Hi Jeff and the group,
It is not possible to isolate the chassis from the PCB ground at high
frequencies. They are coupled by tens of Ohms of impedance. This leads
to a resonant condition where the coupling capacitance and the
connection between chassis and ground at the single point form a tuned
circuit at a much lower frequency than you would expect. Even
multipoint grounding results in a (higher) resonant frequency of the
structure and sometimes removing one or more grounds changes the
frequeny and causes the equipment to work. Then people cite this as a
reason (incorrectly) that multpoint grounding is bad. With multipoint
grounding, suppressing the structural resonant frequency is really
easy, just replace one or more (depending on the size of the structure)
of the grounding points with resistors totalling about 20 Ohms for the
parallel combination (or you could measure exactly what value is
needed), and viola! Resonance gone! Sometimes I do live experiments in
my seminars of this effect. It is easy to reproduce and improves
emissions and immunity of a system.
Sometimes multipoint grounding is not permitted, and once in a while
for a good reason. Anything can be made to work, but in my 50 years of
experience in Electrical Engineering, the cost is higher if you are
prevented from grounding in the best way for the particular design you
are doing. Usually, but not always, this is multipoint grounding.
Personally, I have solved a lot of problems with multi-point grounding
and don't ever recall causing one. Keep in mind there are a few unusual
cases where grounding requirements within a system can be strange (NTSC
video for one, Never The Same Color twice, and ancient telecom systems
before switching power supplies were available).
Douglas C. Smith
--------------------------------------------
University of Oxford Course Tutor
Department for Continuing Education
Oxford, Oxfordshire, United Kingdom
--------------------------------------------
PO Box 60941, Boulder City, NV 89006
Email: doug@xxxxxxxxxx
Web: http://www.dsmith.org
Web: http://emcesd.com
Tel: +1-702-570-6108
Mobile: +1-408-858-4528
On Mon, 9 Jan 2017 15:37:50 +0000, "Loyer, Jeff"
<dmarc-noreply@xxxxxxxxxxxxx> 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
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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