Analog Communication Systems Amplitude Modulation By Dr. Eng. Omar Abdel-Gaber M. Aly Assistant Professor Electrical Engineering Department College of Engineering Al-Majmaa Al-Majmaa University

Communications Principles, Omar Aly2 Outline  Overview of Modulation What is modulation? Why modulation? Classifications of modulations  Definitions of Bandwidth  Linear Modulation:  Amplitude modulation

Communications Systems, Omar Aly3 Overview of Modulation  What is modulation? The process of varying a carrier signal in order to use that signal to convey information.  Why modulation? 1. Efficient transmission of signals using antennas of practical size:  The optimal antenna size is related to wavelength:  Voice signal: 3 kHz  Wavelength: λ = c / f = 3 x 10 8 /(3000) = 300 Km  If modulated by a 100 MHz carrier:  Wavelength λ = 3×10 8 / (10 8 ) = 3m

Communications Systems, Omar Aly4 Overview of Modulation  Why modulation? 2. Utilizing the channel for transmission of more than one signal (multiplexing) 3. Modulation allows us to get better trade-off between bandwidth and signal-to-noise ratio (By choosing the suitable modulation technique)

Communications Systems, Omar Aly5 Overview: Types of Modulation  Analog modulation The input message is continuous in time and value Continuous-wave modulation (focus of this course)  A parameter of a high-freq carrier is varied in accordance with the message signal  If a sinusoidal carrier is used, the modulated carrier is:

Communications Systems, Omar Aly6 Overview: Types of Modulation  Linear modulation: A(t) is linearly related to the message. AM, DSB, SSB  Angle modulation: Phase modulation: Φ(t) is linearly related to the message. Freq. modulation: dΦ(t)/dt is linearly related to the message.

Communications Systems, Omar Aly7 Overview: Types of Modulation Linear modulation (Amplitude modulation) Message Carrier Angle modulation: Phase modulation Freq modulation

Communications Systems, Omar Aly8 Overview: Types of Modulation  Analog pulse modulation Message value is continuous. Transmission happens at discrete times. Transmitted signal is a sequence of pulses The amplitude, width, or position of the pulse can be varied over a continuous range according to the message value at the sampling instant. PAM: Pulse amplitude modulation PWM: Pulse width modulation PPM: Pulse position modulation

Communications Systems, Omar Aly9 Overview: Types of Modulation PAM: Pulse amplitude modulation PWM: Pulse width modulation PPM: Pulse position modulation Message

Communications Systems, Omar Aly10 Problems to be studied  For each modulation scheme, we will study the following topics: How does the modulator work? How does the demodulator work? What is the required bandwidth? What is the power efficiency? What is the performance in the presence of noise?

Communications Systems, Omar Aly11 Definitions of Bandwidth  A measure of the extent of significant spectral content of the signal for positive frequencies.  Band-limited signal: Bandwidth is (BW) = W. BW = 2W.  When the signal is not band-limited: Different definitions exist

Communications Systems, Omar Aly12 Definitions of Bandwidth  Null-to-null bandwidth: Null: A frequency at which the spectrum is zero.  3dB bandwidth:

Communications Systems, Omar Aly13 Definitions of Bandwidth  Radio spectrum is a scarce and expensive resource: US license fee: ~ $77 billions / year  Communications systems should provide the desired quality of service with the minimum bandwidth.

Communications Systems, Omar Aly14 AM and FM Radio  AM radio ranges from 535 to 1605 kHz The bandwidth of each station is 10 kHz. AM signals can travel quite long distance due to ionospheric refraction  The FM radio band goes from 88 to 108 MHz FM stations are 200 kHz apart FM has much better quality than AM Ionospheric refraction doesn't affect FM or TV signals too much (line- of-sight propagation, need tall antenna)

Communications Systems, Omar Aly15 m(t)cos(  c t) (Modulated signal) m(t) (Modulating signal) Amplitude Modulation: Double Sideband (DSB)  Amplitude modulation is characterized by the fact that the amplitude A of the carrier is varied in proportional to the message signal m(t) cos(  c t) (Carrier)

Communications Systems, Omar Aly16

Communications Systems, Omar Aly17

Communications Systems, Omar Aly18 Double Sideband (DSB-SC) m(t) M(  ) m(t)cos(  c t ) BW = B BW = 2B  c ≥ 2  B

Communications Systems, Omar Aly19 (DSB-SC) Demodulation  The demodulation consists of multiplication of the incoming modulated signal m(t)cos(  c t) by a carrier cos(  c t) followed by a low pass filter e(t)m(t)cos(wct) cos(  c t) Low Pass Filter 0.5m(t)

Communications Systems, Omar Aly20 (DSB-SC) Demodulation E(w) LPF  This method of demodulation is called synchronous detection or coherent detection, we use a carrier of exactly the same frequency and phase as the carrier used for modulation

(DSB-SC) Demodulation  In the time domain: Communications Systems, Omar Aly21

Communications Systems, Omar Aly22 Modulator Implementations  Modulation can be achieved in several ways Multiplier Modulators:  Modulation achieved by using an analog multiplier. Nonlinear Modulators:  Modulation can be achieved by using nonlinear device NL Switching Modulators:  Modulation can be achieved by using a simple switching operation.

Communications Systems, Omar Aly23 Nonlinear Modulators:  Let the input-output characteristics of the NL elements be approximated as:

Communications Systems, Omar Aly24 Switching Modulators:  A modulated signal can be obtained by multiplying m(t) by any periodic signal  (t) of the fundamental radian frequency  c.  Using Fourier series  (t) can be expressed as  If the signal is passed through a BPF of BW=2B and centered at  c we can get the modulated signal

Communications Systems, Omar Aly25 Switching Modulators:  Consider the square pulse train w(t) whose Fourier series is  The signal m(t)w(t) is given by:  Multiplication of a signal by a square pulse train is a switching operation

Communications Systems, Omar Aly26

Communications Systems, Omar Aly27 Diode-bridge modulator

Communications Systems, Omar Aly28 Ring modulator