Basics of Common Mode Noise Filters(3/5)

⑥ Major filter characteristic parameters

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 diagramINPUTOUTPUTFilter parameter
(Mixed Mode S-Parameter)
Signal qualitycommon_kiso10DifferentialDifferentialDifferential insertion loss
(0 dB = ideal)
Noise rejectioncommon_kiso11Common modeCommon modeCommon mode attenuation
(-∞ dB = ideal)
common_kiso12DifferentialCommon modeMode conversion
(-∞ dB = ideal)

⑦ Data library

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.

■Download from here

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