The document discusses amplitude modulation (AM), which is the simplest and earliest form of modulation. AM involves varying the amplitude of a carrier signal based on the instantaneous amplitude of an information signal. It describes the basic principles of AM, including modulation index and different types of AM such as double sideband suppressed carrier AM and single sideband AM. Advantages of AM include its simplicity of implementation, while disadvantages include inefficiency in power and bandwidth usage and susceptibility to noise.

1. 2009-CPE-03
UCE&T BZU MULTAN

2. Amplitude Modulation

3. What is Modulation
 Modulation
 In the modulation process, some characteristic of a high-
frequency carrier signal (bandpass), is changed according
to the instantaneous amplitude of the information
(baseband) signal.
 Why Modulation is used
 Suitable for signal transmission (distance…etc)
 Multiple signals transmitted on the same channel
 Capacitive or inductive devices require high frequency AC
input (carrier) to operate.
 Stability and noise rejection
CSULB May 22, 2006 3

4. About Modulation
 Application Examples
 broadcasting of both audio and
video signals.
 Mobile radio communications, such
as cell phone.
• Basic modulation types
– Amplitude Modulation: changes the amplitude.
– Frequency Modulation: changes the frequency.
– Phase Modulation: changes the phase.
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5. Basic Amplitude Modulation
 Amplitude Modulation is
the simplest and earliest
form of transmitters
 The information signal
varies the instantaneous
amplitude of the carrier

6. AMPLITUDE MODULATION (AM)
 In amplitude modulation, the message signal m(t) is impressed on
the amplitude of the carrier signal c(t) = Accos(2fct)
 This results in a sinusoidal signal whose amplitude is a function
of the message signal m(t)
 There are several different ways of amplitude modulating the
carrier signal by m(t)
 Each results in different spectral characteristics for the
transmitted signal
 We will describe these methods, which are called
(a) Double sideband, suppressed-carrier AM (DSB-SC AM)
(b) Single-sideband AM (SSB AM)
Oh-Jin Kwon, EE dept., Sejong Univ., Seoul, Korea:
http://dasan.sejong.ac.kr/~ojkwon/ 6

8. Amplitude Modulation
 The condition for envelope detection of the AM signal
for all t [ A  m(t )]  0
 If m(t )  0 and A=0 also satisfy the above condition
 Let be the peak amplitude of
m p (t ) m(t )
m(t   m p (t )
 This condition)is equivalent to
A  m p (t )
 The min. carrier amplitude required for envelope
detection is
m p (t )

9. Modulation index m (t ) / A
 p
 The modulation index
0   1

10. Modulation Index of AM Signal
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11. Modulation Index of AM Signal
CSULB May 22, 2006 11

12. Double-Sideband Suppressed-Carrier AM
 A double-sideband, suppressed-carrier (DSB-SC) AM signal is
obtained by multiplying the message signal m(t) with the carrier
signal c(t) = Accos(2fct)
 Amplitude-modulated signal
u (t )  m(t )c(t )  Ac m(t ) cos(2 f c t )
 An example of the message signal m(t), the carrier c(t), and the modulated
signal u (t) are shown in Figure 3.1
 This figure shows that a relatively slowly varying message signal m(t) is
changed into a rapidly varying modulated signal u(t), and due to its rapid
changes with time, it contains higher frequency components
 At the same time, the modulated signal retains the main characteristics of the
message signal; therefore, it can be used to retrieve the message signal at the
receiver
Oh-Jin Kwon, EE dept., Sejong Univ., Seoul, Korea:
http://dasan.sejong.ac.kr/~ojkwon/ 12

13. Double-Sideband Suppressed-Carrier AM
 Figure 3.1 An example of message, carrier, and DSB-SC modulated
signals
Oh-Jin Kwon, EE dept., Sejong Univ., Seoul, Korea:
http://dasan.sejong.ac.kr/~ojkwon/ 13

14. Single-Sideband AM
 The two sidebands of an AM signal are mirror images of one
another
 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

15. Single-Sideband AM
.
 A method, illustrated in Figure
3.16, generates a DSB-SC AM
signal and then employs a filter
that selects either the upper
sideband or the lower sideband
of the double-sideband AM
Figure 3.16 Generation of a single-
signal sideband AM signal by filtering one of
the sidebands of a DSB-SC AM signal.
., 15

16. Sideband and carrier power
 Carrier term does not carry information, and hence
the carrier power is wasted
 AM (t )  A cos ct  m(t ) cos ct  carrier  sidebands
 The carrier power Pcis the mean sq. value of
A cos c twhich is A2 / 2
 The sideband power Ps is the mean sq. value
 of m(t ) cos c t which is m 2 (t ) / 2

17. Power Efficiency
 The power efficiency m2 (t )
Ps
  100%
Pc  Ps A  m (t )
2 2
 For the special case of tone modulation
m(t )  A cos mt m (t )  A / 2
2 2
 Hence Ps
  2
A2 / 2 100%   2 100%
Pc  Ps A  A / 2 2
2 2
  1, m ax  33 %

18. Quadrature AM
 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 modulators

19. Quadrature Operation

20. Advantages/disadvantages
Advantages of Amplitude Modulation, AM
There are several advantages of amplitude modulation, and some of these reasons
have meant that it is still in widespread use today:
 It is simple to implement
 it can be demodulated using a circuit consisting of very few components
 AM receivers are very cheap as no specialized components are needed.
Disadvantages of amplitude modulation
Amplitude modulation is a very basic form of modulation, and although its
simplicity is one of its major advantages, other more sophisticated systems
provide a number of advantages. Accordingly it is worth looking at some of the
disadvantages of amplitude modulation.
 It is not efficient in terms of its power usage
 It is not efficient in terms of its use of bandwidth, requiring a bandwidth
equal to twice that of the highest audio frequency
 It is prone to high levels of noise because most noise is amplitude based and
obviously AM detectors are sensitive to it.