c. AM Receiving System.
In figure 2-16, the block diagram of a typical AM
receiver is illustrated. The RF amplifier selects one of the many signals on the
antenna, amplifies it, and sends the amplified signal to the mixer. Here, a second
signal is applied from the local oscillator.
The two signals are mixed in this
stage, producing four predominant frequencies: the two inputs, the sum of the two,
and the difference between the two. For example, if the frequency at the antenna
is 1,500 kHz and the local oscillator frequency is 1,045 kHz, the frequencies at
the output of the mixer are:
- 1,500 kHz
Local oscillator
- 1,045 kHz
Sum
- 2,545 kHz
Difference
- 455 kHz
The difference frequency is usually selected as the output and is known as the IF
signal.
It carries the information of the received RF signal.
The IF amplifier
stage amplifies this frequency and sends it to the detector. Here, the information
(AF voltage) and the carrier are separated. The carrier is filtered out, leaving
only the audio.
The audio is amplified by the audio amplifier stages and
reproduced by the speaker.
Figure 2-16.
Block diagram of an AM receiver.
d. CW Reception.
The receiver in figure 2-16 can be used to receive CW
signals. All stages of the receiver operate the same except the detector. In this
stage the IF is mixed with a signal from the beat-frequency oscillator
309 L2
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