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.
CSULB May 22, 2006 4

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
CSULB May 22, 2006 10

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.