TABLE OF CONTENTS Filters are frequency-selective circuits that pass some frequencies and reject others. Filters are available in several different flavors: low-pass, high-pass, bandpass, and notch. All of these filters are classified according to the frequencies that they pass (or reject). The breakpoint between the accept band and the reject band is usually taken to be the frequency at which the passband response falls off 3 dB. The four different types are characterized below.
Low-pass filters pass all frequencies below the cutoff frequency defined by the 3-dB point (Fig. 23-1A). These filters are useful for removing the harmonic content of signals or eliminating interfering signals above the cutoff frequency.
High-pass filters pass all frequencies above the 3-dB cutoff point (Fig. 23-1B). These filters are useful for eliminating interference from strong signals below the cutoff point. For example, a person using a shortwave receiver might wish to install a 1800-kHz high-pass filter to eliminate signals from strong AM broadcast stations.
Bandpass filters pass all frequencies between lower ( F L) and upper ( F H) 3-dB points, while rejecting those outside the F H F L range. Bandpass filters are either wideband (low Q) or narrow band (high Q), as shown in Fig. 23-1C and 23-1D, respectively.
Q is the quality factor of the bandpass filter and is...
