JNTU B.Tech II PRINCIPLES OF COMMUNICATIONS Previous Question Paper (July/August - 2021 - R13)

An electrical communication system is designed to transmit information from a source to a destination over a channel. The block diagram consists of the following functional components:

• Information Source: Generates the message signal, which can be analog (e.g., audio) or digital (e.g., data).
• Input Transducer: Converts the physical message into an electrical signal.
• Transmitter: Processes the electrical signal for efficient transmission. This includes amplification, filtering, and modulation to shift the signal to a suitable frequency band.
• Channel: The physical medium (e.g., copper wire, optical fiber, or free space) that carries the signal. It introduces attenuation, distortion, and noise.
• Receiver: Extracts the desired signal from the received waveform. It performs amplification, filtering, and demodulation to recover the original baseband signal.
• Output Transducer: Converts the electrical signal back into the original physical form (e.g., sound or visual display) for the destination.

When a noise signal with power spectral density (PSD) S_i(f) is passed through a linear time-invariant filter with a frequency response H(f), the PSD of the output noise S_o(f) is determined by the magnitude squared of the filter's transfer function.

The effect of filtering on the noise PSD can be summarized as follows:

• Spectral Shaping: The filter modifies the frequency distribution of the noise power. Frequencies within the passband of the filter are passed to the output, while frequencies in the stopband are attenuated.
• Noise Bandwidth Reduction: If the filter is a band-pass or low-pass filter, it restricts the total noise power at the output by limiting the effective bandwidth.
• White Noise Case: If the input is white noise with a constant PSD of N_0/2, the output PSD becomes (N_0/2)|H(f)|^2, meaning the output noise takes on the shape of the filter's power transfer function.