This "Also see" article below Lee's article says:
EMI and emissions: rules, regulations, and options
<http://www.edn.com/design/systems-design/4338748/EMI-and-emissions-rules-regulations-and-options>
"Common-mode currents in the signal ground can also contribute to
emissions. In that case, it is advantageous to make a solid connection
between the signal ground and chassis ground (assuming you're using a
metal chassis) at the I/O connectors. (The objective is to short any
voltage signal ground and chassis ground at the cable connection.)
Better yet, ground your high-speed boards as often as you can, through
multiple short, fat, flat connections, which helps minimize resonances
due to the board size."
On 01/09/2017 07:37 AM, Loyer, Jeff (Redacted sender jwloyer for DMARC)
wrote:
Hi Lee,
Can you provide an example where making multiple ground connections between a
backplane and chassis caused issues, and why? Section 9.9.1 of "Grounds for
Grounding" advocates stitching return planes to the chassis wherever
possible. Can you explain the flaw in their recommendation (which includes
substantiating figures from Bruce Archambeault), or the difference in your
situation?
Thanks,
Jeff Loyer
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On ;
Behalf Of Lee Ritchey
Sent: Saturday, January 07, 2017 11:26 AM
To: dmarc-noreply@xxxxxxxxxxxxx
Cc: 'si-list' <si-list@xxxxxxxxxxxxx>
Subject: [SI-LIST] Re: An Article on what ground is
I use the sides of the card cage as part of my Faraday cage. This is what
is sometimes called chassis ground.
There are several books on EMI that instruct engineers to connect logic
ground in the backplane to the card cage and then to connect the faceplates
to logic ground. When this is done, the usual complaint is that EMI is
"leaking" at the cracks.
When I get an EMI call with this complaint I know what to do to fix it
without even going on site. But, as you know, if you solve the problem over
the phone, people think that this is free advice and won't pay for it, so I
go on site and solve the problem for a reasonable fee!
Another one of these bad rules is connecting logic ground in a PCB to the
case at every mounting screw.
Sadly, there are two books on EMI published under the IEEE banner that list
both of these methods along with about a dozen other "rules of thumb" that
are technically incorrect.
When I see an EMI book with either of these rules I expect it to have other
bad advice on EMI as well and recommend that my students not buy them..
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On ;
Behalf Of Loyer, Jeff (Redacted sender "jwloyer" for DMARC)
Sent: Saturday, January 7, 2017 10:32 AM
To: leeritchey@xxxxxxxxxxxxx
Cc: 'si-list' <si-list@xxxxxxxxxxxxx>
Subject: [SI-LIST] Re: An Article on what ground is
Hi Lee,
Two questions:
1) Do you use a designated Faraday cage that is separate than the
chassis? That seems to be the implication if you distinguish between your
Faraday cage and "chassis ground". My guess is that you use the chassis as
your Faraday cage, but maybe not?
2) Can you provide an example where making multiple connections between
the backplane and the Faraday cage (chassis?) caused issues, and how? I.E.,
why would you insist on only a single connection in this instance? That
sounds very similar to what is recommended when parts manufacturers call for
separate "analog" and "digital" grounds, and is different from my
understanding (I would instead encourage more connections between the
backplane ground plane and the chassis).
Thanks,
Jeff Loyer
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On ;
Behalf Of Lee Ritchey
Sent: Saturday, January 07, 2017 9:57 AM
To: bbakshan@xxxxxxxxx
Cc: 'si-list' <si-list@xxxxxxxxxxxxx>
Subject: [SI-LIST] Re: An Article on what ground is
Thanks for the nice feedback.
Aha! You have asked an EMI question! I use the ground plane in a backplane
as one side of my Faraday cage. Where that backplane meets the flanges on
the card cage, I leave the copper on the top layer and connect that with vias
to the ground plane in the backplane. Now, there is the one connection
between logic ground and the Faraday cage. There can be no other connections
such as on face places if one is to avoid "leaking" at the cracks which is a
common complaint when more than one connection is made between logic ground
and the Faraday cage. Notice, I did not use the phrase "chassis ground" in
this discussion.
I have used this method in dozens of systems with very good EMI results.
Hope this helps.
Lee
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx<mailto:si-list-bounce@xxxxxxxxxxxxx>
[mailto:si-list-bounce@xxxxxxxxxxxxx] On Behalf Of Boris Bakshan
Sent: Saturday, January 7, 2017 5:24 AM
To: Lee Ritchey <leeritchey@xxxxxxxxxxxxx<mailto:leeritchey@xxxxxxxxxxxxx>>
Cc: si-list <si-list@xxxxxxxxxxxxx<mailto:si-list@xxxxxxxxxxxxx>>
Subject: [SI-LIST] Re: An Article on what ground is
Hi Lee,
Very good, comprehensive article, thank you.
What is your recommendation when it comes to backplanes? a backplane is
attached to the chassis with bolts/other conductive mechanical parts. Do you
recommend plating these PCB holes in the backplane? If you do, should these
holes be connected to (any) electrical net?
Thanks Lee.
On Fri, Jan 6, 2017 at 10:43 PM, Lee Ritchey
<leeritchey@xxxxxxxxxxxxx<mailto:leeritchey@xxxxxxxxxxxxx>>
wrote:
I have written an article on ground following the brisk exchange on'unsubscribe' in the Subject field
this subject last month. I hope it clears up some of the confusion
around this subject.
Here is where it is on the web.
http://ubm.io/2ikEDat
Lee Ritchey
Speeding Edge
P.O. Box 2194
Glen Ellen, CA
95442
707-568-3983
I just used the energy it takes
To get mad and wrote some blues.
Count Basie
Or:
Worry is like a rocking chair,
It keeps you busy but.
It doesn't get you anywhere.
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