Figure 2-16. Block diagram of an AM receiver

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:

Antenna

- 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