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

2. 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

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

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

5. 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

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

7. 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

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

9. 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

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

11. 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.

12. 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.

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

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

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

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

17. 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.

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

19. 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.

20. 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)

21. Mathematical analysis of phasing method

22. 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.

23. Cont’d
AM wave is applied to a vestigial sideband filter, producing a modulation scheme – VSB + C
Mainly used for television video transmission.

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

25. Advantages/Benefits of SSB
Power consumption
Bandwidth conservation
Selective fading
Noise reduction

26. Disadvantages of SSB
Complex receivers
Tuning difficulties

27. 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

28. END OF CHAPTER 2.3 AM SINGLE SIDEBAND COMMUNICATIONS