LC composite EMI filter is the key to noise countermeasures

This time, we will start with the LC compound type EMI filter series.

＜When C and L are combined, the insertion loss will decrease sharply＞

Previously, we introduced that the insertion loss of a capacitor and an inductor combined decreases more dramatically than that of a single capacitor.

Figure 1 shows its graphical features.

Figure 1 Filter component count and frequency characteristics

As shown in the figure, the greater the number of filter elements, the more sharply the filter insertion loss decreases.

＜The greater the slope of the filter's insertion loss characteristics, the higher the selectivity between signal and noise.＞

As the slope of the filter's insertion loss characteristic increases, when the signal is close to the frequency of the noise, it is difficult for the signal to be adversely affected. Figure 2 shows an example where the signal frequency is high and close to the noise frequency. When the two frequencies are close, if a filter with a relatively flat insertion loss is used and a constant that can sufficiently reduce the noise is selected, the high-order harmonic components of the signal with a frequency close to it are also sufficiently weakened. As a result, as shown in Figure 3, the signal waveform becomes flat. If a constant that does not affect the signal frequency is selected, the noise cannot be fully reduced. On the other hand, if a filter with a large slope of insertion loss characteristic change is used, the signal and noise can be separated, and the influence of the two on the signal can be more effectively controlled. For this reason, the noise suppression effect of the LC composite filter can be used to suppress the influence on the signal waveform as much as possible in high-speed signal lines.

Figure 2 Different effects of different filter characteristics on the signal

Figure 3 Example of waveform trend