Question #90178

Derive mathematical expression of an A.M. wave and indicate the sideband components. Why is the S.S.B. transmission beneficial ?

Expert's answer

Modulation is a technique which modifies waves of high frequency by the wave of low frequency.

Consider the carrier signal:

the modulating signal:

and the signal after modulation look like

Putting this all together gives

Now multiply the terms with cosines by @$V_c/V_c@$:

@$+\frac{V_m}{2}\frac{V_c}{V_c}\text{cos}(\omega_ct-\omega_mt)-\frac{V_m}{2}\frac{V_c}{V_c}\text{cos}(\omega_ct+\omega_mt),@$

and substitute @$V_m/V_c@$ with @$m_a@$ - the amplitude modulation index.

The sideband components are

and

@$ \frac{\omega_c+\omega_m}{2\pi}=f_c+f_m.@$The single-sideband transmission reduces the bandwidth transmitted and it allows to double the number of channels within the same frequency band.

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