LINEAR MODULATION

Amplitude ModulationAmplitude Modulation = Change the

amplitude of carrier in proportion with the instantaneous value of the information signal

AM is inexpensive but low-qualityUsed for commercial broadcasting of audio &

videoTechniques:

Double Sideband Full Carrier AM (DSBFC AM)Double Sideband Suppressed Carrier AM (DSBSC

AM)Single Sideband Suppressed Carrier (SSBSC AM)AM Vestigial Sideband

Double Sideband Full Carrier AM (DSBFC AM)

Modulated wave envelope has same frequency as info signal

Amplitude of modulated wave varies according to amplitude of information signal

Time-domain representation

Output envelope contains dc voltage, carrier frequency and sum (fc + fm) and difference (fc – fm) frequencies

Bandwidth, B = difference between USB and LSB, i.e. B = 2fm(max)

fc = carrier frequencyfm(max) = highest info signal frequency

Frequency-domain representation

Example 1The carrier frequency of a DSBFC AM

modulator is 250 kHz. The maximum information signal frequency is at 15 kHz. FindFrequency limits of the upper and lower

sidebandsBandwidthUpper- and lower-side frequencies produced

when the information signal is of a single frequency of 7 kHz

Also, draw the output frequency spectrum

DSBFC AM has two major disadvantages:About 2/3 total power transmitted is taken by

carrier, which does not contain infoInfo in USB = info in LSB. Hence, wasteful

bandwidth consumptionOther improved AM techniques:

AM double-sideband suppressed carrier (DSBSC AM)

AM single-sideband suppressed carrier (SSBSC AM)

AM vestigial sideband (VSB)

AM Double-sideband Suppressed Carrier (DSBSC AM)

Generated by circuit called balanced modulator, where it produces the sum and difference frequencies but cancel the carrier

Transmitted power is reduced

Bandwidth is still same as DSBFC (because both sidebands are still transmitted)

AM Single-sideband Suppressed Carrier (SSBSC AM)The carrier and one sideband are

suppressedTransmitted power is reducedBandwidth is reduced by half

Advantages of SSBPower conservation

Suppressing carrier and one sideband gives more power available for the unsuppressed sideband

Thus, signal power for the transmitted sideband can be increased, and this improves the signal-to-noise ratio

Bandwidth conservation50% reduction in bandwidth usage

Advantages of SSBElimination of selective fading effect

Selective fading happens when carrier and the two sidebands take different paths through the channel, which may bring about signal attenuation or distortion

When only one sideband is transmitted, selective fading will not happen

Noise reductionSince the bandwidth is halved, thermal noise power is also halved (compared to DSB)

Disadvantages of SSBComplex receivers

Require more expensive receivers because envelope detection cannot be used

SSB requires carrier recovery circuit and synchronization circuit

Tuning difficultiesReceivers need more precise tuning, thus

more complex

AM Vestigial Sideband (VSB)Normally used for TV broadcasting where

picture/video and audio signal have different carrier frequencies

(in TV broadcasting)Audio carrier is frequency-modulated, but video info amplitude-modulates the picture carrier

AM TransmitterTransmitter = combination of electronic

devices and circuitry that accepts info signal then converts it into RF signal capable of being transmitted over long distances

Audio oscillator

Carrier generator

Amplitude modulator

Output amplifier

Antenna

Transmitter for double-sideband full carrier AM

Amplitude modulatorInfo signal & carrier are ‘multiplied’ resulting in modulation envelopeModulated signal amplitude varies according to info signal but frequency is same as carrier

frequency Output amplifier

Increases the strength of the signal before transmission Antenna

Electrical conductors that radiate radio waves (at transmitter) or collect radio waves (at receiver)

Audio oscillator

Carrier generator

Amplitude modulator

Output amplifier

Antenna

Transmitter for double-sideband full carrier AM

For generating DSBSC waves, amplitude modulator is replaced by balanced modulator

Audio oscillator

Carrier generator

Balancedmodulator

Output amplifier

Antenna

Transmitter for double-sideband suppressed carrier AM

For generating SSBSC waves, a bandpass filter is added

Audio oscillator

Carrier generator

Balancedmodulator

Output amplifier

Antenna

Transmitter for single-sideband suppressed carrier AM

Bandpass filter

AM ReceiverMost commonly used is the superheterodyne receiver

AM Demodulat

or

Local oscillator

RF Section

Speaker

Antenna

Audio-frequency amplifier

MixerIF Section

AntennaElectromagnetic waves strike antenna and generate

small voltages in the antennaRF Section

Amplifies the received modulated wavesIs tuned to the carrier frequency of the incoming wave

Mixer + Local oscillatorConverts incoming signal to a predetermined fixed

intermediate frequency (IF)IF section

Provides amplification and selectivityMay consist of a few stages

DemodulatorExtracts and recovers info signal after signal

amplificationAudio-frequency amplifier

Amplifies the power of the recovered info signal and filters the demodulator output

Low-pass filter is used to remove IF ripples and a capacitor blocks the dc voltage level

AM Demodulator: Diode detector

To extract audio signal from output of the IF section

Output of demodulator contains 3 components:The wanted info

signalSome ripple in IFA positive dc

voltage level

In many cases, total gain is far greater than that required for adequate reception

Excessive gain can distort signalThe solution is to place a gain control in the

receiver, called the Automatic Gain Control (AGC)

AGC adjusts gain based on amplitude of received signal

AGC takes received signal from output of the demodulator

AGC operationTake received signal from

output of demodulator

DC voltage level increase

Signal strength increase?

Voltage applied at RF and IF amplifier

to reduce gain

DC level > threshold?

RF and IF amplifier in normal operation

Y

Y

N

N