Side Bands

Author: J.B. Hoag

A wave of the same shape as that shown in Fig. 16 C can be produced by combining three radio-frequency waves in a "nonlinear device" or rectifying tube. Conversely, an amplitude-modulated carrier wave can be thought of as though it consisted of three radio-frequency waves, as in Fig. 16 N.¹

Fig. 16 N. Side bands

One of the waves has a fixed amplitude and the same frequency as that of the unmodulated carrier. The other two waves have amplitudes equal to each other. This value changes in proportion to the strength of the audio signal. Their frequencies differ from that of the carrier by the frequency of the audio wave. The frequency of one of the waves is greater, the other is less, than that of the carrier. Since the audio frequency changes with the pitch of the sound waves which enter the microphone, the frequencies of these two waves increase or decrease, from that of the carrier (for zero a.f.) to values which are greater or less by the highest a.f. Thus, these frequencies change over an upper and a lower side band on the sides of the carrier. Therefore, in order to transmit an intelligible signal, a channel or band of frequencies is needed equal to twice the highest pitch of the sound. A band 5 kHz on each side of the carrier, or 10 kHz total width, is used in broadcasting. All of the electrical circuits which handle the modulated-carrier wave in the transmitter and the receiver should have equal amplification of all frequencies over this 10-kHz band, with zero amplification at all other frequencies. In practice this can only be approximated.

1	This viewpoint may not appear simple to the beginner, but it actually proves to be so in the practical analysis of transmitters.