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ElectronicsElectronics

Principles & ApplicationsPrinciples & ApplicationsEighth EditionEighth Edition

Chapter 12Communications(student version)

Charles A. Schuler

©2013

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• Modulation and Demodulation• Simple Receivers• Superheterodyne Receivers• Frequency Modulation• Single Sideband• Wireless Data• Troubleshooting

INTRODUCTION

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Dear Student:

This presentation is arranged in segments. Each segmentis preceded by a Concept Preview slide and is followed by aConcept Review slide. When you reach a Concept Reviewslide, you can return to the beginning of that segment byclicking on the Repeat Segment button. This will allow youto view that segment again, if you want to.

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Concept Preview• Modulation is the process of adding information to

an RF signal.• The information signal controls the amplitude of

the RF signal when amplitude modulation is used.• The envelope of an AM signal has the same shape

as the information signal (oscilloscope display).• AM produces upper and lower sidebands.• A spectrum analyzer displays an AM signal’s

carrier and sidebands.

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Oscillator

A high-frequency oscillator can launch a radio wave.

The process of adding information to the radio signal is called modulation.

High frequencies are often called radio frequencies.

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Audio Frequency (AF)

Radio Frequency (RF)

AM = RF x AF + RF

Amplitude Modulation

Modulator

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Since the RF carrier frequency is muchhigher than the modulating frequency,

an actual oscilloscope displayof AM looks like this:

On a spectrum analyzer,AM looks like this:

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time

amp

litu

de

Oscilloscope

amp

litu

de

frequency

Spectrum Analyzer

fC = carrier frequency

LSB = fC - fAUDIO

USB = fC + fAUDIO

AM produces sum and difference frequencies called sidebands.

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+VCC

L C

2 LC

1fC =

AF

RF

(fC)

An amplitude modulator

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AM quiz

The process of placing information on acarrier wave is _________. modulation

With AM, the _________ of the carrier waveis controlled or varied. amplitude

The oscilloscope displays a graph of ________versus time. amplitude

The spectrum analyzer displays a graph of_________ versus time. frequency

A spectrum analyzer display of AM showsa carrier plus two ________. sidebands

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Concept Review• Modulation is the process of adding information to

an RF signal.• The information signal controls the amplitude of

the RF signal when amplitude modulation is used.• The envelope of an AM signal has the same shape

as the information signal (oscilloscope display).• AM produces upper and lower sidebands.• A spectrum analyzer displays an AM signal’s

carrier and sidebands.

Repeat Segment

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Concept Preview• Information signal recovery is called detection.

AM receivers often use a diode detector.• Tuned amplifiers provide selectivity so that only

the desired station will be received.• Superheterodyne receivers use an intermediate

frequency (IF) before detection.• A local oscillator is mixed with the desired station

to convert it to the intermediate frequency.• An image frequency will also mix with the

oscillator and produce the intermediate frequency.• Selectivity before the mixer eliminates the image.

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An AM detector

This capacitorapproaches a short

circuit at the carrier frequency.

AM in Audio outDiode

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Transmitter Diode

Antenna

Headphones

A very basic AM receiver

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A practical receiver needs tuned amplifiersto provide selectivity and sensitivity.

gain

frequency

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IF amplifier

Oscillator

Mixer

Antenna

Detector

It’s too difficult to simultaneously tuneseveral circuits. The IF amplifier is

permanently tuned to one frequency.

IF passbandCarrier andsidebands

The desired station frequency is mixed to the IF frequency.

Audio

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Frequency mixing is also called convertingor heterodyning. Receivers like this are

known as superheterodyne types.

IF amplifier

Oscillator

Mixer

Antenna

Detector

This is called the local oscillatorand it is tuned above the

station frequency by an amountequal to the IF frequency.

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IF amplifier

Oscillator

Mixer Detector

fSTATION = 1020 kHz

fLO = 1475 kHz

fIF = 455 kHz

Some typical frequencies:

Note: the two inputs to the mixer have a difference of 455 kHz.

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A tuned circuit before the mixer is required.

IF amplifier

Oscillator

Mixer Detector

fSTATION = 1020 kHz

fLO = 1475 kHz

fIF = 455 kHz

Superheterodyne receivers can also respond to the image frequency.

fIMAGE = 1930 kHz

(1930 - 1475 = 455)

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Receiver quiz

Recovering the information from a modulatedsignal is called __________. detection

AM detection is often accomplished with a_________ rectifier. diode

Radio receivers employ tuned amplifiers toprovide sensitivity and ______. selectivity

Superheterodyne receivers convert eachsignal to an _______ frequency. intermediate

A superhet can respond to one additionalfrequency called the _______. image

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Concept Review• Information signal recovery is called detection.

AM receivers often use a diode detector.• Tuned amplifiers provide selectivity so that only

the desired station will be received.• Superheterodyne receivers use an intermediate

frequency (IF) before detection.• A local oscillator is mixed with the desired station

to convert it to the intermediate frequency.• An image frequency will also mix with the

oscillator and produce the intermediate frequency.• Selectivity before the mixer eliminates the image.

Repeat Segment

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Concept Preview• With frequency modulation (FM), the information

signal controls the frequency of the carrier.• FM produces more sidebands than AM and thus

has greater bandwidth.• Noise and static can be removed from an FM

signal by clipping.• The carrier in an AM signal can be eliminated by

using a balanced modulator.• Single sideband AM also eliminates one of the

sidebands.

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Audio Frequency (AF)

Frequency Modulation

RFOscillator

One way to accomplish this is to use a varicap diode in the oscillator tank circuit.

The audio signal changes thevaricap bias and the resonantfrequency of the tank circuit.

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On a spectrum analyzer, FM shows more sidebands than AM.

fC

Uppersidebands

Lowersidebands

FM usually requires more bandwidth than AM.

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Noise is always a problem in any communication system. FM has an advantage over AM since it offers better noise rejection.

LIMITER

FM signalplus noise

Noiseremoved

An FM receiver can use an amplitude limiter to remove noise. An AM receiver cannot since the modulation would be defeated.

Modulationpreserved

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Audio Frequency (AF)

Radio Frequency (RF)

DSBSC = RF x AF

DSBSC Modulation

Balancedmodulator

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Audio Frequency (AF)

Radio Frequency (RF)

Spectrum analyzer

DSBSC Modulation

Balancedmodulator

LSB USBNo carrier

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frequency

Balancedmodulator

Bandpassfilter

The lowersideband

is not in thepassband.

Since the sidebands are redundant, one can be filtered out to decrease bandwidth.

SSBSC

Only theupper

sideband istransmitted.

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IF amplifierMixer

Oscillator

Detector

Oscillator

A superheterodyne SSB receiver requires a second oscillator to replace the missing carrier.

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FM and SSB quiz

With FM, amplitude noise can be removedwith a ___________. limiter

FM needs more bandwidth than AM sincethere are more _________. sidebands

A balanced modulator produces sidebandsbut no ___________. carrier

In SSB, one of the sidebands can be eliminatedby using a ____________. filter

SSB demodulation requires an oscillator toreplace the missing _________. carrier

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Concept Review• With frequency modulation (FM), the information

signal controls the frequency of the carrier.• FM produces more sidebands than AM and thus

has greater bandwidth.• Noise and static can be removed from an FM

signal by clipping.• The carrier in an AM signal can be eliminated by

using a balanced modulator.• Single sideband AM also eliminates one of the

sidebands.

Repeat Segment

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Wireless data devices are providing faster and more convenient communication and operation in several key areas. Here, the discussion is limited to WiFi, Bluetooth, and RFID technology.

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1 Multiple input and multiple output antennas at both transmitter and receiver (smart antenna technology)2 The original specification was released as IEEE 802.11 in 1997 followed by 802.11a and 802.11b in 1999.3 Direct-sequence spread spectrum4 Frequency-hopping spread spectrum5 Orthogonal frequency-division multiplexing6 IEEE 802.11y is licensed by the FCC in the United States.

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Bluetooth (IEEE 802.15)Range: 10 to 100 metersPower: 1 to 100 mWSensitivity: 0.1 nW (-70 dBm)Data rate: 1 MbpsBluetooth II: 3 MbpsFrequency: 2.4 GHz (same as 802.11b and 802.11g)

Bluetooth technology is designed for personal area networks (PANs) and for appliances that don't require large data flows (printers, keyboards, mice, personal computers, and mobile phones). Bluetooth enable automobiles allow hands-free phone operation.

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Radio Frequency Identification Devices (RFID) extend barcode technology into many new application areas.

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Inductive Reader and Tag

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Backscatter Reader and Tag

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Receiver troubleshooting

• Signal injection is standard practice.

• Both AF and RF signal generators may be required.

• Some receivers may require adjustments of their tuned circuits. This is called alignment.

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Wireless troubleshooting

Software problems include:

• Adapters are disabled

• Adapters are not authenticated

• Adapters are not configured properly

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Wireless network troubleshooting

Hardware problems include:

• Adapters are physically turned off or missing

• Interference

• Distance and obstructions to the signal

• Multipath signal distortion

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A field-strength meter can be used to determine if transmitters are working at all. However, this type of go/no-go test will only identify a limited range of possible problems.

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More advanced wireless network troubleshooting can be accomplished with dedicated test equipment or with special software running on general purpose equipment such as notebook computers and some portable devices.

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REVIEW

• Modulation and Demodulation• Simple Receivers• Superheterodyne Receivers• Frequency Modulation• Single Sideband• Wireless Data• Troubleshooting