Amplitude modulation

1. Muhammad Uzair Rasheed 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. CSULB May 22, 2006 4 Basic modulation types Amplitude Modulation: changes the amplitude. Frequency Modulation: changes the frequency. Phase Modulation: changes the phase.

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 ) = A c cos ( 2  f c t ) 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 Double sideband, suppressed-carrier AM (DSB-SC AM) Single-sideband AM (SSB AM) Oh-Jin Kwon, EE dept., Sejong Univ., Seoul, Korea: http://dasan.sejong.ac.kr/~ojkwon/ 6

7. Amplitude Modulation The condition for envelope detection of the AM signal for all t If and A=0 also satisfy the above condition Let be the peak amplitude of This condition is equivalent to The min. carrier amplitude required for envelope detection is

8. Modulation index The modulation index

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

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

11. 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 ) = A c cos ( 2  f c t ) Amplitude-modulated signal 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

12. 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

13. 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

14. 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 signal ., 15 . Figure 3.16 Generation of a single-sideband AM signal by filtering one of the sidebands of a DSB-SC AM signal.

15. Sideband and carrier power Carrier term does not carry information, and hence the carrier power is wasted The carrier power is the mean sq. value of which is The sideband power is the mean sq. value of which is

16. Power Efficiency The power efficiency For the special case of tone modulation Hence

17. 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

18. Quadrature Operation

19. 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.