311 linear modulation

Amplitude Modulation
Amplitude Modulation = Change the amplitude of
carrier in proportion with the instantaneous value of the
information signal
AM is inexpensive but low-quality
Used for commercial broadcasting of audio & video
Techniques:
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 frequency
fm(max) = highest info signal
frequency
Frequency-domain
representation

Example 1
The carrier frequency of a DSBFC AM modulator is 250
kHz. The maximum information signal frequency is at 15
kHz. Find
Frequency limits of the upper and lower sidebands
Bandwidth
Upper- 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 info
Info in USB = info in LSB. Hence, wasteful bandwidth
consumption
Other 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 suppressed
Transmitted power is reduced
Bandwidth is reduced by half

Advantages of SSB
Power 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 conservation
50% reduction in bandwidth usage

Advantages of SSB
Elimination 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 reduction
Since the bandwidth is halved, thermal noise
power is also halved (compared to DSB)

Disadvantages of SSB
Complex receivers
Require more expensive receivers because envelope detection
cannot be used
SSB requires carrier recovery circuit and synchronization
circuit
Tuning difficulties
Receivers 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 Transmitter
Transmitter = 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

Audio oscillator
Carrier generator
Amplitude
modulator
 Amplitude modulator
Output
amplifier
Antenna
full carrier AM
Info signal & carrier are ‘multiplied’ resulting in modulation envelope
Modulated 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)

For generating DSBSC waves, amplitude modulator is
replaced by balanced modulator
Audio oscillator
Carrier generator
Balanced
modulator
Output
amplifier
Antenna
suppressed carrier AM

For generating SSBSC waves, a bandpass filter is added
Audio oscillator
Carrier generator
Balanced
modulator
Output
amplifier
Antenna
Bandpass
filter
Transmitter for single-sideband
suppressed carrier AM

AM Receiver
Most commonly used is the superheterodyne receiver
AM
Local oscillator
Demodulator
RF
Section
Speaker
Antenna
Audio-frequency
amplifier
Mixer
IF
Section

Antenna
Electromagnetic waves strike antenna and generate small voltages in the
antenna
RF Section
Amplifies the received modulated waves
Is tuned to the carrier frequency of the incoming wave
Mixer + Local oscillator
Converts incoming signal to a predetermined fixed intermediate
frequency (IF)
IF section
Provides amplification and selectivity
May consist of a few stages
Demodulator
Extracts and recovers info signal after signal amplification
Audio-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 signal
Some ripple in IF
A positive dc voltage level

In many cases, total gain is far greater than that required for
adequate reception
Excessive gain can distort signal
The 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 operation
Take received signal from output of
demodulator
Signal strength
increase?
DC voltage level
increase
DC level >
threshold?
Voltage applied at RF and
IF amplifier to reduce gain
RF and IF amplifier in
normal operation
Y
Y
N
N

311 linear modulation

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311 linear modulation

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