[SI-LIST] Re: Radiating Ceramic Bulk Capacitors
- From: "Bart Bouma" <bart.bouma@xxxxxxxxx>
- To: Daniel.Paradis@xxxxxxxxxx
- Date: Thu, 10 Apr 2003 10:00:13 +0200
Hi Daniel,
I'm not an EMC expert, but I have some RF-experience and now I'm dealing
wih ceramic caps.
What Istvan and Larry said is most probably the cause for your EMI
problems: high impedance peaks caused by parallel resonance (aka
"anti-resonance") of a circuit formed by an capacitor that went inductive
(above SRF) and a smaller valued capacitor - that still is capacitive,
together with other components and pcb tracks etc.
But, I would like to emphasize that capacitors in general, including
ceramic caps, can radiate!
A capacitor can couple RF/EMI to an adjacent capacitor, or to a track that
runs along the capacitor (due to inductive coupling).
And a track - of course - can act as an effective antenna.
This coupling starts at frequencies above some tens of MHz, and is quite
low, but starts to increase at GHz frequencies.
There are ceramic caps available that don't radiate: X2Y ceramic
multilayer capacitors. These caps are three terminal IPD's.
X2Y-caps have a Faraday-cage around their electrodes, each active electode
is surrounded by a grounded plate, effectively screening the "hot"
electrodes.
Radiation and coupling is almost eliminated.
Due to this internal structure the X2Y-caps also have excellent common
mode supression, leading all CM-EMI to ground.
X2Y info: www.yageo.com or www.x2y.com
regards, Bart Bouma
application engineer
Yageo Europe
"Paradis, Daniel" <Daniel.Paradis@xxxxxxxxxx>
09-04-03 16:45
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Please respond to Daniel.Paradis
To: "'si-list@xxxxxxxxxxxxx'" <si-list@xxxxxxxxxxxxx>
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Subject: [SI-LIST] Radiating Ceramic Bulk Capacitors
Category:
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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