Figure 2-10.
Carrier modulated at an audio rate.
(6) The double-sideband system is used by most broadcast radio stations.
The carrier is transmitted together with the upper sideband (USB) and
lover sideband (LSB) frequencies. All three are received in the radio
receiver at the same time.
A demodulator extracts the audio signal.
The Signal is amplified by an amplifier and reproduced by the speaker.
(7) If the receiver can locally produce the same frequency as the carrier
to beat with the sidebands, there is no need to transmit the carrier.
Also, since both the USB and LSB carry the same information, it is not
necessary to transmit both.
A single-sideband (SSB) transmitter
filters one sideband (either upper or lower), suppresses the carrier,
and makes available the power that normally is used to transmit the
carrier and both sidebands, to transmit only one sideband frequency.
This is illustrated in figure 2-11. To produce at the SSB receiver a
signal whose power is equivalent to the power of the signal received by
the DSB receiver, the SSB transmitter requires only one-sixth of the
output power required by the DSB transmitter.
Since the SSB
transmitter does not have to produce as much power as the DSB
transmitter, the SSB transmitter can be lighter and more compact.
In
addition, SSB transmission also saves space in the transmitting
frequency spectrum, and is less subject to interference from adjacent
frequency channels.
d. Frequency Modulation.
Frequency modulation (FM) is a second method of
adding information to an RF carrier. This is accomplished by varying the frequency
of the carrier using the audio signal developed by the microphone.
309 L2
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