Lecture 3

Amplitude Modulation Agenda

 Bandwidth-efficient Amplitude Modulations

 Single SideBand (SSB)

 Quadrature Amplitude Modulation (QAM)

Amplitude Modulation 1-2 Amplitude Modulation  Single Sideband (SSB)

 Either the LSB or the USB can be suppressed from the DSB signal via bandpass filtering

 One sideband is transmitted

 Requires only one-half the bandwidth of the DSB-SC signal

 An SSB signal can be coherently demodulated like DSB-SC signals

 Since no additional carrier accompanies the modulated SSB signal  SSB-SC Amplitude Modulation 1-3

Single Sideband (SSB)

Amplitude Modulation 1-4 Single Sideband (SSB)

 Time Domain Representation of SSB Signals  Obtaining a time domain expression for each sideband (building blocks of an SSB signal)  For USB

Amplitude Modulation 1-5 Single Sideband (SSB)

 Time Domain Representation of SSB Signals  From the frequency-shifting property, the inverse transform yields

• If the phase of every component of m(t) is changed by π/2 and without changing the amplitude, the resulting signal is

mh(t), the Hilbert transform of m(t)  Similarly for LSB,

 Generally, for a SSB signal

Amplitude Modulation 1-6

SSB-SC

 Given the time domain expression of SSB- SC signals, the coherent demodulation of such signals can be confirmed analytically

 Apply low pass filter to remove unwanted SSB

terms (i.e., signals with frequency 2fc)

Amplitude Modulation 1-7 SSB Modulation Systems

 Three common methods

 Phase shifting

 Selective filtering

 Weaver method

Amplitude Modulation 1-8 SSB Modulation Systems

 Phase shifting

Phase shifter

Amplitude Modulation 1-9 Example

Consider the following circuit: (i) An upper-sideband single-sideband (SSB) can be generated by feeding a message signal m(t) into the input port of the above circuit. This is known as phase-shifted method for SSB signal generation. Determine the modulated signal s(t) at the output port. (ii) Suggest one appropriate modification of the circuit so that the modified circuit can be used to demodulate the upper-sideband SSB signal obtained in part (i) and retrieve the message signal m(t). Prove and verify your modified circuit as an upper-sideband SSB signal demodulator Amplitude Modulation 1-10 SSB Modulation Systems

 Selective-filtering

 The most commonly used method of generating SSB signals

 A DSB-SC signal is passed through a sharp cutoff filter to eliminate the undesired sideband

 To obtain USB, the filter should pass all

components above fc unattenuated and

completely suppress all components below fc Amplitude Modulation 1-11 SSB Modulation Systems

 Selective-filtering

Amplitude Modulation 1-12 SSB Modulation Systems

 Selective-filtering

Amplitude Modulation 1-13 SSB Modulation Systems

 Weaver’s method

 Two stages of SSB amplitude modulation is used

 First, the modulation is carried out by using a

smaller carrier frequency fc1

 The resulting SSB signal effectively widens the

gap to 2fc1

 Now by treating this signal as the new baseband signal, it is possible to achieve SSB-modulation at a higher carrier frequency Amplitude Modulation 1-14 SSB Modulation Systems

 Weaver’s method

Amplitude Modulation 1-15 Detection of SSB Signals with a Carrier (SSB+C)  Consider SSB signals with a carrier

 m(t) can be recovered by synchronous detection

[multiplying by Acos2πfct or Acosωct]

 If A is large enough, m(t) can also be recovered by envelope or rectifier detection (as in AM-TC)

Amplitude Modulation 1-16 Detection of SSB Signals with a Carrier (SSB+C)  Recovering SSB signals by envelope detector

 Where E(t) is the envelope of the signal

Amplitude Modulation 1-17 Detection of SSB Signals with a Carrier (SSB+C)

Amplitude Modulation 1-18 Quadrature Amplitude Modulation

 Due to limitations of generating accurately SSB-SC signals, Quadrature Amplitude Modulation (QAM) or quadrature multiplexing is presented  QAM can be adopted without requiring sharp-cutoff bandpass filters  Operates by transmitting two DSB signals using carriers of same frequency but in phase quadrature

Amplitude Modulation 1-19 QAM

Amplitude Modulation 1-20 QAM  Two modulated DSB signals occupy the same band

 Baseband signals can be separated at the receiver by synchronous detection if two local carriers are used in phase quadrature

Amplitude Modulation 1-21 QAM

 Two baseband signals m1(t) and m2(t), each of bandwidth B Hz, can be transmitted simultaneously over a bandwidth 2B by using DSB transmission and quadrature multiplexing  The upper channel is the in-phase (I) channel and the lower channel is the quadrature (Q) channel

 m1(t) and m2(t) can be separately demodulated

Amplitude Modulation 1-22 QAM – Demodulation

 The QAM demodulation must be totally synchronous  An error in the phase or the frequency of the carrier at the demodulator in QAM will result in loss and interference between the two channels  Let the carrier at the demodulator

Amplitude Modulation 1-23 QAM - Demodulation  In this carrier case

 The low pass filter suppresses the two signals modulated by carrier of angular frequency resulting in the first output

 The same will happen for the second output  This is called co-channel interference (undesirable)  Unequal attenuation of the USB and LSB during transmission leads to cross talk or co-channel interference Amplitude Modulation 1-24 QAM

 Quadrature multiplexing is used in analog color television to multiplex the so-called chrominance signals, which carry information about colors

 Digital satellite television transmission applies QAM

Amplitude Modulation 1-25 QAM

 In terms of bandwidth requirement, SSB is similar to QAM but less exacting in terms of the carrier frequency and the phase or the requirement of distortionless medium

 SSB has difficulties if the baseband signal m(t) has significant spectral content near DC

Amplitude Modulation 1-26 Lecture Summary

Covered material  Bandwidth-efficient Amplitude Modulations  Single SideBand (SSB)  Quadrature Amplitude Modulation (QAM)

Material to be covered next lecture  Vestigial Sideband (VSB) Modulation  Local Carrier Synchronization for Suppressed Carrier Signals Transmission

Amplitude Modulation 1-27