Basics of Common Mode Noise Filters(3/5)

Frequency response has conventionally been used for selecting common mode filters. However, mixed mode S-parameters are now currently used. They are more useful parameters derived from S-parameters conventionally used for high-frequency circuits.
The parameters are specified by the input mode (common/differential) and output/reflection mode (common/differential). Therefore, they are ideal for describing the characteristics of common mode noise filters, which act differently depending on the input mode.

The table below shows the major parameters of filter characteristics.

The insertion loss of differential mode (Sdd21) is a parameter used to indicate the signal quality loss of differential mode transmission. The frequency at which the Sdd21 loss becomes 3 dB, which is defined as the cut-off frequency, is a key indicator for selecting a filter.

In addition, the common mode attenuation (Scc21) indicates the noise rejection performance, and mode conversion (Scd21) indicates the conversion of part of the differential signals into common mode signals.

The graph below shows an example of the frequency response of common mode filters. The performance is better as the Sdd21 loss is lower, and the Scc21 and Scd21show larger attenuation in a wider frequency range.

	Circuit diagram	INPUT	OUTPUT	Filter parameter (Mixed Mode S-Parameter)
Signal quality		Differential	Differential	Differential insertion loss Sdd21 (0 dB = ideal)
Noise rejection		Common mode	Common mode	Common mode attenuation Scc21 (-∞ dB = ideal)
		Differential	Common mode	Mode conversion Scd21 (-∞ dB = ideal)

We provide the following data as parameters to indicate filter characteristics.
- S-parameter
- SPICE model

The S-parameters are provided based on actual measurement data and can be converted into mixed mode S-parameters by calculations shown in the figure below to obtain the frequency response of a variety of parameters.

SPICE models are provided as equivalent circuit models.

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