JNTU B.Tech III PRINCIPLES OF ELECTRONIC COMMUNICATIONS Previous Question Paper (August - 2022 - R16)

Modulation is the process of varying one or more parameters of a high-frequency carrier signal, such as amplitude, frequency, or phase, in accordance with the instantaneous values of a low-frequency message signal (baseband signal). This process shifts the frequency spectrum of the baseband signal to a higher range suitable for transmission.

The need for modulation arises from several practical requirements:

• Antenna Size Reduction: For efficient radiation, an antenna must be a fraction of the wavelength (typically λ/4). Low-frequency signals have very large wavelengths, requiring impractically large antennas. Modulation shifts the signal to a higher frequency, significantly reducing the required antenna size.
• Multiplexing: Modulation allows multiple signals to be transmitted simultaneously over the same medium by assigning different carrier frequencies to each signal, preventing interference.
• Improved Quality of Reception: Modulation helps in reducing noise and interference, as high-frequency signals are less susceptible to atmospheric noise than baseband signals.
• Narrow Bandwidth Requirement: By shifting signals to higher frequencies, the ratio of bandwidth to carrier frequency becomes small, simplifying the design of tuned circuits and filters.

Given: Modulation index (μ) = 0.5 (50%), Sideband frequencies = 5.005 MHz and 4.995 MHz, Amplitude of sidebands (A_USB = A_LSB) = 40 V.

1. Carrier Frequency (f_c): The sideband frequencies are defined as f_c + f_m = 5.005 MHz and f_c - f_m = 4.995 MHz. Adding these two equations: (f_c + f_m) + (f_c - f_m) = 5.005 + 4.995 = 10 MHz. Therefore, 2f_c = 10 MHz, which gives f_c = 5 MHz.

2. Carrier Amplitude (A_c): The amplitude of each sideband in an AM signal is given by the formula:

Substituting the given values:

The carrier signal has a frequency of 5 MHz and an amplitude of 160 V.