Moreover, the time response would be slow and oscillatory. Assuming tight coupling of the filter components and reasonable coupling of the choke itself (conditions we would expect to achieve), the gain near the cutoff point may be very large (several dB). One of the critical factors involved in the operation of higher order filters is the attenuating character at the corner frequency. The design of a second order filter requires more care and analysis than a first order filter to obtain a suitable response near the cutoff point, but there is less concern needed at higher frequencies, as previously mentioned. Ideally, a second order filter: 1) provides 12 dB per octave attenuation (four times that of a first order filter) after the cutoff point and 2) provides greater attenuation at frequencies above inductor self-resonance (see Figure 4 ).įigure 4: Analysis of a second order (two pole) common mode low pass filter Second Order FiltersĪ second order filter uses two reactive components and has two advantages over the first order filter. However, some care should be observed in choosing a choke for a first order low pass filter because an inductance with a much higher than typical or minimum value may limit the choke’s useful band of attenuation. It is typical for the inductance value of a common mode choke to be specified as a minimum requirement, thus ensuring that the crossover frequency not be shifted too high. For example, a ± 20% variation of rated inductance means that the nominal 3 dB frequency of 4000 Hz could be anywhere from 3332 Hz to 4999 Hz. Because of the predominant inductor dependence of a first order filter, the variations of actual choke inductance must be considered. The attenuation at 4000 Hz would be 3 dB, increasing at 6 dB per octave. įigure 3: A first order (single pole) common mode filter The resulting common mode filter configuration is shown in Figure 3. For example, attenuation at and above 4000 Hz into a 50Ω load would require a 1.99 mH (50/(2π x 4000)) inductor. The value of inductance required of the choke is simply the load in Ohms divided by the radian frequency at and above which the signal is to be attenuated. In the case of a low pass common mode filter, a common mode choke is the reactive element employed. This type of filter uses a single reactive component to store certain bands of spectral energy without passing this energy to the load. The simplest and least expensive filter to design is a first order filter. The inductors thus perform as independent components with their mutual inductance responding to the common signal: the mutual inductance then attenuates this common signal. When inductors L1 and L2 encounter an identical signal of the same polarity referred to ground (common mode signal), they each contribute a net, non-zero flux in the shared core. They perform as independent components with their leakage inductances responding to the differential signal, and the leakage inductances attenuate the differential signal.

For a differential input current ((A) to (B) through L1 and to (A) through L2), the net magnetic flux which is coupled between the two inductors is zero.Īny inductance encountered by the differential signal is then the result of imperfect coupling of the two chokes.

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