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Explain generation of SSB using phase shift method.

Mumbai University > Computer Engineering > Sem 3 > Electronic Circuits and Communication Fundamentals

Marks: 10 Marks

Year: Dec 2015

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  • The phaseing method of SSB generation uses a phase shift technique that causes one of the side bands to be conceled out. A block diagram of a phasing type SSB generator is shown in fig.

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  • The carrier signal is $V_c\sin2πf_ct$ the modulating signal is $V_m\sin2πf_mt$. Balanced modulator produces the product of these two signals. $$(V_m\sin2πf_mt)( V_c\sin2πf_ct)$$
  • Applying a trigonometric identity. $$(V_m\sin2πf_mt)( V_c\sin2πf_ct)=1/2[\cos(2πf_c-2πf_m)t-\cos(2πf_c+2πf_m)t]$$
  • Note that these are the sum and different frequencies or the upper and lower side bands.
  • It is important to remember that a cosine wave is simply a sine wave shifted by 90. A cousine wave has exactly the same shape as a sine wave, but it occurs 90
  • The 90 phase shifters create cousine waves of the carrier and modulating signal which are multiplied in balanced modulator to produce. $$(V_m-\cos2πf_mt)(V_c\cos2πf_ct) (V_m-\cos2πf_mt)(V_c\cos2πf_ct)$$
  • Another common trigonometric identity translates this to $$(V_m\cos2πf_mt)( V_c\cos2πf_ct) 1/2[\cos(2πf_c-2πf_m)t+\cos(2πf_c+2πf_m)t]$$
  • Now if you add these two expressions together the sum frequencies cancel while the difference frequencies add producing only the lower side band. $$\cos(2πf_c-2πf_m)t$$
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