of a =
pcb to
chassis ground.
The recommendation is to have no more than 1/20 of a wavelength between
grounding points.
So at 500 MHz this is about 1 inch which clearly is not feasible in most
applications.
I'm guessing the reason for 1/20 wavelength is so that common mode noise
generated on the pcb ground plane has a low impedance return path =
through
the chassis structure from one section of the board to another. If the
distance is much greater than that then there could be a high impedance
(resonance) in the chassis return path and the common mode noise will go
elsewhere. Am I right in my thinking here? What benefit is there to =
using
multipoint chassis grounding when the spacing between ground points is =
much
greater than 1/20 wavelength which is what represents most design today?
=20
Thanks - Joel
=20
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=20
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