(SEM VII) THEORY EXAMINATION 2018-19 ANALOG AND DIGITAL COMMUNICATION

ANALOG AND DIGITAL COMMUNICATION (NEC-702A)

B.Tech – Semester VII

SECTION A

(Attempt all questions)

(a) Basic block diagram of an analog communication system

An analog communication system consists of an information source, input transducer, transmitter, communication channel, receiver, and output transducer. The information source generates the message signal, which is converted into an electrical signal by the input transducer. The transmitter modulates this signal onto a high-frequency carrier to make transmission efficient. The modulated signal travels through the communication channel, where it may be affected by noise. At the receiver, demodulation extracts the original message signal, which is finally converted into a usable form by the output transducer.

(b) Advantage of SSB-SC over DSB-SC

Single Sideband Suppressed Carrier (SSB-SC) modulation is superior to Double Sideband Suppressed Carrier (DSB-SC) because it requires only half the bandwidth for transmission. In SSB-SC, only one sideband is transmitted, reducing power consumption and improving transmission efficiency. It also reduces noise and interference, making it suitable for long-distance and high-quality communication.

(c) Transmission bandwidth of FM signals

The transmission bandwidth of an FM signal depends on the frequency deviation and the highest frequency present in the modulating signal. According to Carson’s rule, the bandwidth is approximately twice the sum of the maximum frequency deviation and the modulating frequency. FM signals require much larger bandwidth compared to AM signals, but they provide better noise immunity and signal quality.

(d) Angle modulation

Angle modulation is a type of modulation in which the angle of the carrier wave is varied in accordance with the message signal while keeping the amplitude constant. It includes frequency modulation and phase modulation. Angle modulation provides improved noise performance compared to amplitude modulation.

(e) Frequency deviation

Frequency deviation is the maximum change in the carrier frequency from its unmodulated value due to the modulating signal. It is an important parameter in FM systems as it determines bandwidth and signal quality. Larger frequency deviation improves noise immunity but increases bandwidth.

(f) Demodulation

Demodulation is the process of extracting the original message signal from the modulated carrier at the receiver. It is the reverse of modulation and is essential for recovering the transmitted information accurately. Different modulation schemes require corresponding demodulation techniques.

(g) Need for modulation

Modulation is required to make signal transmission efficient by reducing antenna size, increasing transmission range, preventing signal mixing, and improving noise immunity. Without modulation, direct transmission of low-frequency signals would be impractical.

SECTION B

(Attempt any questions as per paper)

Functioning of modulation and demodulation with Quadrature Amplitude Modulation (QAM)

Modulation is used to shift the baseband signal to a higher frequency suitable for transmission, while demodulation recovers the original signal at the receiver. Quadrature Amplitude Modulation combines both amplitude and phase modulation to transmit multiple bits per symbol. In QAM, two carrier waves with a phase difference of 90 degrees are modulated separately and then combined. This technique increases data transmission rate without increasing bandwidth, making it widely used in modern digital communication systems such as broadband and wireless networks.

Differential Pulse Code Modulation (DPCM)

DPCM is a digital modulation technique that encodes the difference between successive samples of a signal instead of the absolute sample values. At the transmitter, the input signal is sampled, predicted, and the difference is quantized and encoded. At the receiver, the original signal is reconstructed using the decoded difference and predictor. DPCM reduces the number of bits required for transmission compared to PCM, resulting in efficient bandwidth utilization.

SECTION C

(Attempt any one)

Delta Modulation and Adaptive Delta Modulation

Delta modulation is a simplified form of PCM where only the change in signal amplitude is transmitted using a single bit per sample. The transmitter compares the input signal with a predicted value and transmits a bit indicating whether the signal has increased or decreased. Although delta modulation is simple, it suffers from slope overload distortion and granular noise. Adaptive Delta Modulation improves performance by varying the step size according to the rate of change of the input signal, thereby reducing distortion and improving signal quality.

Need for digital modulation and types of digital modulation

Digital modulation is required for reliable transmission of digital data over communication channels. It offers better noise immunity, efficient bandwidth utilization, and compatibility with digital systems. Common digital modulation techniques include Amplitude Shift Keying, Frequency Shift Keying, and Phase Shift Keying. In these techniques, digital data is transmitted by varying amplitude, frequency, or phase of the carrier wave. These methods form the foundation of modern digital communication systems.