PRINCIPLES OF COMMUNICATION (REC-502)
B.Tech (SEM-V) – AKTU
SECTION A
(Attempt all questions – 2 × 7 = 14 marks)
Q1 (a) What are the differences between NBFM and WBFM?
Narrowband FM (NBFM) has a small modulation index and occupies less bandwidth. It is used in communication systems like walkie-talkies.
Wideband FM (WBFM) has a large modulation index, occupies higher bandwidth, and provides better noise immunity. It is used in FM broadcasting.
Q1 (b) Define Sampling Theorem used in communication system.
Sampling theorem states that a continuous-time signal can be completely represented by its samples if the sampling frequency is at least twice the maximum frequency present in the signal. This minimum rate is called Nyquist rate.
Q1 (c) What is aliasing effect? How it can be reduced?
Aliasing occurs when a signal is sampled at a rate lower than the Nyquist rate, causing overlap of frequency components. It can be reduced by using an anti-aliasing low-pass filter and increasing the sampling frequency.
Q1 (d) Determine the Nyquist rate and Nyquist interval of the signal sinc²(100t).
For sinc²(100t), maximum frequency = 100 Hz.
Nyquist rate = 2 × 100 = 200 Hz
Nyquist interval = 1 / 200 = 0.005 seconds
Q1 (e) Mention the exact data rates for T-1, T-2, T-3 and T-4 digital carrier systems.
T-1 = 1.544 Mbps
T-2 = 6.312 Mbps
T-3 = 44.736 Mbps
T-4 = 274.176 Mbps
Q1 (f) List the disadvantages of SSB modulation scheme.
SSB modulation requires complex transmitters and receivers. Carrier suppression is difficult and tuning is critical. It is also more sensitive to frequency errors.
Q1 (g) Write the expression for μ-law compander.
The μ-law companding expression is:
F(x) = (ln(1 + μ|x|)) / ln(1 + μ)
SECTION B
(Attempt any three – 7 × 3 = 21 marks)
Q2 (a) What is delta modulation? How delta modulation differs from PCM and PAM?
Delta modulation is a digital modulation technique where only the change in signal amplitude is encoded. Unlike PCM, it uses one bit per sample and has simpler circuitry. Compared to PAM, delta modulation is less sensitive to noise and distortion.
Q2 (b) For the given binary sequence 011010110, construct unipolar NRZ, bipolar NRZ, bipolar RZ, AMI and Manchester formats.
Unipolar NRZ represents binary 1 with positive voltage and 0 with zero voltage.
Bipolar NRZ uses alternate polarity for successive ones.
Bipolar RZ returns to zero between bits.
AMI uses alternate positive and negative pulses for binary 1 and zero voltage for binary 0.
Manchester encoding represents binary data using mid-bit transitions.
Q2 (c) Explain the working of ratio detector used to demodulate the FM signal with neat sketch.
Ratio detector demodulates FM signals using amplitude variations produced by frequency changes. It uses a tuned transformer and diodes. Output voltage is proportional to frequency deviation and it offers good noise rejection without requiring amplitude limiters.
SECTION C
(Attempt any one part from each question – 7 × 2 = 14 marks)
Q5 (a) Describe PAM and PPM generation and demodulation with neat labeled diagrams.
Pulse Amplitude Modulation (PAM) varies amplitude of pulses according to message signal.
Pulse Position Modulation (PPM) varies position of pulses according to signal amplitude.
Demodulation involves low-pass filtering and synchronization circuits to recover original message signal.
Q6 (b) What is Adaptive Delta Modulation? Explain ADM transmitter, receiver and advantages.
Adaptive Delta Modulation varies step size based on signal slope. The transmitter adjusts step size dynamically and receiver reconstructs signal accordingly. ADM reduces slope overload distortion and granular noise compared to basic delta modulation.
Q7 (a) Calculate the output signal-to-noise ratio of frequency modulation. Calculate figure of merit for frequency modulation.
The SNR of FM improves with modulation index and bandwidth.
Figure of merit of FM is defined as the ratio of output SNR to input SNR and is greater than unity, showing superior noise performance compared to AM.
