1-37 Figure 1-18B.—Components of a simple high-pass filter. Figure 1-18C.—Components of a simple high-pass filter. RESONANT CIRCUITS AS FILTERS Resonant circuits can be made to serve as filters in a manner similar to the action of individual capacitors and inductors. As you know, the series-LC circuit offers minimum opposition to currents that have frequencies at or near the resonant frequency, and maximum opposition to currents of all other frequencies. You also know that a parallel-LC circuit offers a very high impedance to currents that have frequencies at or near the resonant frequency, and a relatively low impedance to currents of all other frequencies. If you use these two basic concepts, the BANDPASS and BAND-REJECT filters can be constructed. The bandpass filter and the band-reject filter are two common types of filters that use resonant circuits. Bandpass Filter A bandpass filter passes a narrow band of frequencies through a circuit and attenuates all other frequencies that are higher or lower than the desired band of frequencies. This is shown in figure 1-19 where the greatest current exists at the center frequency (f_{r}). Frequencies below resonance (f_{1}) and frequencies above resonance (f_{2}) drop off rapidly and are rejected.