ElectronicsElectronics

Principles & ApplicationsPrinciples & ApplicationsFifth EditionFifth Edition

Chapter 12Radio Receivers

©1999 Glencoe/McGraw-Hill

Charles A. Schuler

• Modulation and Demodulation• Simple Receivers• Superheterodyne Receivers• Frequency Modulation• Single Sideband• Receiver Troubleshooting

INTRODUCTION

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 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

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 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

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.

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 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

REVIEW

• Modulation and Demodulation• Simple Receivers• Superheterodyne Receivers• Frequency Modulation• Single Sideband• Receiver Troubleshooting