bandwidth, which limits noise interference. This is the primary reason that CW signals can be
received at greater distances than are possible with the same power by other means of radio
transmission.
(2) The manual CW system is normally a slow method of transmission and reception. Its speed is limited
primarily by the skill and experience of the operator. This transmission rate may be extended by
adapting automatic transmitting and receiving devices to the CW radio system.
(3) If a keyed tone signal is impressed on the carrier at the transmitter, the process is called tone
modulation. A BFO is not needed to receive tone-modulated radiotelegraph signal pulses; a BFO is
needed to receive CW signals.
c. Radiotelephone. Two commonly used methods of modulation in radiotelephone transmission are
amplitude modulation (AM) and frequency modulation (FM). In either system, a microphone converts voice
waves or other sound waves to weak electrical impulses. These impulses are strengthened through a series of
amplifiers. A modulator serves to combine the AF with the RF carrier. The form of modulation (AM or FM)
determines to a large extent the signal characteristics.
(1) Amplitude modulation. In AM, the modulator heterodynes the AF signal with the carrier signal of the
radio transmitter. This process results in development of a radio wave that maintains a fixed carrier
frequency but varies in amplitude according to the AF signal amplitude and frequency. The
amplitude variations are caused by the addition of intelligence power in the form of upper and lower
sidebands. The carrier itself contains no intelligence. This is a double-sideband (DSB) composite
radio signal.
(2) Single sideband. Single-sideband (SSB) transmission is a special form of AM wherein only one
sideband is transmitted. The carrier and other sideband are suppressed. This form of transmission is
possible because identical intelligence appears in both upper and lower sidebands. By eliminating the
carrier and one sideband, SSB transmitters can produce the same intelligence as AM with a much
smaller output power. A further advantage is gained by the reduction of the composite signal
bandwidth to one-half its former value for DSB transmission. Since the carrier has been suppressed,
it is necessary to furnish a locally generated carrier in the receiver to achieve demodulation.
(3) Frequency modulation. In FM, the modulator causes the signal frequency to shift to either side of the
resting frequency (frequency of the carrier without modulation). The deviation from resting
frequency follows the amplitude and frequency patterns in the AF signal. The overall RF signal has a
relatively constant amplitude value, but varies in frequency in accordance with the signal intelligence
placed on the carrier during the modulation process.
d. Radio Teletypewriter. Radio teletypewriter (radiotelegraph) signals are produced by a complex system of
mechanical and electronic equipment.
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