Abstract :
The general problem of selectivity, as always, is to amplify the wanted frequencies and reject the unwanted frequencies. This is the function of the i-f amplifier and the main reason for using the superheterodyne system at all. In television, the closest unwanted frequencies are the picture carrier of the next higher channel and the sound carrier of the next lower channel. Channels 2, 3, 5, 7, 8, 9, 10, 11 and 12 would be interfered with by the picture carrier of Channels 3, 4, 6, 8, 9, 10, 11, 12 and 13, respectively, and their roles would be interchanged as far as the sound carriers are concerned. Naturally, we want, to reject all the information in the adjacent channel, but the carriers are by far the strongest. Figure 1 illustrates how an improvement in adjacent channel rejection can increase the service area of a transmitter 100 miles from a transmitter on an adjacent channel.
