Modulation; analog communication system block diagram

Modulation

Modulation is the process by which some characteristic of a high-frequency carrier $c(t) = A_c\cos(2\pi f_c t + \phi)$, namely its amplitude, frequency or phase, is varied in accordance with the instantaneous amplitude of the message signal $m(t)$. The results are amplitude modulation (AM), frequency modulation (FM) and phase modulation (PM) respectively. For example, in AM, $s(t) = A_c[1 + k_a m(t)]\cos 2\pi f_c t$.

Modulation shifts the message spectrum from baseband to a band around $f_c$, which allows practical antenna sizes, multiplexing of many signals, and better noise performance.

Functional block diagram of an analog communication system

+--------+   +----------+   +-------------+
| Source |-->|  Input   |-->| Transmitter |
|        |   |transducer|   | mod + amp   |
+--------+   +----------+   +------+------+
                                   |
                     Noise   +-----v-----+
                     ------->|  Channel  |
                             +-----+-----+
                                   |
+--------+   +----------+   +------v------+
| Desti- |<--|  Output  |<--|  Receiver   |
| nation |   |transducer|   | amp + demod |
+--------+   +----------+   +-------------+

Blocks

1. Information source: generates the message, e.g. speech, music, picture. Its output is usually non-electrical.
2. Input transducer: converts the message into an electrical baseband signal $m(t)$, e.g. microphone, camera, temperature sensor.
3. Transmitter: processes $m(t)$ for efficient transmission. It band-limits and amplifies the message, modulates it on a carrier generated by an oscillator, then amplifies the modulated signal (power amplifier) and couples it to the antenna or cable.
4. Channel: the medium between transmitter and receiver: wire pair, coaxial cable, optical fibre or free space. It attenuates the signal and introduces distortion; noise and interference are added mainly here.
5. Receiver: selects the wanted signal from the antenna, amplifies it (RF amplifier), converts it to an intermediate frequency (mixer + local oscillator in a superheterodyne receiver), filters and amplifies it, and demodulates it to recover $m(t)$, followed by audio/video amplification.
6. Output transducer: converts the recovered electrical signal back into the original form, e.g. loudspeaker, display.
7. Destination: the listener, viewer or machine that uses the message.

Example: FM radio: microphone, FM transmitter at 88–108 MHz, free space, FM receiver with discriminator, loudspeaker.