11.4: Filters

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A microwave filter can consist solely of lumped elements, solely of distributed elements, or a mix. Loss of lumped elements, particularly above a few gigahertz, means that the performance of distributed filters nearly always exceeds that of lumped-element filters. However, since the basic component of a distributed filter is a one-quarter wavelength long transmission line, distributed filters can be prohibitively large below a few gigahertz. The basic types of responses required at RF as follows:

Lowpass—providing maximum power transfer at frequencies below the corner frequency, \(f_{0}\). See Figure 11.5.1(a).
Highpass—passing signals at frequencies above \(f_{0}\). Below \(f_{0}\), transmission is blocked. See Figure 11.5.1(b).
Bandpass—passing signals at frequencies between lower and upper corner frequencies (defining the passband) and blocking transmission outside the band. This is the most common type. See Figure 11.5.1(c).
Bandstop (or notch)—which blocks signals between lower and upper corner frequencies (defining the stopband). See Figure 11.5.1(d).
Allpass—which equalizes a signal by adjusting the phase generally to correct for phase distortion elsewhere. See Figure 11.5.1(e).

In the passband, filters can be treated as though they are attenuators with typical losses, or attenuation, of \(0.1\text{ dB}\) for a large basestation filter to \(3\text{ dB}\) for a miniature filter in a handset. Out-of-band losses are typically at least \(40\text{ dB}\) for a handset filter to \(120\text{ dB}\) or more for a basestation filter.