Filtering can take on different meanings for differenr situations, in general, most EMI filters are low-pass filters, although hi(jh~pa$s and bandpass filters exist. In some cases of a particular frequency being the cause of interference, a notch filter may be used. In general, a perfect, ideal single comjwnent filter (either a capacitor or inductor) has a theoretical rolj-off or gain of - 20 dB/decade, with a practical maximum of something between and -I2<> dB. In fact, real components do not achieve that theoretical goal. Capacitors are more useful in high-importance circuits, whereas inductors are more useful in lower-impedance circuits.
Perhaps the simplest form of single component filter is rhe feedthrough capacitor (a.k.a, "EMI filter"). When combined with good shielding, the use of such a capacitor am be <}uite sufficient. Figure 1(5-5 shows two methods of passing a feedthrough capacitor through a sliielded panel. In Fig. 15-5A we see the screw in variety. The threaded nut is cinched tight against the chassis or panel. In Fig. 15-oB we see the installation of a solder-in type of feedthrough capacitor. A small filiet of solder is used to hold the capacitor against the chassis or panel. This type of capacitor assumes a solderable chassis or panel, thus it eliminates the use of aluminum.
15-S TVo types of feedthrough capacitor.
Where greater suppression is needed, a combination of L and C elements is needed, A two-component L-section filter is shown in Fig, 15-6A. This filter produces a theoretical gain of —40 dB/decadet which means 100:1 per decade between input and output signals. This filter can be used at any frequency, although the values will tend to differ between, say, LF and VHF, The ideal is to keep the lead lengths as short as possible to prevent radiation of the signal. As an alternative, a higher-order filter can be realized by replacing the grommet in Fig. 15-GA with a second feedthrough capacitor. In that case, a theoretical gain of -60 dB/decade is realized.
In Fig. 15-6B we see a case where the opposite situation occurs, i.e., the inductor input Lr-section filter. In this case the inductor or RF choke is mounted external to the chassis and directly drives the feedthrough capacitor.
Figure 15-7 shows a graphic of which filters to use when the source impedances are known. For example, in the case where the input impedance and the output impedance are both low then use either a single inductor or RF choke or a T- filter con sisting of two inductors and a capacitor. When the source impedance is high and the output impedance is low then use a single capacitor input L-section filter. Similarly, when the input impedance is low and the output impedance is high use an inductor input L-section filter. Finally, when the input and output impedances are both high use either a single capacitor or a pi-section filter as shown.
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