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

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

3. 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

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

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

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

7. Comparison of time domain representation of three common AM transmission systems:
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8. 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.

9. 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.

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

11. 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

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

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

14. 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

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

16. (i) Balanced ring modulator
(a) schematic diagram; (b) D1 and D2 biased on; (c) D3 and D4 biased on
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17. Balanced Modulator Waveforms
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18. (ii) FET Push-Pull Balanced Modulator
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19. FET Push-Pull Balanced Modulator Phasor Diagrams
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20. (iii) Balanced Bridge Modulator
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21. (iv) Linear IC Balanced Modulators
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22. 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.

23. Filtering method

24. 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.

25. 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

26. Phasing method..cont’d

27. Phasing method..cont’d
Blok digram 2

28. 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.

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

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

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

32. Disadvantages of SSB
Complex receivers
Tuning difficulties