Lecture 4 1

The AM Radio

Lecture 4 2

The AM Radio

• Understanding the AM radio requires knowledge of several EE subdisciplines:– Communications/signal processing (frequency

domain analysis)– Electromagnetics (antennas, high-frequency

circuits)– Power (batteries, power supplies)– Solid state (miniaturization, low-power

electronics)

Lecture 4 3

The AM Radio “System”

Transmitter Receiver

Lecture 4 4

Signal

• The radio system can be understood in terms of its effect on signals.

• A signal is a quantity that may vary with time.– Voltage or current in a circuit– Sound (pressure wave traveling through air)– Light or radio waves (electromagnetic energy

traveling through free space)

Lecture 4 5

Frequency

• The analysis and design of AM radios (and communication systems in general) is usually conducted in the frequency domain using Fourier analysis.

• Fourier analysis allows us to represent signals as combinations of sinusoids (sines and cosines).

Lecture 4 6

Frequency

Frequency is the rate at which a signal oscillates.

High Frequency Low Frequency

Lecture 4 7

Electromagnetic Waves

• Visible light is electromagnetic energy with frequency between 380THz (Terahertz) and 860THz.– Our visual system perceives the frequency of the

electromagnetic energy as color.– Red is 460THz, green is 570THz, and blue is

630THz.• An AM radio signal has a frequency of between

500kHz and 1.8MHz.• FM radio and TV uses different frequencies.

Lecture 4 8

Sound Waves

• Sound is a pressure wave in a transmission medium such as air or water.

• We perceive the frequency of the wave as the “pitch” of the sound.

• A single frequency sound sounds like a clear whistle.

• Noise (static) is sound with many frequencies.

Lecture 4 9

Fourier Analysis

• Mathematical analysis of signals in terms of frequency

• Most commonly encountered signals can be represented as a Fourier series or a Fourier transform.

• A Fourier series is a weighted sum of cosines and sines.

Lecture 4 10

Example-Fourier Series

Square wave

Fourier Series representation of the square wave

tkkk

]24[cos)12(

41

Lecture 4 11

Fourier Series Example (Cont.)One term

Five terms

Lecture 4 12

Frequency-Summary

• Signals can be represented in terms of their frequency components.

• The AM transmitter and receiver are analyzed in terms of their effects on the frequency components signals.

Lecture 4 13

AM Transmitter

• Each AM station is allocated a frequency band of 10kHz in which to transmit its signal.

• This frequency band is centered around the carrier frequency of the station– A station at 610 on your dial transmits at a

carrier frequency of 610kHz– The signal that is broadcast occupies the

frequency range from 605kHz to 615kHz

Lecture 4 14

AM Transmitter

• Transmitter input (signal source) is an audio signal.– Speech, music, advertisements

• The input is modulated to the proper carrier frequency.

• Modulated signal is amplified and broadcast

Lecture 4 15

Transmitter Block Diagram

Signal

SourceModulator

Power

Amplifier

Antenna

Lecture 4 16

Modulator

The modulator converts the frequency of the input signal from the audio range (0-5kHz) to the carrier frequency of the station (i.e.. 605kHz-615kHz)

frequency5kHz

Frequency domain representation of input

Frequency domain representation of output

frequency610kHz

Lecture 4 17

Modulator-Time DomainInput Signal

Output Signal

Lecture 4 18

Power Amplifier

• A typical AM station broadcasts several kW– Up to 50kW-Class I or class II stations– Up to 5kW-Class III station– Up to 1kW-Class IV station

• Typical modulator circuit can provide at most a few mW

• Power amplifier takes modulator output and increases its magnitude

Lecture 4 19

Antenna

The antenna converts a current or a voltage signal to an electromagnetic signal which is

radiated throughout space.

Lecture 4 20

AM Receiver

• The AM receiver receives the signal from the desired AM station as well a signals from other AM stations, FM and TV stations, cellular phones, and any other source of electromagnetic radiation.

• The signal at the receiver antenna is the sum of all of these signals (superposition).

• The AM receiver separates the desired signal from all other received signals using its frequency characteristics.

Lecture 4 21

AM Receiver

• We present a superhetrodyne receiver-this is the type used in most modern radio and TV receivers.

• The desired signal is first translated to an Intermediate Frequency (IF).

• The desired signal is then recovered by a demodulator.

Lecture 4 22

Receiver Block Diagram

RF

Amplifier

IF

Mixer

IF

Amplifier

Envelope

Detector

Audio

Amplifier

Antenna

Speaker

Lecture 4 23

Antenna

• The antenna captures electromagnetic energy-its output is a small voltage or current.

• In the frequency domain, the antenna output is

0 frequency

Undesired SignalsDesired Signal

Carrier Frequencyof desired station

Lecture 4 24

RF Amplifier

• RF stands for radio frequency.• RF Amplifier amplifies small signals from the

antenna to voltage levels appropriate for transistor circuits.

• RF Amplifier also performs a bandpass filter operation on the signal– Bandpass filter attenuates the frequency

components outside the frequency band containing the desired station

Lecture 4 25

RF Amplifier-Frequency Domain

• Frequencies outside the desired frequency band are attenuated

• Frequency domain representation of the output:

0 frequency

Undesired SignalsDesired Signal

Carrier Frequencyof desired station

Lecture 4 26

IF Mixer

• The IF Mixer shifts its input in the frequency domain from the carrier frequency to an intermediate frequency of 455kHz:

IF Mixer

0 frequency

Undesired Signals

Desired Signal

455 kHz

Lecture 4 27

• The IF amplifier bandpass filters the output of the IF Mixer, eliminating essentially all of the undesired signals.

IF Amplifier

0 frequency

Desired Signal

455 kHz

Lecture 4 28

Envelope Detector

• Computes the envelope of its input signal

Lecture 4 29

Audio Amplifier

• Amplifies signal from envelope detector

• Provides power to drive the speaker

Lecture 4 30

Hierarchical System Models

• Hierarchical modeling is modeling at different levels of abstraction

• We can “divide and conquer”• Higher levels of the model describe overall

function of the system• Lower levels of the model describe detail

necessary to implement the system

Lecture 4 31

Systems in EE

• In EE, a system is an electrical and/or mechanical device, a process, or a mathematical model that relates one or more inputs to one or more outputs.

• In the AM receiver, the input is the antenna voltage and the output is the sound energy produced by the speaker.

SystemInputs Outputs

Lecture 4 32

Top Level Model

AM ReceiverInput Signal Sound

Lecture 4 33

Second Level Model

RF

Amplifier

IF

Mixer

IF

Amplifier

Envelope

Detector

Audio

Amplifier

Antenna

Speaker

Power Supply

Lecture 4 34

Low Level ModelEnvelope Detector.

Half-wave

Rectifier

Low-pass

Filter

Lecture 4 35

Circuit Level ModelEnvelope Detector

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