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

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.

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 Solution for DSBFC; i) ii) iii) Bandwidth=2xfmmax=2(10kHz)=20kHz

Example 1..cont’d ii) iii)Bandwidth=2xfmmax=2(10kHz)=20kHz iv) 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) Solution:For SSB-FC ii) iii)Bandwidth=fmmax=10kHz iv) 100kHz 110kHz fc-fm

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

Comparison DSB-FC DSB-SC SSB-FC SSB-SC 7.5 2.5 6.25 1.25 20 10 66.67 Power (W) 7.5 2.5 6.25 1.25 BW(kHz) 20 10 Power Saving(%) 66.67 16.67 83.33 Modulating signal = 10kHz Unmodulated carrier frequency=100kHz Unmodulated carrier voltage = 10Vp

Methods of Generating SSB Balanced Ring Modulator FET Push-Pull Balanced Modulator Balanced Bridge Modulator Linear IC Balanced Modulators

(i) Balanced ring modulator (a) schematic diagram; (b) D1 and D2 biased on; (c) D3 and D4 biased on Tomasi Electronic Communications Systems, 5e Copyright ©2004 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved.

Balanced Modulator Waveforms Tomasi Electronic Communications Systems, 5e Copyright ©2004 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved.

(ii) FET Push-Pull Balanced Modulator Tomasi Electronic Communications Systems, 5e Copyright ©2004 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved.

FET Push-Pull Balanced Modulator Phasor Diagrams Tomasi Electronic Communications Systems, 5e Copyright ©2004 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved.

(iii) Balanced Bridge Modulator Tomasi Electronic Communications Systems, 5e Copyright ©2004 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved.

(iv) Linear IC Balanced Modulators Tomasi Electronic Communications Systems, 5e Copyright ©2004 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved.

SSB Transmitters 2 types, i) Filtering method ii) Phasing 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

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 a1(t) Information signal Ac cos (wct + 90) Am cos wmt a1(t) Balanced Modulator 1 Information signal Ac cos (wct + 90) Phase shifter + Output Signal, ao(t) Carrier signal Phase shifter Balanced Modulator 2 a2(t) Am cos (wmt + 90) Blok digram 1

Phasing method..cont’d

Phasing method..cont’d Blok digram 2

AM 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 + C 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 Selective fading Noise reduction

Disadvantages of SSB Complex receivers Tuning difficulties