AMPLITUDE MODULATION SYSTEM
Review of Spectral Characteristics of Periodic and aperiodic
Why use modulation?
• “Carrying one signal on another” - uses carrier
• Modulated carrier transmitted
• Problems with transmitting baseband signals
– Antennas difficult at low frequencies
– Noise and interference at low frequencies
– Can’t share with others
• Easier to transmit carrier at higher frequency
– Can choose convenient frequency
• Antennas can be smaller
• May be useful propagation effects
– Fractional bandwidth much smaller
• Antennas and other components easier to design
• Can have many frequency channels
GENERATION OF AMPLITUDE MODULATION AND
• Amplitude Modulation is the simplest and earliest form of
• AM applications include broadcasting in medium- and high-
frequency applications, CB radio, and aircraft communications
Basic Amplitude Modulation
The information signal varies the instantaneous amplitude of the
carrier,it involves translating a base band signal to a band pass
signal at frequencies that are very high when compared to the base
AMPLITUDE MODULATED WAVE
Mathematical Representation of an AM Wave.
• In amplitude modulation, the amplitude of a high frequency
carrier signal is varied in accordance to the instantaneous
amplitude of the modulating message signal.
• Let Accos(2пfct) be the carrier signal
• Modulating signal = Am cos(2П fmt)
• AM signal can be represented as
SAM(t)=Ac[1+ Am/ Ac cos(2П fmt) ] cos(2П fct)
SAM (t)=Ac[1+m(t)]cos(2П fct)
• The modulation index k is given as the ratio of max
amplitude of the modulating signal and the max amplitude of
the carrier signal. It is also sometime expressed as %
• K = Am/Ac
• Ac[1+ Am/ Ac cos(2П fmt) ] cos(2П fct)
= Ac cos(2П fct) +Am/2 cos(2П (Fc+ Fm)t)+ Am/2
cos(2П (Fc- Fm)t)
• The modulated signal has a carrier freq, and upper and lower
• SAM(t)=Ac[1+m(t)]cos(2П fct)
• The power = Ac2/2+Am2/4+Am2/4
• AM is a nonlinear process
• Sum and difference frequencies are created that carry the
Full-Carrier AM: Time Domain
•Modulation Index - The ratio between the amplitudes between the
amplitudes of the modulating signal and carrier, expressed by the
m = Em/Ec
• When the modulation index is greater than 1, over modulation is
Modulation Index for Multiple Modulating Frequencies
• Two or more sine waves of different, uncorrelated frequencies
modulating a single carrier is calculated by the equation
Measurement of Modulation Index
Under modulated (<100%) 100% modulated
Can be used
Gives Distorted signal
Over Modulated (>100%)
Full-Carrier AM: Frequency Domain
fusb fc fm
flsb fc fm
Time Domain Frequency Domain Modulation Index
m = 0.5
m = 1.0
m = 1.25
•Signal bandwidth is an important characteristic of any modulation
•In general, a narrow bandwidth is desirable
•Bandwidth is calculated by:
•Power in a transmitter is important, but the most important power
measurement is that of the portion that transmits the information
•AM carriers remain unchanged with modulation and therefore are
•Power in an AM transmitter is calculated according to the formula
at the right.
Pt Pc 1
• The total power in an AM signal is
PAM= (½)Ac2 [1+2 <m(t)> + <m2(t)>]
Where <> represents the average value.
• If the modulating signal is m(t)=kcos(2Πfmt),then total
Where Pc=Ac2/2 is the power in the carrier signal.
Pm=<m2(t)> is the power in the modulating signal.
and k is the modulation index.
Quadrature AM and AM Stereo
•Two carriers generated at the same frequency but 90º out of phase
with each other allow transmission of two separate signals.
•This approach is known as Quadrature AM (QUAM or QAM)
•Recovery of the two signals is accomplished by synchronous
detection by two balanced modulator
Generation of Suppressed-Carrier AM
•Full-carrier AM is simple but not efficient
•Removing the carrier before power amplification allows full
transmitter power to be applied to the sidebands
•Removing the carrier from a fully modulated AM systems results
in a double-sideband suppressed-carrier transmission.
SSB Can be generated using two techniques
1. Phasing method
2. Filter Method
This method is a special modulation type of IQ canonical form
of Generalized transmitters
The filtering method is a special case in which RF processing (with
a sideband filter) is used to form the equivalent g(t), instead of
using base band processing to generate g(m) directly.
The filter method is the most popular method because excellent
sideband suppression can be obtained when a crystal oscillator is
used for the sideband filter.
Crystal filters are relatively inexpensive when produced in quantity
at standard IF frequencies.
Weaver’s Method for Generating SSB.
•The two sidebands of an AM signal are mirror images of one
•As a result, one of the sidebands is redundant
•Using single-sideband suppressed-carrier transmission results in
reduced bandwidth and therefore twice as many signals may be
transmitted in the same spectrum allotment
•Typically, a 3dB improvement in signal-to-noise ratio is achieved
as a result of SSBSC
DSBSC and SSB Transmission
The DSB-SC can be generated using either the balanced
modulator or the ‗ring-modulator‘.
The balanced modulator uses two identical AM generators
along with an adder.
The two amplitude modulators have a common carrier
with one of them modulating the input message , and the
other modulating the inverted message .
Generation of AM is not simple, and to have two AM
generators with identical operating conditions is extremely
Properties of DSB-SC Modulation:
(a) There is a 180 phase reversal at the point where +A(t)=+m(t)
goes negative. This is typical of DSB-SC modulation.
(b) The bandwidth of the DSB-SC signal is double that of the
message signal, that is,
BWDSB-SC =2B (Hz).
(c) The modulated signal is centered at the carrier frequency ωc
with two identical sidebands (double-sideband) – the lower
sideband (LSB) and the upper sideband (USB). Being identical,
they both convey the same message component.
(d) The spectrum contains no isolated carrier. Thus the name
(e)The 180 phase reversal causes the positive (or negative) side of
the envelope to have a shape different from that of the message
signal. This is known as envelope distortion, which is typical of
(f) The power in the modulated signal is contained in all four
Carrier Recovery for DSBSC Demodulation
Coherent reference for product detection of DSBSC can not
be obtained by the use of ordinary PLL because there are no
spectral line components at fc.
A squaring loop can also be used to obtain coherent reference
carrier for product detection of DSBSC. A frequency divider
is needed to bring the double carrier frequency to fc.
Power in Suppressed-Carrier Signals
•Carrier power is useless as a measure of power in a DSBSC or
•Instead, the peak envelope power is used
•The peak power envelope is simply the power at modulation
peaks, calculated thus:
Generation of VSB
1. One sideband is not rejected fully.
2. One sideband is transmitted fully and a small part (vestige)of the
other sideband is transmitted.
The transmission BW is BWv = B + v. where, v is the vestigial
DEMODULATION OF AM SIGNALS
AM demodulationtechniquesare of two type:
– coherent demodulation
– non coherent demodulation(Envelope
• Coherent demodulationrequires knowledge of
the transmitted carrier frequency and phase at the
• whereas non coherent detectionrequires no
Peak (envelope)detectorfor AM
Input is rectified (negative half removed)
Capacitor is charged up on each peak, then slowly
• It is a down converter circuit which converts the input band
pass signal to a base band signal
• If the input to the product detector is an AM signal of the
form R(t)cos(2Πfct+θr),the output of the multiplier can be
v1(t)=R(t) cos(2Πfct+θr)A0 cos(2Πfct+θ0)
• where fc is the oscillator carrier frequency, and θr and θ0 are
• received signal phase and oscillator phases respectively.
• v1(t)=1/2 A0 R(t)cos(θr - θ0)+ 1/2 A0
• The output obtained after passing through a LPF is
Vout(t)=1/2 A0 R(t)cos(θr - θ0)=KR(t)
• Where K is a gain constant
Non-coherent Envelope Detector:
• If the input to the envelope detector is represented as
R(t)cos(2Πfct+θr),then the output is given by
• Vout(t) = K|R(t)|
• where K is a gain constant.
• Envelope detectors are useful when the input signal power is
at least10dB greater than the noise power, whereas product
detectors are able to process AM signals within signal-to-
noise ratio well below 0 dB.
Frequency Divison Multiplexing