AMPLITUDE MODULATION (AM) 2-2 AM Single Side Band Communications CHAPTER 2 AMPLITUDE MODULATION (AM) 2-2 AM Single Side Band Communications

Applications of SSB 1) For wireless communications, especially to amateur radio band, citizen band and navigational communication equipment. 2) In telephone system between an exchange station to other station (trunk-call) to maximize (double) the useful allocated medium or band.

Review: conventional AM(DSB-FC) Frequency spectrum: Bandwidth=2Xfmmax Total Power=Pcarrier +Pusb +Plsb fc fc+fm fc-fm

Two major Disadvantages/Drawbacks of DSBFC Large power consumption, where carrier power constitutes >2/3 transmitted power.{remember:carrier does not contain any information} Large bandwidth utilized. Thus, DSBFC is both power and bandwidth inefficient

Double side band suppressed carrier(DSB-SC) Frequency spectrum: Bandwidth:2 x fmmax Total Power= Pusb + Plsb fc fc+fm fc-fm

Single Side Band Full Carrier (SSB-FC) Frequency spectrum: Bandwidth=fmmax Total Power=Pcarrier +Pusb fc fc+fm fc-fm

Single Side band Suppress Carrier (SSB-SC) Frequency spectrum: Bandwidth=fmmax Total Power=+Pusb fc fc+fm fc-fm

Comparison of time domain representation of three common AM transmission systems: Tomasi Electronic Communications Systems, 5e Copyright ©2004 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved.

AM DSBFC Waveforms

SSB Waveforms

Example 1 For an AM DSBFC wave with a peak unmodulated carrier voltage Vc = 10 Vp,frequency of 100kHz, a load resistor of RL = 10 , frequency of modulating signal of 10kHz and m = 1, determine the following Powers of the carrier and the upper and lower sidebands. ii) Total power of the modulated wave. iii) Bandwidth of the transmitted wave. iv) Draw the power and frequency spectrum.

Example 1..cont’d For the same given values, determine questions (ii)-(iv) for a AM DSB-SC, AM SSB-FC and AM SSB-SC systems. Determine also the percentage of power saved in each of the system design.

Example 1..cont’d i) ii) iii) Bandwidth=2xfmmax=2(10kHz)=20kHz Solution for DSBFC; i) ii) iii) Bandwidth=2xfmmax=2(10kHz)=20kHz

Example 1..cont’d ii) iii)Bandwidth=2xfmmax=2(10kHz)=20kHz iv) 90kHz Solution:For DSB-SC ii) iii)Bandwidth=2xfmmax=2(10kHz)=20kHz iv) 110kHz 90kHz fc

Example 1..cont’d ii) iii)Bandwidth=fmmax=10kHz iv) fc-fm 100kHz Solution:For SSB-FC ii) iii)Bandwidth=fmmax=10kHz iv) 100kHz 110kHz fc-fm

Example 1..cont’d ii) iii)Bandwidth=fmmax=10kHz iv) fc-fm 110kHz fc Solution:For SSB-sC ii) iii)Bandwidth=fmmax=10kHz iv) fc-fm 110kHz fc

Methods of Generating SSB There are 2 common methods: i) Filtering method A filter removes the undesired sideband producing SSB. Quartz crystal filters are the most widely used sideband filters since they are very selective and inexpensive. ii) Phasing method A balanced modulator eliminates the carrier and provides DSB.

Filtering method Filter response curve Sideband filter Balanced modulator Carrier oscillator Microphone Audio amplifier Linear Antenna Upper sidebands DSB signal SSB Lower

Phasing methods-using two balance modulator Another way to produce SSB uses a phase shift method to eliminate one sideband. Two balanced modulators driven by carriers and modulating signals 90º out of phase produce DSB. Adding the two DSB signals together results in one sideband being cancelled out.

Phasing method..cont’d Balanced Modulator 1 Modulator 2 Phase shifter + Information signal Carrier signal Output Signal, aot Am cos wmt Am cos (wmt + 90) Ac cos (wct + 90) A2(t) A1(t)

Phasing method..cont’d

VESTIGIAL SIDEBAND (VSB) Also called asymmetric sideband system. Compromise between DSB & SSB. Easy to generate. Bandwidth is only ~ 25% greater than SSB signals. Derived by filtering DSB, one pass band is passed almost completely while just a trace or vestige of the other sideband is included.

Cont’d…vsb AM wave is applied to a vestigial sideband filter, producing a modulation scheme  VSB + fc Mainly used for television video transmission.

Cont’d…vsb VSB Frequency Spectrum fc LSB MSB Carrier VSB

Advantages/Benefits of SSB Power consumption Bandwidth conservation Immune to or less affected by Selective Fading Noise reduction

Disadvantages of SSB Complex receivers Tuning difficulties