IT-404 Analog And Digital Communication - Jun 2026 Question Paper

Under each question, How to answer lists the points an answer should cover, outlined by CampusPrep from the syllabus. Check the details against your textbook.

1. 7 Marks
   aDraw the block diagram of a communication system and describe the function of each block.
   How to answer

   • Block diagram with source, transmitter, channel, receiver, destination
   • Function of transducer, modulator, channel impairments
   • Function of demodulator, decoder and output device
   • Signal flow from information source to destination
2. 7 Marks
   bClassify signals as energy and power signals. Sketch the standard signals: unit step, ramp, signum and impulse.
   How to answer

   • Criteria for energy vs power signals using total energy and average power integrals
   • Classification logic for aperiodic vs periodic and bounded signals
   • Mathematical expressions for unit step, ramp, signum and impulse
   • Sketches of amplitude vs time for the four standard signals
3. 7 Marks
   aDefine linear, time-invariant, causal and stable systems with one example each.
   How to answer

   • Definition of linearity with superposition and homogeneity
   • Definition of time-invariance with shifted input-output test
   • Definition of causality and stability with BIBO criterion
   • One example for each property with justification
4. 7 Marks
   bState the properties of the Fourier transform and find the Fourier transform of a unit impulse and a constant.
   How to answer

   • List of Fourier transform properties: linearity, shifting, scaling, duality, convolution
   • Fourier transform integral definition
   • Derivation of $F[\delta(t)] = 1$ using sifting property
   • Derivation of Fourier transform of constant $A$ as $2\pi A\delta(\omega)$
5. 7 Marks
   aDefine modulation index of an AM wave and draw the frequency spectrum of a DSB-FC signal.
   How to answer

   • Definition of modulation index $\mu = A_m/A_c$ for AM
   • Expression of DSB-FC AM wave in time and frequency domain
   • Frequency spectrum with carrier and two sidebands at $f_c \pm f_m$
   • Labelled amplitude and frequency positions in spectrum diagram
6. 7 Marks
   bA carrier of 100 V (peak) is amplitude modulated to a depth of 50%. Calculate the total power delivered to a $50 \Omega$ load and the transmission efficiency.
   How to answer

   • Formula for carrier power $P_c = V_c^2/(2R)$ and total power $P_t = P_c(1+\mu^2/2)$
   • Substitution of $V_c=100$ V peak, $\mu=0.5$, $R=50\,\Omega$
   • Numerical value of total power delivered to load
   • Transmission efficiency $\eta = \mu^2/(2+\mu^2)$ calculation
7. 7 Marks
   aWith a neat diagram, describe the working of a square law modulator.
   How to answer

   • Neat circuit/block diagram of square-law modulator with nonlinear device and bandpass filter
   • Square-law characteristics $v_o = a v_{in}+b v_{in}^2$ with sum of carrier and message
   • Generation of desired AM terms and unwanted terms
   • Role of bandpass filter centred at carrier to extract DSB-FC
8. 7 Marks
   bCompare DSB-SC, SSB-SC and VSB-SC systems on the basis of bandwidth and power requirement.
   How to answer

   • Bandwidth of DSB-SC, SSB-SC and VSB-SC in terms of message bandwidth
   • Power requirement and carrier suppression in each system
   • Tabular comparison of bandwidth and power
   • Application implication of comparison
9. 7 Marks
   aDraw the block diagram of a superheterodyne receiver and explain its working.
   How to answer

   • Block diagram of superheterodyne receiver from antenna to loudspeaker
   • Function of RF amplifier, mixer, local oscillator and IF stage
   • Frequency translation to fixed intermediate frequency and demodulation
   • Role of AGC, audio amplifier in receiver working
10. 7 Marks
    bDefine sensitivity, selectivity and fidelity of a radio receiver.
    How to answer

    • Definition of sensitivity as minimum input for standard output
    • Definition of selectivity as ability to select desired station
    • Definition of fidelity as faithful reproduction over audio band
    • Factors affecting each performance parameter
11. 7 Marks
    aDefine frequency deviation, modulation index and deviation ratio of an FM wave.
    How to answer

    • Definition of frequency deviation $\Delta f$ as peak departure from carrier
    • Definition of modulation index $\beta = \Delta f/f_m$ for FM
    • Definition of deviation ratio for worst-case modulating frequency
    • Relevant formulas linking deviation to modulating signal
12. 7 Marks
    bAn FM wave has a carrier frequency of 100 MHz, frequency deviation of 75 kHz and modulating frequency of 10 kHz. Calculate the modulation index and the bandwidth. Also state the difference between FM and PM.
    How to answer

    • Modulation index $\beta = \Delta f/f_m$ calculation from 75 kHz and 10 kHz
    • Bandwidth by Carson's rule $BW = 2(\Delta f+f_m)$
    • Difference in definition: FM varies frequency vs PM varies phase
    • Comparison of modulation index dependence on modulating frequency
13. 7 Marks
    aState the sampling theorem for low pass signals. Distinguish between natural and flat top sampling.
    How to answer

    • Statement of sampling theorem $f_s \ge 2f_m$ for low-pass signals
    • Conditions for exact reconstruction and Nyquist rate
    • Principle of natural sampling with pulse-top following signal
    • Differences from flat-top sampling and aperture effect
14. 7 Marks
    bIn a PCM system, a signal band-limited to 3.4 kHz is sampled at 8 kHz and encoded using 8 bits per sample. Determine the bit rate and the signal-to quantization noise ratio in dB.
    How to answer

    • Bit rate formula $R_b = f_s \times n$ with $f_s=8$ kHz, $n=8$
    • Quantization SNR formula $SQNR \approx 1.76+6.02n$ dB
    • Substitution to get bit rate in kbps
    • Numerical SQNR in dB for 8-bit encoding
15. 7 Marks
    aDraw the waveforms of ASK, BPSK and BFSK for the data 1011.
    How to answer

    • Time alignment of data bits 1-0-1-1 with carrier
    • ASK waveform: carrier present for 1, absent for 0
    • BPSK waveform: phase 0 for 1, $\pi$ shift for 0
    • BFSK waveform: frequency f1 for 1, f2 for 0
16. 7 Marks
    bDefine information and entropy. Calculate the channel capacity of a channel with 3 kHz bandwidth and SNR of 30 dB using the Shannon-Hartley theorem.
    How to answer

    • Definitions of information $I = \log_2(1/p)$ and entropy $H$ as average information
    • Shannon-Hartley law $C = B\log_2(1+SNR)$
    • Conversion of 30 dB SNR to linear ratio 1000
    • Channel capacity calculation for 3 kHz bandwidth