CHAPTER 2. AMPLITUDE MODULATION (AM) 2.3 AM Single Side Band Communications

Transcription

1 CHAPTER AMPLITUDE MODULATION (AM).3 AM Single Side Band Couniations

2 OBJECTIVES To define and desribe AM single sideband To opare single sideband transission to onventional double sideband AM The explain the ethods of generating SSB To define and desribe AM vestigial sideband To desribe the advantages and disadvantages of SSB

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

4 REVIEW: CONVENTIONAL AM(DSB-FC) Frequeny spetru: f -f f f f Bandwidth x f ax Total PowerP arrier P usb P lsb

5 TWO MAJOR DISADVANTAGES/DRAWBACKS OF DSBFC Large power onsuption, where arrier power onstitutes >/3 transitted power.{reeber:arrier does not ontain any inforation} Large bandwidth utilized. Thus, DSBFC is both power and bandwidth ineffiient

6 SINGLE SIDE BAND FULL CARRIER (SSB-FC) Frequeny spetru: f -f Bandwidthf (ax) f f f Total PowerP arrier P usb

7 CONT D A for of aplitude odulation in whih the arrier is transitted at full power but only one of the sidebands (either the upper or lower) is transitted Requires less bandwidth than DSBFC but also produes a deodulated signal with a lower aplitude

8 SINGLE SIDE BAND SUPPRESS CARRIER (SSB-SC) Frequeny spetru: f -f Bandwidthf (ax) f f f Total PowerP usb

9 CONT D A for of aplitude odulation in whih the arrier is totally suppressed and one of the sidebands reoved. Therefore, SSBSC requires half as uh bandwidth as onventional DSB AM and onsiderably less transitted power

10 COMPARISON OF TIME DOMAIN REPRESENTATION OF THREE COMMON AM TRANSMISSION SYSTEMS: Toasi

11 EXAMPLE 1 For an AM DSCFC wave with a peak unodulated arrier voltage V 10 Vp,frequeny of 100kHz, a load resistor of RL 10 Ω, frequeny of odulating signal of 10kHz and 1, deterine the following i) Powers of the arrier and the upper and lower sidebands. ii) Total power of the odulated wave. iii) Bandwidth of the transitted wave. iv) Draw the power and frequeny spetru.

12 EXAMPLE 1..CONT D For the sae given values, deterine questions (ii)-(iv) for a AM DSB-SC, AM SSB-FC and AM SSB-SC systes. Deterine also the perentage of power saved in eah of the syste design.

13 EXAMPLE 1..CONT D Solution for DSBFC; i) ii) iii) Bandwidthxf ax (10kHz)0kHz W P P P P t 7.5 (5) 4 1 (5) W P P P W R V R V P lsb usb (10) ) / (

14 EXAMPLE 1..CONT D Solution:For DSB-SC ii) iii)bandwidthxf ax (10kz)0kHz iv) 66.67% 100% % x W W Power W W W Power saved saved W P P P t.5 (5) 4 1 (5) kHz 90kHz

15 EXAMPLE 1..CONT D Solution:For SSB-FC ii) P t P P (5) 4 6.5W Power saved 1.5W % Power 16.67% 7.5W saved 6.5W 1.5W x100% 7.5W iii)bandwidthf ax 10kHz iv) f -f 100kHz 110kHz

16 EXAMPLE 1..CONT D Solution:For SSB-sC ii) P t P (5) 1.5W Power saved 6.5W % Power 83.33% saved 7.5W 1.5W 6.5W x100% 7.5W iii)bandwidthf ax 10kHz iv) f -f f 110kHz

17 METHODS OF GENERATING SSB i) Filtering ethod A filter reoves the undesired sideband produing SSB. Balaned odulators is used to suppress the unwanted arrier and filters to suppress the unwanted sidebands Quartz rystal filters are the ost widely used sideband filters sine they are very seletive and inexpensive.

18 BLOCK DIAGRAM OF FILTERING METHOD Carrier osillator Balaned odulator DSB signal Sideband filter SSB signal Linear aplifier Antenna Mirophone Audio aplifier Filter response urve Lower sidebands Upper sidebands

19 CONT D ii) Phasing ethod using two balane odulator Another way to produe SSB uses a phase shift ethod to eliinate one sideband. Two balaned odulators driven by arriers and odulating signals 90º out of phase produe DSB. Adding the two DSB signals together results in one sideband being anelled out.

20 BLOCK DIAGRAM OF PHASING METHOD A os wt Balaned Modulator 1 A 1 (t) Inforation signal A os (wt 90) Output Signal, a o t Phase shifter Carrier signal Phase shifter Balaned Modulator A os (wt 90) A (t)

21 MATHEMATICAL ANALYSIS OF PHASING METHOD { } ) ( ) 90 os( ) 90 os( 1 os )* 90 os( ) ( (1) ) ( ) ( ) ( > > wt wt AA wt wt AA wt A wt A t a t a t a t a { } ) (3 ) 90 os( ) 90 os( 1 90 ) os( )* os( ) ( > wt wt AA wt wt AA wt A wt A t a { } ) 90 os( (3) ( ) ) ( 0 0 t w t w A A t a

22 VESTIGIAL SIDEBAND (VSB) Also alled asyetri sideband syste. Coproise between DSB & SSB. Easy to generate. Bandwidth is only ~ 5% greater than SSB signals. Derived by filtering DSB, one pass band is passed alost opletely while just a trae or vestige of the other sideband is inluded.

23 CONT D AM wave is applied to a vestigial sideband filter, produing a odulation shee VSB C Mainly used for television video transission.

24 CONT D VSB VSB Frequeny Spetru Carrier VSB LSB MSB f

25 ADVANTAGES/BENEFITS OF SSB Power onsuption Bandwidth onservation Seletive fading Noise redution

26 DISADVANTAGES OF SSB Coplex reeivers Tuning diffiulties

27 AT THE END OF THIS CHAPTER, YOU SHOULD BE ABLE, To define and desribe AM single sideband To opare single sideband transission to onventional double sideband AM The explain the ethods of generating SSB To define and desribe AM vestigial sideband To desribe the advantages and disadvantages of SSB

28 END OF CHAPTER.3 AM SINGLE SIDEBAND COMMUNICATIONS