9.2 RECEIVERS

The transmitter forms one half of the communications link. Its performance must complement that of the receiver—whose sensitivity, selectivity, and demodulation characteristics determine the overall fidelity, spectral efficiency, and robustness of the complete radio system.

At the end of the communications channel is the receiver, which must be able to select the desired signal from all the signals present at the antenna, amplify and demodulate, and present it to the source in the appropriate format. There are three main features that describe how well the receiver performs these functions:

Sensitivity—how effectively the receiver recovers weak signals.
Selectivity—how well the desired signal can be distinguished from adjacent and interfering signals.
Fidelity—how accurately the recovered signal represents the original information.

These performance measures—sensitivity, selectivity, and fidelity—are formalized and quantified in Section 0, but it is worth keeping them in mind when considering the basic design of a receiver in the following sections.

Receiver architectures have evolved progressively to overcome limitations in gain, stability, and bandwidth control. Early receivers relied on passive detection, followed by tuned radio-frequency (TRF) amplification. The introduction of heterodyning and, subsequently, the superheterodyne principle enabled stable high gain and constant bandwidth independent of tuning frequency. Modern receivers extend these concepts into the digital domain through software-defined and adaptive architectures. The following sections trace this evolution before examining each functional stage in detail.