Pozar, David M. Department of Electrical and Computer Engineering, University of Massachusetts, Amherst, Massachusetts.
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A two-port component used to provide frequency selectivity in satellite and mobile communications, radar, electronic warfare, metrology, and remote-sensing systems operating at microwave frequencies (1 GHz and above). Four types of filter responses are commonly used in such systems (Fig. 1): low-pass, high-pass, band-pass, and band-stop. For example, a low-pass filter allows the passage of signals at frequencies below the cutoff frequency, fc (the passband), while reflecting signals at higher frequencies (the stopband). While an ideal filter would have zero loss in its passband and infinite attenuation in its stopband, such characteristics can only be approximated for realistic filters. Microwave filters perform the same function as electric filters at lower frequencies, but differ in their implementation because circuit dimensions are on the order of the electrical wavelength at microwave frequencies. Thus, in the microwave regime, distributed circuit elements such as transmission lines must be used in place of the lumped-element inductors and capacitors used at lower frequencies. This can make microwave filter design more difficult, but it also introduces a variety of useful coupling and transmission effects that are not possible at lower frequencies. In addition, the use of distributed elements often introduces spurious stopband regions of low attenuation at frequencies far above the passband.
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