Chapter 6: Filter Design

Overview

A filter is used to selectively pass or attenuate a particular band of frequencies, and can be constructed of LC, RC, LCR, LR, or distributed components, and can be either active or passive in nature. Active filters will contain some sort of amplifier combined with any of the above lumped passive components, while passive filters will simply employ lumped or distributed components, with ceramic and crystal filters found in many passband and stopband applications. Any of these will dramatically improve a filter s shape factor (steepness of its skirts), as well as provide a variety of bandwidths all the way from ultranarrowband to wideband. Surface acoustic wave (SAW) passive filters are also becoming more common, and are available with superb shape factors from narrowband to ultrawideband.

The RF spectrum contains quite a broad range of frequencies. Obviously, a method had to be found that would allow us to segregate a small chunk of this spectrum for transmission and reception, without interfering with other communications channels. This can be done with the use of untuned and tuned filters. The basic passive untuned LC filter can function as a low-pass or high-pass filter because of the inductor s ability to increase its reactance, and a capacitor s ability to decrease its reactance, as the frequency is increased. This makes the untuned LC filter frequency-selective. To act as a low-pass filter and attenuate higher frequencies (Fig. 6.1), an inductor will be arranged in series (blocking the high frequencies), while a...