1-45
for transmitting voice signals with those for transmitting music reveals that much less spectrum space is
required for voice communications.
Radio stations in the standard broadcast band are assigned carrier frequencies by the Federal
Communications Commission (FCC). When two stations are located near each other, their carriers must
be spaced some minimum distance apart in the radio spectrum. Otherwise, the sideband frequencies of
one station will interfere with sideband frequencies of the other station. The standard AM broadcast band
starts at 535 kilohertz and ends at 1,605 kilohertz. Carrier assignments start at 540 kilohertz and continue
in a succession of 10-kilohertz increments until the upper limit of the broadcast band is reached. This adds
up to a total of 107 carrier assignments, or CHANNELS, over the entire broadcast band. If stations were
assigned to all 107 channels (in a given geographical area), each station would be allotted a channel width
of 10 kilohertz. This leaves 5 kilohertz on each side of each carrier for sidebands. Since interference
between such closely spaced stations would be nearly impossible to prevent, the FCC avoids assigning
adjacent channels to stations in the same area. As a consequence of this policy, one or more vacant
channels normally exist between stations in the broadcast band. In the interest of better fidelity, the
stations are permitted to use modulating frequencies higher than 5 kilohertz as long as no interference
with other stations is produced.
Q-31. What are the two major sections of a typical AM transmitter?
Q-32. When 100 kilohertz and 5 kilohertz are heterodyned, what frequencies are present?
Q-33. What is the upper sideband of an AM transmission?
Q-34. Where is the intelligence in an AM transmission located?
Q-35. What determines the bandwidth of an AM transmission?
ANALYSIS OF AN AM WAVE.—A significant amount of information concerning the basic
principles of amplitude modulation can be obtained from a study of the properties of the modulation
envelope.
A carrier wave which has been modulated by voice or music signals is accompanied by two
sidebands; each sideband contains individual frequencies that vary continuously. Since a wave of this
nature is nearly impossible to analyze, you can assume in the following sections that the modulating
signal, unless otherwise qualified, is a single-frequency, constant-amplitude sine wave.
PERCENT OF MODULATION IN AN AM WAVE.—The degree of modulation is defined in
terms of the maximum permissible amount of modulation. Thus, a fully modulated wave is said to be
100-PERCENT MODULATED. The modulation envelope in figure 1-40, view (A), shows the conditions
for 100-percent sine-wave modulation. For this degree of modulation, the peak audio voltage must be
equal to the dc supply voltage to the final power amplifier. Under these conditions, the rf output voltage
will reach 0 on the negative peak of the modulating signal; on the positive peak, it will rise to twice the
amplitude of the unmodulated carrier.
