Communication system and AM block diagram

A communication system transfers information from a source to a destination through a channel using electrical signals.

Amplitude modulation (AM)

In AM, the amplitude of a high-frequency carrier wave is varied in proportion to the instantaneous amplitude of the low-frequency message (modulating) signal. The carrier frequency and phase stay constant.

Carrier: vc = Vc sin ωc t. Message: vm = Vm sin ωm t.

AM wave: v = Vc (1 + m sin ωm t) sin ωc t, where the modulation index m = Vm/Vc (0 < m ≤ 1).

Bandwidth = 2 fm. The spectrum has the carrier fc and the sidebands fc − fm and fc + fm.

Block diagram of an AM transmitter

 +-----+   +---------+   +-----------+   +---------+   +---------+
 | Mic |-->|Audio amp|-->| Modulator |-->|Power amp|-->| Antenna |
 +-----+   +---------+   +-----------+   +---------+   +---------+
                               ^
                               |
           +-----------+   +--------+
           |Carrier osc|-->| RF amp |
           +-----------+   +--------+

1. The microphone converts sound to an electrical message signal, which is amplified by the audio amplifier.
2. The oscillator generates the high-frequency carrier, which is boosted by the RF amplifier.
3. The modulator combines the two to give the AM wave.
4. The power amplifier raises its power, and the antenna radiates it.

AM receiver (superheterodyne)

 Ant->[RF amp]->[Mixer]->[IF amp]->[Detector]->[Audio amp]->[Speaker]
                  ^
              [Local osc]

The RF stage selects the station, the mixer converts its frequency to a fixed IF (455 kHz), the IF amplifier gives gain and selectivity, the detector recovers the audio, and the audio amplifier drives the loudspeaker.