ElectronicsElectronics

Principles & ApplicationsPrinciples & ApplicationsSixth EditionSixth Edition

Chapter 12Communications(student version)

©2003 Glencoe/McGraw-Hill

Charles A. Schuler

• Modulation and Demodulation

• Simple Receivers

• Superheterodyne Receivers

• Frequency Modulation

• Single Sideband

• Receiver Troubleshooting

INTRODUCTION

Dear Student:

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

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.

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.

Audio Frequency (AF)

Radio Frequency (RF)

AM = RF x AF + RF

Amplitude Modulation

Modulator

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:

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.

+VCC

L C

2 LC

1fC =

AF

RF

(fC)

An amplitude modulator

AM Quiz

The process of placing information on a carrier wave is __________. modulation

With AM, the __________ of the carrier wave is 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 shows a carrier plus two __________. sidebands

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

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.

An AM Detector

This capacitorapproaches a short

circuit at the carrier frequency.

AM in Audio outDiode

Transmitter Diode

Antenna

Headphones

A very basic AM receiver

A practical receiver needs tuned amplifiersto provide selectivity and sensitivity.

gain

frequency

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

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.

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.

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)

Receiver QuizRecovering the information from a modulated signal is called __________. detection

AM detection is often accomplished with a __________ rectifier. diode

Radio receivers employ tuned amplifiers to provide sensitivity and __________. selectivity

Superheterodyne receivers convert eachsignal to an __________ frequency. intermediate

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

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

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.

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.

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

fC

Uppersidebands

Lowersidebands

FM usually requires more bandwidth than AM.

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

Audio Frequency (AF)

Radio Frequency (RF)

DSBSC = RF x AF

DSBSC Modulation

Balancedmodulator

Audio Frequency (AF)

Radio Frequency (RF)

Spectrum analyzer

DSBSC Modulation

Balancedmodulator

LSB USBNo carrier

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.

IF amplifierMixer

Oscillator

Detector

Oscillator

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

Bluetooth

Range: 10 to 100 meters

Power: 1 to 100 mW

Sensitivity: 0.1 nW (-70 dBm)

Frequency: 2.4 GHz

Data rate: 1 Mbit/s

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.

FM and SSB Quiz

With FM, amplitude noise can be removed with a __________. limiter

FM needs more bandwidth than AM since there are more __________. sidebands

A balanced modulator produces sidebands but no __________. carrier

In SSB, one of the sidebands can be eliminated by using a __________. filter

SSB demodulation requires an oscillator to replace the missing __________. carrier

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

• Modulation and Demodulation

• Simple Receivers

• Superheterodyne Receivers

• Frequency Modulation

• Single Sideband

• Receiver Troubleshooting

INTRODUCTION