Jawaharlal Nehru Technological University Hyderabad B.Tech III Principles of Electronic Communications Previous Question Paper (February - 2022 - R16)

Modulation is the process of varying one or more properties of a periodic waveform, called the carrier signal, with a modulating signal that contains information to be transmitted. The primary advantages of modulation include:

• Antenna Size Reduction: For efficient radiation of electromagnetic waves, the antenna length must be a fraction of the wavelength (typically λ/4). Without modulation, low-frequency baseband signals would require antennas several kilometers long. By shifting the signal to a high-frequency carrier, the wavelength decreases, allowing for practical, compact antenna designs.
• Multiplexing: Modulation allows multiple signals to be transmitted simultaneously over the same medium by assigning each signal a different carrier frequency. This is essential for radio, television, and telecommunications.
• Improved Signal-to-Noise Ratio (SNR): Modulation techniques, particularly frequency modulation (FM), allow for noise reduction at the receiver end, enhancing the quality of the signal compared to baseband transmission.
• Narrow Bandwidth Requirement: By transmitting at high frequencies, the ratio of bandwidth to carrier frequency remains small, which simplifies the design of tuned circuits and filters.
• Long-Distance Communication: High-frequency signals propagate over much greater distances through free space compared to low-frequency baseband signals, which suffer from high attenuation.

Given: Modulation index (μ) = 0.5, Sideband frequencies (f_USB) = 10.005 MHz, (f_LSB) = 9.995 MHz, Sideband amplitude (A_SB) = 20 V.

1. Carrier Frequency (f_c):

2. Carrier Amplitude (A_c):

3. Frequency Domain Representation: The spectrum consists of three spikes: one at 10 MHz with amplitude 80 V, and two sidebands at 9.995 MHz and 10.005 MHz, each with amplitude 20 V.

4. Time Domain Representation: The signal is represented by the equation:

Note: The envelope of the time-domain signal varies between a maximum of 120 V (80 + 40) and a minimum of 40 V (80 - 40) at a rate of 5 kHz.