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Sec. 5–2 AM Broadcast Technical Standards and Digital AM Broadcasting 319
power level with efficient Class C or Class D amplifiers and then amplitude modulating the last
high-power stage. This is called high-level modulation. One example is shown in Fig. 5–2a, in
which a pulse width modulation (PWM) technique is used to achieve the AM with high conversion
efficiency [DeAngelo, 1982]. The audio input is converted to a PWM signal that is used to control
a high-power switch (tube or transistor) circuit. The output of this switch circuit consists of a highlevel
PWM signal that is filtered by a low-pass filter to produce the “DC” component that is used
as the power supply for the power amplifier (PA) stage. The PWM switching frequency is usually
chosen in the range of 70 to 80 kHz so that the fundamental and harmonic components of the
PWM signal can be easily suppressed by the low-pass filter, and yet the “DC” can vary at an audio
rate as high as 12 or 15 kHz for good AM audio frequency response. This technique provides
excellent frequency response and low distortion, since no high-power audio transformers are
needed, but vacuum tubes are often used in the PA and electronic switch circuits, as transistors do
not have sufficiently larger dissipation.
Another technique allows an all solid-state high-power transmitter to be built. It uses digital
processing to generate AM. A 50-kW AM transmitter can be built that uses 50 to 100 transistor PA
(power amplifier) modules, each of which produces either 100 W, 300 W, 500 W, or 1,000 W. See
http:www.broadcast.harris.com. Each module generates a constant-amplitude square wave at
the carrier frequency (which is filtered to produce the sine wave fundamental). To synthesize the
AM signal, the analog audio signal is sampled and digitized via an ADC. The samples are used
to determine (compute) the combination of modules that need to be turned on (from sample to
sample) in order to generate the required amplitude on the combined (summed) signal. If one
module fails, another module (or a combination of modules) is substituted for it, ensuring excellent
on-the-air transmitter reliability, since the transmitter will continue to function with failed
modules. Any failed modules can be replaced or repaired later at a convenient time. This AM
transmitter has an AC-power-to-RF conversion efficiency of 86% and excellent audio fidelity.
5–2 AM BROADCAST TECHNICAL STANDARDS AND DIGITAL
AM BROADCASTING
Some of the FCC technical standards for AM broadcast stations are shown in Table 5–1.
Since the channel bandwidth is 10 kHz, the highest audio frequency is limited to 5 kHz if the
resulting AM signal is not to interfere with the stations assigned to the adjacent channels. This
low fidelity is not an inherent property of AM, but occurs because the channel bandwidth was
limited by the 10 kHz standard the FCC chose instead of, say, 30 kHz, so that three times the
number of channels could be accommodated in the AM broadcast band. In practice, the FCC
allows stations to have an audio bandwidth of 10 kHz, which produces an AM signal bandwidth
of 20 kHz. This, of course, causes some interference to adjacent channel stations. There
are about 4,825 AM stations in the United States.
In the United States, the carrier frequencies are designated according to the intended
coverage area for that frequency: clear-channel, regional channel, or local channel frequencies.
Table 5–1 shows the clear-channel and local-channel frequencies. The others are
regional. Class A clear-channel stations operate full time (day and night), and most have
a power of 50 kW. These stations are intended to cover large areas. Moreover, to accommodate
as many stations as possible, nonclear-channel stations may be assigned to operate on