This document discusses linear modulation techniques, specifically amplitude modulation (AM). It describes the main types of AM including double sideband full carrier (DSBFC), double sideband suppressed carrier (DSBSC), and single sideband suppressed carrier (SSBSC). It provides details on how each is generated and transmitted, their advantages and disadvantages, and the basic components and functioning of AM transmitters and receivers.
2. 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
3. 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
4. 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
5. 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
6. 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)
7. 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)
8. AM Single-sideband Suppressed
Carrier (SSBSC AM)
The carrier and one sideband are suppressed
Transmitted power is reduced
Bandwidth is reduced by half
9. 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
10. 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)
11. 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
12. 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
13. 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
14. Audio oscillator
Carrier generator
Amplitude
modulator
Amplitude modulator
Output
amplifier
Antenna
Transmitter for double-sideband
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)
15. For generating DSBSC waves, amplitude modulator is
replaced by balanced modulator
Audio oscillator
Carrier generator
Balanced
modulator
Output
amplifier
Antenna
Transmitter for double-sideband
suppressed carrier AM
16. 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
17. AM Receiver
Most commonly used is the superheterodyne receiver
AM
Local oscillator
Demodulator
RF
Section
Speaker
Antenna
Audio-frequency
amplifier
Mixer
IF
Section
18. 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
19. 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
20. 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
21. 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