[SI-LIST] Re: Radiating Ceramic Bulk Capacitors
- From: "Paradis, Daniel" <Daniel.Paradis@xxxxxxxxxx>
- To: 'Istvan Novak - Board Design Technology' <inovak@xxxxxxxxxxxxxxxxxxx>
- Date: Wed, 9 Apr 2003 14:07:16 -0400
Istvan
This is a good suggestion.
Did you ever tested the impedance of a board over a wide range of frequency
to get the curves?
We did it here for individual caps but never for a board.
I would imagine a scenario where the impedance is measured in several
different points
in a check board configuration following the cap network.
It would be interesting to take the measurements with only caps placed and
nothing else
to compare with the theoretical models of network caps provided by Laplace
transform templates (Excell or MathCad)
Another case would be with the regular board with all the parts.
I wonder how different it could be from the simplified "only cap" model.
-----Original Message-----
From: Istvan Novak - Board Design Technology
[mailto:inovak@xxxxxxxxxxxxxxxxxxx]
Sent: Wednesday, April 09, 2003 1:48 PM
To: Daniel.Paradis@xxxxxxxxxx
Cc: si-list@xxxxxxxxxxxxx; inovak@xxxxxxxxxxxxxxxxxxx
Subject: Re: [SI-LIST] Radiating Ceramic Bulk Capacitors
Daniel,
One possible scenario is that your boards radiate, and the ceramic
capacitors just create an
impedance profile such that one board will excite mostly the impedance peaks
(this is where the
radiation gets worse with the capacitors) and the other board excites mostly
the new impedance
minima (this is where radiation goes down with capacitors). To
prove/disprove this possibility
you would need to simulate or measure the impedance profile of the two
boards.
Regards,
Istvan Novak
Delivered-To: si-list@xxxxxxxxxxxxx
From: "Paradis, Daniel" <Daniel.Paradis@xxxxxxxxxx>
To: "'si-list@xxxxxxxxxxxxx'" <si-list@xxxxxxxxxxxxx>
Subject: [SI-LIST] Radiating Ceramic Bulk Capacitors
Date: Wed, 9 Apr 2003 10:45:45 -0400
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To all EMC experts.
I have been experimenting a little with bulk capacitors recently.
And I would like to get your opinion on the following subject.
I got 2 different boards with about nine 10uF bulk ceramic capacitor
(1206
packages)
distributed evenly in a matrix configuration in the digital section
of the
design.
I took the EMI signature of the 2 different designs and I saw the
following.
One of the board showed a significant reduction in emission with all
the
bulks in place.
The other board showed the exact opposite; The radiation level was
much
lower when the bulk were removed.
I took 3 different measurements for each board/scenarios and compare
the
average values.
I remember having read a document from Dr. Johnson saying that
capacitors
can sometimes be a very effective EMI radiator.
I am assuming that common mode noise can sometimes get trough the
bulk to
migrate from one power plane to another.
Since the impedance of bulk capacitors is quite high at EMI
frequencies,
a drop of voltage caused by this common mode noise can occur and
produce
more radiation.
My preliminary conclusion is that bulk capacitors cannot be placed
arbitrary
in a matrix configuration.
They should be placed away from high frequency current paths.
Also I was thinking that since bulk capacitors (and decoupling or
bypass
caps) can act as radiators...
they should always be placed at the bottom of the board to use the
chassis
as a Faraday cage.
Any thoughts you want to share on the subject?
Thanks
Daniel Paradis
Staff Electrical Engineer
Digital Subscriber Networks
Scientific-Atlanta, Inc.
5030 Sugarloaf Parkway, ATL 1.3468
Lawrenceville, GA 30042
Tel: (770) 236-7896
Fax: (770) 236-2449
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