AMPLITUDE MODULATION (AM) Part 3 - AM Single Side Band Communications CHAPTER 2 AMPLITUDE MODULATION (AM) Part 3 - AM Single Side Band Communications

OBJECTIVES To define and describe AM single sideband To compare single sideband transmission to conventional double sideband AM The explain the methods of generating SSB To define and describe AM vestigial sideband To describe the advantages and disadvantages of SSB

LECTURE OVERVIEW Single sideband and double sideband Power calculation in SSB Method of generating SSB Vestigial sidebands Advantages and disadvantages of SSB

Review: conventional AM(DSB-FC) Frequency spectrum: Bandwidth=2 x fmmax 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 or more of the total transmitted power. Carrier does not contain any information, the sidebands contain information. Large bandwidth utilized. With double-sideband transmission, the information contained in the upper sideband is identical to the information contained in the lower sideband. Therefore, transmitting both sidebands is redundant. Thus, DSBFC is both power and bandwidth inefficient

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

Cont’d… A form of amplitude modulation in which the carrier is transmitted at full power but only one of the sidebands (either the upper or lower) is transmitted Requires less bandwidth than DSBFC but also produces a demodulated signal with a lower amplitude

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

Cont’d… A form of amplitude modulation in which the carrier is totally suppressed and one of the sidebands removed. Therefore, SSBSC requires half as much bandwidth as conventional DSB AM and considerably less transmitted power

Comparison of time domain representation of three common AM transmission systems: Tomasi

Example 1 For an AM DSCFC 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; ii) iii) Bandwidth=2xfm(max)=2(10kHz)=20kHz

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

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

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

Methods of Generating SSB i) Filtering method A filter removes the undesired sideband producing SSB. Balanced modulators is used to suppress the unwanted carrier and filters to suppress the unwanted sidebands Quartz crystal filters are the most widely used sideband filters since they are very selective and inexpensive.

Block diagram of filtering method response curve Sideband filter Balanced modulator Carrier oscillator Microphone Audio amplifier Linear Antenna Upper sidebands DSB signal SSB Lower

Cont’d… ii) Phasing method 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.

Block diagram of phasing method 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)

Mathematical analysis of phasing method

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 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

AT THE END OF THIS CHAPTER, YOU SHOULD BE ABLE, To define and describe AM single sideband To compare single sideband transmission to conventional double sideband AM The explain the methods of generating SSB To define and describe AM vestigial sideband To describe the advantages and disadvantages of SSB

END OF CHAPTER 2, PART 3