a valuable link in communications and entertainment. cheap when the amplifier is used indoors as it does not Early receivers were insensitive, inefficient, cumber- have to be protected from extreme weather conditions. some, and required large power supplies. Modem designs The main disadvantage of indoor mounting is that signals using as little as one integrated circuit have overcome are attenuated by the aerial down-lead cables before most of these disadvantages and relatively inexpensive amplification. This signal loss decreases the available receivers are readily available. signal before amplification and therefore a poorer signal- to-noise ratio is available to the distribution points than if 2.1.8.4 Fundamental radio receiver the amplifier was to be mounted on the masthead. requirements 2.1.8 Radio receivers In the AM medium wave band, broadcasting stations transmit their signals centred on assigned carrier fre- 2.1.8.1 Aims quencies. These carrier frequencies are spaced 9 kHzapart from each other as in Figure 2.1-27 and range from The aims of this section are to introduce you to: 522 kHz to 1620 kHz. The information bandwidth allo- cated for each AM transmission is 9 kHz. This means that the tuned r.f. receiver modulation frequencies greater than 4.5 kHz are not the superhet receiver normally used. To receive information from a broadcast the double superhet receiver signal, an AM broadcast receiver must be tuned to the selectivity requirements in receivers correct carrier frequency, have a bandwidth that will pass sensitivity requirements in receivers the required modulated signal, and be capable of extracting concepts of signal-to-noise and sinad ratios information from the required radio signal to operate de- noise figures of receivers sired output devices such as loudspeakers and earphones. CHAPTER 2.1 Basic features of radio communication systems Broadcasting stations are spaced 9 kHz in the medium wave band Fig 2.1-27 Spacing of broadcast stations in the medium wave band. Radio Post- Demodulation freque