TABLE OF CONTENTS Until now we have treated the radio signal as though it were something having value in itself; actually it is only variations in a radio signal which are meaningful to a person receiving the signal. A constant, unchanging electromagnetic wave conveys no information whatever to the listener; in order that some information be transmitted, the amplitude, frequency, or phase of the radio wave must be varied in some manner. Any such variation of the wave to impress information is called modulation.
The only kind of signal that is confined to a single frequency is one which is purely sinusoidal in waveform and which is not being varied in any manner. Any time that we impress modulation upon a signal, frequencies other than the original signal frequency will be produced in the process of modulation. In particular, frequencies equal to the sum and difference of the radio signal and the modulation frequencies will appear in the radiated energy. The original, unmodulated radio signal is known as the carrier, and the additional signals which are generated in the modulation process are called sidebands.
Figure 59-1 shows the relationship between the carrier and sideband frequencies of a carrier which is modulated in amplitude by a single audio tone. In Fig. 59-1 a we see the actual waveform of the modulated carrier and in Fig. 59-1 b its frequency spectrum. By "frequency spectrum" is meant a graph of amplitude versus frequency; this is what we would observe if we...
