[SI-LIST] Radiating Ceramic Bulk Capacitors
- From: "Paradis, Daniel" <Daniel.Paradis@xxxxxxxxxx>
- To: "'si-list@xxxxxxxxxxxxx'" <si-list@xxxxxxxxxxxxx>
- Date: Wed, 9 Apr 2003 10:45:45 -0400
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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