ncy demodulation section section section Fig. 2.1-28 Three main sections of a radio receiver. Discussions that follow pertain mainly to receivers approx.) to pass the entire frequency spectrum of the operating in this band. This is not a limitation because desired broadcast signal. Second, the passband should many of the principles involved apply equally well to present equal transmission characteristics to all frequen- other frequency bands. When the need arises, specific cies of the desired broadcast signal. In addition, the principles applying to a particular frequency band will be bandwidth should be no wider than that required for the mentioned but these occasions will be clearly indicated. desired signal because any additional bandwidth will allow extraneous signals and noise from adjacent channels to 2.1.9 Radio receiver properties impinge on the receiver. Notice that the skirts of the ideal selectivity curve are vertical, SO that the attenuation of any signal outside the passband is infinitely high. A radio receiver has three main sections (see Figure In practice, costs and stability constraints prevent the 2.1-28). ideal selectivity response curve from ever being attained Ar.f. section to select and if necessary to amplify a de- and it is more rewarding to examine what is achieved by sired r.f. signal to an output level sufficient to operate commercial receivers. a demodulator. An overall receiver selectivity response curve for A demodulator section to demodulate the required a typical domestic transistor receiver for the reception of radio signal and extract its modulated information. AM broadcast signals is shown in Figure 2.1-30. In the A post-demodulation section to amplify demodu- table supplied with Figure 2.1-30, you should note that lated signals to the required level to operate output the selectivity curve is not symmetrical about its centre devices such as loudspeakers, earphones and/or TV frequency. This is true of most tuned circuits because the screens. effective working quality factor (Qw) 