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Basic specifications and filtering characteristics for a common mode choke are represented in terms of impedance vs. frequency, as shown in this example of the wirewound type PLT5B products (four different part numbers altogether). Note that two kinds of impedance are shown on a single chart: common mode (solid lines) and differential mode (dotted lines). The specification of greatest interest is usually the common mode impedance, because that is what is going to provide the desired filtering action on the balanced transmission line. In contrast, the differential mode impedance is generally quite small (10 to 100 times smaller for this particular design). This is because of the small amount of magnetic flux generated in the choke core by the differential mode signal flowing in opposing directions, as previously shown. But even this is an important specification, because it defines how much filtering is actually applied to the differential mode signal that needs to be preserved. Both specifications are important to the circuit designer and are typically shown on the same chart. For specification purposes, 100 MHz is chosen as a standard reference frequency for common mode choke impedances. However, to determine the impedance at other operating frequencies, a full set of impedance vs. frequency curves can be obtained from Murata’s website.
