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Fundamentals of Microelectronics, 2nd Edition by Behzad Razavi

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Analog Filters

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.

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15.1   GENERAL CONSIDERATIONS

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)].

Figure 15.1   (a) Desired channel in ...

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