Our treatment of microelectronics thus far has mostly concentrated on the problem of amplification. Another important function vastly used in electronic systems is “filtering.” For example, a cellphone incorporates filters to suppress “interferers” that are received in addition to the desired signal. Similarly, a high-fidelity audio system must employ filters to eliminate the 60 Hz (50 Hz) ac line interference. This chapter provides an introduction to analog filters. The outline is shown below.
In order to define the performance parameters of filters, we first take a brief look at some applications. Suppose a cellphone receives a desired signal, X(f ), with a bandwidth of 200 kHz at a center frequency of 900 MHz [Fig. 15.1(a)]. As mentioned in Chapter 1, the receiver may translate this spectrum to zero frequency and subsequently “detect” the signal.
Now, let us assume that, in addition to X(f), the cellphone receives a large interferer centered at 900 MHz + 200 kHz [Fig. 15.1(b)].1 After translation to zero center frequency, the desired signal is still accompanied by the large interferer and cannot be detected properly. We must therefore “reject” the interferer by means of a filter [Fig. 15.1(c)].