311 linear modulation
DESCRIPTION
Basic Communication theoryTRANSCRIPT
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LINEAR MODULATION
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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
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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
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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
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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
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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)
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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)
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AM Single-sideband Suppressed Carrier (SSBSC AM)The carrier and one sideband are
suppressedTransmitted power is reducedBandwidth is reduced by half
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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
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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)
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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
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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
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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
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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
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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
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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
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AM ReceiverMost commonly used is the superheterodyne receiver
AM Demodulat
or
Local oscillator
RF Section
Speaker
Antenna
Audio-frequency amplifier
MixerIF Section
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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
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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
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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
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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