Chapter 18 - Planning a Receiver System

Abstract

Planning a receiver system means planning a radio receiver design and architecture. No parameter of the receiver is unimportant, but several have predominance. First is the sensitivity required of the receiver. Whether it is described in terms of microvolts or dBm to achieve a given signal-to-noise ratio (SNR), the sensitivity is important. Second, the dynamic range of the receiver is very important. Some form of input filtering is needed to improve the image rejection of the receiver as well as suppress spurious signals. Two designs are used. A 32–35 MHz low-pass filter may be used to keep the response within range. Or, individual bandpass filters may be used instead of the low-pass filter. If an RF amplifier is used, it will have a low gain (3 to 20 dB) and relative broad selectivity (maybe a wideband amplifier). All radio reception faces the problem of SNR control. Indeed, when signals are weak, the SNR is of paramount importance to the success of the system. A certain amount of noise is contributed from sources outside the receiver, but the chapter focuses with the noise generated inside it. A basic level of noise is inherent to any circuit, equal to the thermal noise of the resistive component of its impedance. The sensitivity of a radio receiver is probably one of the most important aspects to its design. The selectivity of the receiver is set largely by the IF amplifier filtering, but the filtering that goes up front is very important in reducing certain spurious responses.