0.16 Lecture 17:modulation, broadcast radio

Lecture #17:

MODULATION, BROADCAST RADIO

Motivation:

• Modulation is widely used to encode a signal so as to more effectively utilize it.

• Modulation is fundamental to electronic communication systems—radio, TV, satellite communications, cell phones, etc.

Outline:

• General description of modulation

• Amplitude modulation

• Broadcast AM radio

• Conclusions

I. GENERAL DESCRIPTION OF MODULATION

1/ Overview

The word modulation in an electronic context means to recode a signal for the purpose of more effectively manipulating that signal. For example, suppose we have some signal xm(t) that we wish to process in some way.

For example, we wish to

• transmit it through a channel,

• filter it,

• amplify it,

• display it,

• record it.

However, it is not efficient, convenient, economical, or possible to do so directly. Then we encode the signal and process the encoded signal to improve some aspect of the processing.

2/ Wave-parameter modulation

Modulation can involve varying some feature of a CT signal to encode the signal. Varying the amplitude of a sinusoid (amplitude modulation or AM) or its frequency (frequency modulation or FM) in proportion to a signal is called wave-parameter modulation.

3/ Pulse-parameter modulation

Modulation can also encode the CT signal with the parameters of pulses called pulse-parameter modulation. A number of different pulse-parameter modulation schemes are shown below.

In PWM, the width of pulses encodes the amplitude of the CT signal. In PAM the amplitude of pulses encodes the CT signal. In PCM the amplitude of the quantized CT signal is encoded as a binary number that is represented by a pulse code.

4/ Example of the use of modulation — pigeon telemetry

An ornithologist wishes to record the sounds made by a Lahore pigeon (shown below) while in flight.

Typical pigeon sounds have a spectrum in the frequency range 0.1-3 kHz. Since the pigeon is in flight, we need to make a small (light weight) system consisting of a microphone and a telemetering system that will transmit the sound information.

One might simply transduce the audio signal from the microphone and transmit the electrical signal to the ground. A question arises — what size antenna is needed to transmit the signal in an energetically efficiently manner?

For energetic efficiency, the dimensions of the antenna cannot be orders of magnitude smaller than the wavelength of the transmitted signal. The wavelength λ of the transmitted signal is

$\lambda =\frac{c}{f}\approx \frac{3\times {\text{10}}^{8}m/s}{3\times {\text{10}}^3\text{Hz}}\approx \text{100}\text{km}$

If we make the antenna λ/10, then the antenna dimensions are at least 10 km. Thus, the antenna dimensions exceed that of the pigeon by a factor of more than 104!

On the left is a scale drawing of the pigeon (in red) and the antenna (in dark blue).

The pigeon will not get off the ground!

One solution is to move the spectrum of the transduced pigeon sounds to a high frequency, to transmit this modulated signal to the ground, and then to demodulate to audio frequencies.