AM radio waves

Electrical Network IS AN INTERCONNECTION OF ELECTRICAL COMPONENTS

+-

L

C

1R

2R

Sv −

+

Ov

TYPICAL LINEAR CIRCUIT

• To analyze, design and measure a number of quantities (e.g. current, voltage) of linear analog electrical network systems, across engineering disciplines and within sub-disciplines of Electrical Engineering.

OBJECTIVES

EE SubdisciplinesEE Subdisciplines

• Power• Electromagnetics• Communication/

Signal Processing• Digital • Controls• Solid State

The AM RadioThe AM Radio&&

The Telephone SystemThe Telephone System

The AM RadioThe 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)

The AM Radio “System”The AM Radio “System”

Transmitter Receiver

SignalSignal

• 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)

FrequencyFrequency

• 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).

FrequencyFrequency

Frequency is the rate at which a signal oscillates.

High Frequency Low Frequency

Sound WavesSound 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.

Fourier AnalysisFourier 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.

Example-Fourier SeriesExample-Fourier Series

Square wave

Fourier Series representation of the square wave

∞

=−∑ − tk

kkππ ]24[cos

)12(4

1

Fourier Series Example (Cont.)Fourier Series Example (Cont.)

One term

Five terms

Frequency-SummaryFrequency-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.

AM TransmitterAM 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

AM TransmitterAM 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

Transmitter Block DiagramTransmitter Block Diagram

Signal

SourceModulator

Power

Amplifier

Antenna

ModulatorModulator

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

Modulator-Time DomainModulator-Time Domain

Input Signal

Output Signal

AntennaAntenna

The antenna converts a current or a voltage signal to an electromagnetic signal which is radiated throughout space.

AM ReceiverAM Receiver

• The AM receiver receives the signal from the desired AM station as well as 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.

AM ReceiverAM 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.

Receiver Block DiagramReceiver Block Diagram

RF

Amplifier

IF

Mixer

IF

Amplifier

Envelope

Detector

Audio

Amplifier

Antenna

Speaker

AntennaAntenna

• 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

RF AmplifierRF 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

RF Amplifier-Frequency DomainRF 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

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

IF MixerIF Mixer

0 frequency

Undesired Signals

Desired Signal

455 kHz

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

IF AmplifierIF Amplifier

0 frequency

Desired Signal

455 kHz

Envelope DetectorEnvelope Detector• Computes the envelope of its input signal

Input Signal

Output Signal

Audio AmplifierAudio Amplifier

• Amplifies signal from envelope detector

• Provides power to drive the speaker

Hierarchical System ModelsHierarchical 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

Systems in EESystems 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

Top Level ModelTop Level Model

AM ReceiverInput Signal Sound

Second Level ModelSecond Level Model

RF

Amplifier

IF

Mixer

IF

Amplifier

Envelope

Detector

Audio

Amplifier

Antenna

Speaker

Power Supply

Circuit Level ModelCircuit Level ModelEnvelope DetectorEnvelope Detector

+

-R C

+

-VoutVin



The modern telephone system draws from these Electrical Engineering subdisciplines:

• Signal processing: Speech compression, noise reduction, A/D and D/A conversion..

• Communications and networking: transmission technologies, network architectures and protocols.

• Digital and computer: configurable switching hardware.

• Electromagnetics: microwave transmission hardware.• Solid state: miniaturization, integration of complex

systems onto a single chip.• Power Electronics: extremely reliable power supplies.

Old Versus NewOld Versus New

• The early telephone system provided (what today is know as) POTS-”plain old telephone service”.

• The only service provided by the early telephone system was voice transmission.

• The modern telephone system provides voice transmission as well as a host of other services:– data transmission and video transmission– sophisticated billing and feature capabilities

such as call waiting and call forwarding.

An Early Phone SystemAn Early Phone System

Telephone

Telephone

Speaker

Mic.

Telephone

Speaker

Mic.

Central Office

Switchboard

Speaker

Mic.

Power Supply

The Early Phone SystemThe Early Phone System

• The major components of a telephone were a carbon microphone and a speaker made from an electromagnet and a paramagnetic diaphragm.

• Telephones were connected to the central office by twisted-pair wires.

• At the central office, calls were completed by a human operator at a switchboard-a physical connection between two telephones was made.

An Early Phone CircuitAn Early Phone Circuit

Telephone Handset

Carbon Microphone

Earphone

Central Office

Battery

Telephone Handset

Carbon Microphone

Earphone

The Phone CircuitThe Phone Circuit

• Electrical current flows in this circuit in a loop from the battery at the central office, through the components of the two telephones (the speaker and the microphone), and back into the battery.

• This circuit is a series connection of the components in the two telephones and the battery.

• All of the current that flows through the battery also flows through the components in the two telephones.

MicrophoneMicrophone

• The microphone consists of loosely packed carbon granules in a box with a diaphragm on one side

• The electrical resistance of the carbon in the box is related to the displacement of the diaphragm-when the carbon granules are compressed, the resistance is reduced.

• Thus, the microphone converts changes in pressure to changes in resistance.

• The microphone is modeled electrically as a variable resistor.

SpeakerSpeaker

• The speaker was made from an electromagnet and a paramagnetic diaphragm.

• Changes in the current flowing through the electromagnet result in changes of the magnetic field strength, which in turn results in a change of the position of the diaphragm.

• Thus, the speaker converts changes in current to movement of a diaphragm which produces sound energy.

• The speaker is modeled electrically as an inductor.

Central OfficeCentral Office

• Switchboard: the switchboard connects two telephones electrically.

• Battery: the battery provides the power necessary to create an electrical current flowing in the loop.

The Modern Telephone SystemThe Modern Telephone System

• Fundamentally, the modern telephone systems appears much the same as the early system to handset users.

• There are very significant differences:– Digital data, video, and other signals are

transmitted along with speech.– Calls are routed automatically under software

control.– Most transmission is digital.

A Modern Telephone A Modern Telephone ConnectionConnection

PCM Encoder

PCM Decoder

Switching Network

PCM Decoder

PCM Encoder

Analog AnalogDigital

Analog Vs. DigitalAnalog Vs. Digital

• An analog signal is a continuous-time signal:

• A digital signal is a sequence of 1’s and 0’s:

1101001010011100100110001001110

time

Why Digital?Why Digital?

• Transmission over long distances degrades both analog and digital signals-digital signals can be “cleaned up”, allowing repeaters to be used without any signal distortion.

• Can mix many types of information (phone, video, data, etc.)

• Digital hardware is less expensive.• Digital data can be encrypted.

PCM-Pulse Code ModulationPCM-Pulse Code Modulation

• A PCM encoder converts an analog signal into a digital signal with a particular format.

• A PCM decoder converts a digital signal into an analog signal.

• PCM is one form of quantization.

• PCM is one form of analog-to-digital (A/D) conversion.

PCM EncoderPCM Encoder

A continuous signal is converted into a bit stream:

0000010100000000111111

Involves three operations:

Sampling, Quantization, and Encoding

SamplingSampling

Value of the signal is obtained at equally spaced points in time:

time

QuantizerQuantizer

• Each sample is quantized to one of a finite number of values.

Quantizer input/output relationship:

input voltage

output voltage

EncodingEncoding

• A pattern of bits is assigned to each possible output level of the quantizer.

• n bits can represent 2n quantizer output levels.

PCM DecoderPCM Decoder

PCM decoder is one type of digital-to-analog (D/A) converter.

0000010100000000111111

Telephone NetworkTelephone Network

• A house or business is called a subscriber.• Typically, phone lines to houses or small

businesses are analog twisted-pair wire connections.

• Subscribers’ analog lines are connected to a Regional Terminal (RT) or to a Central Office (CO).

• At the RT or CO, the analog signal is converted to a digital signal.

Network ArchitectureNetwork Architecture

Subscriber

Subscriber

RT

Subscriber

Subscriber

RT

COLong-distance

Network