JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech II PRINCIPLES OF COMMUNICATIONS Previous Question Paper (March - 2022 - R13)

Noise is an unwanted electrical disturbance that interferes with the signal. In linear systems, noise is categorized into external and internal sources. External noise includes atmospheric, extraterrestrial, and man-made noise. Internal noise arises from the physical components of the system, primarily thermal noise (Johnson noise) and shot noise.

Thermal Noise: Generated by the random motion of electrons in a conductor, given by the formula:

where k is Boltzmann's constant, T is absolute temperature, and B is bandwidth. Shot Noise: Caused by the discrete nature of charge carriers in semiconductor devices, expressed as:

where q is electron charge and I_d is the average current. In linear systems, the total noise power at the output is calculated by multiplying the input noise power by the system's power gain G and adding the noise generated by the system itself, often represented by the noise figure F, where F = (S_i/N_i) / (S_o/N_o).

A square law modulator generates Amplitude Modulation (AM) by utilizing the non-linear transfer characteristics of a device, such as a diode or a FET, operating in its square-law region. The input voltage v_i(t) is the sum of the carrier signal A_c cos(ω_c t) and the message signal m(t).

The output current i_o(t) is related to the input by the non-linear relationship:

Substituting v_i(t) = m(t) + A_c cos(ω_c t) into the equation, the squared term a_2 v_i^2(t) produces components including 2 a_2 m(t) A_c cos(ω_c t). By passing this output through a band-pass filter centered at the carrier frequency ω_c, the unwanted DC components and baseband signals are removed, leaving the standard AM wave:

This process effectively shifts the message signal spectrum to the vicinity of the carrier frequency.