Common-Mode Filter Design
Common-mode filters are useful in the control of both radiated and
conducted emission. They are used on signal lines as well as power
lines (AC or DC). However, the majority of common-mode filters are
designed empirically. One of the reasons for this is that the source
and load impedances are not known, and, without these, the filter's attenuation
cannot be determined. The attenuation of commercial common-mode filters
is usually specified assuming 50 ohm source and load impedances and, therefore,
does not relate to the filter's performance in an actual circuit.
This one-day course develops the theory required to design and
apply common-mode filters successfully. It explains why common-mode
filter design is different than differential-mode filter design.
It provides information on typical common-mode source and load impedances.
Filters are designed using the graphical attenuation method. This
graphical method allows the designer to calculate and understand the effects
of varying load and source impedances, and to design complex filters one
stage at a time. Another advantage of this method is the ease in
which parasitic and complex component characteristics can be handled.
The method also provides the designer with an intuitive understanding of
how the filter operates and, therefore, what changes are necessary to vary
This course will answer the questions: Should I use a ferrite
choke or a shunt capacitor for a filter? If I use a ferrite and a
capacitor, should the capacitor face the cable or the circuit? Why
do ferrites only work over a narrow frequency range that seems to change
in every application? What are the advantages of multi-element filters?
Examples are given using the graphical attenuation method to design both
power-line and digital data-line filters. Both single stage and multi-stage
filter designs are discussed.
THEORY OF COMMON-MODE FILTER DESIGN
Basic Filter Theory
Special Characteristics of C-M Filters
Placement and Grounding of C-M Filters
The Filter as an Impedance Mismatching Device
THE GRAPHICAL ATTENUATION METHOD OF FILTER
Single Stage/Multi-Stage Filters
Resonance and Q
Comparison of Graphical Results to Numerical Calculations
EXAMPLES OF DIGITAL DATA-LINE FILTER DESIGNS
Typical Source and Load Impedance
Single Element Filter Examples
"L,"T," and "pi" Filter Examples
Attenuation Curves for "L," "T," and "pi" Filters
EXAMPLES OF POWER-LINE FILTER DESIGNS
Example of a Power Line Filter to Meet FCC/CISPR Requirements
Example of a Power Line Filter to Meet MIL-STD-461 Requirements
Page 2, Advantages of Graphical
Attenuation Approach to Common Mode Filter Design
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Henry Ott Consultants
48 Baker Road Livingston, NJ 07039
Phone: 973-992-1793, FAX: 973-533-1442
June 6, 2000