f. The "Automatic Gain Control" or "AGC" will compensate for the different signal strength
levels sent to the radio receiver from the transmitter, and at the same time the AGC system monitors the
amplification of the detector's signal. At this point the AGC returns a portion of the detected signal to
the IF, and if RF amplifiers are either too high or too low, the AGC will act to either decrease or
increase the amplification at these stages.
Learning Event 3:
DESCRIBE AND IDENTIFY THE CHARACTERISTICS OF FM RADIO
1.
FM radio. The antenna serves the same purpose as it does in the AM radio; however, the "radio
frequency" amplifier performs a duel function in the FM radio.
a. The first is to choose the incoming signal and then amplify this signal to a height that will
allow the rest of the radio receiver to process it. This amplifier acts the same in an FM receiver as in an
AM receiver, although some of the basic circuitry is different because of the different frequencies being
received. In other words, the RF amplifier's function is the same in the AM radio and the FM radio,
with the only difference being the frequency to be processed.
b. The purpose of the mixer is the same as in the AM radio. It mixes, as its name states, the
signal received which contains the carrier signal with the signal from the local oscillator. These signals
mixed together produce a third signal called the IF or Intermediate Frequency. This signal contains the
IF carrier and operates on a frequency of 10.7 megahertz, and the intelligence found on the broadcast
stations carrier.
c. The local oscillator generates a tunable electronic signal. This signal and the incoming signal
in the RF amplifier are tuned at the same rate. The main purpose of the local oscillator is to generate
and inject a carrier signal into the mixer block to develop the two signals which are required to produce
the third signal known as the IF signal. The signal of 10.7 megahertz is produced in the mixer stage, and
contains in it the carrier and the intelligence received from the broadcast station. This amplifier
amplifies the signal from the very low level at the output of the mixer through several stages in order to
get a large signal for the detector.
d. The limiter's functions is to expose the detector to a signal of constant amplitude. This
process eliminates unwanted information in the signal, such as static, before it goes to the detector.
e. The purpose of the detector is to remove the broadcast information from the IF signal carrier.
The carrier is no longer needed since it has taken the broadcast information through the receiver, in an
amplified form and has presented this information to the detector. After the detector has extracted the
carrier from the information, the information is sent to the audio amplifier, automatic frequency control,
and the automatic gain control.
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