IT-404 Analog & Digital Communication - Jun 2023 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
   aWrite down the function of the unit impulse function. also evaluate $x(t) = \int_{0}^{5} \delta(t) \sin 2\pi t \, dt$
   How to answer

   • Definition/properties of unit impulse function delta(t): sifting, area, scaling
   • Sifting property: integral delta(t)*f(t) = f(0) if 0 in limits
   • Evaluate sin 2 pi t at t=0 gives 0
   • Result x(t)=0 with limits 0 to 5 including impulse
2. 7 Marks
   bExplain rectangular pulse, triangular pulse and signum functions with suitable waveform pulse diagram and their expressions.
   How to answer

   • Mathematical expressions for rectangular, triangular and signum functions
   • Waveform sketches with amplitude vs time
   • Key parameters: width, symmetry, discontinuity
   • Use in signal analysis
3. 7 Marks
   aExplain the block diagram of a Superheterodyne receiver. Also define sensitivity and selectivity of receivers.
   How to answer

   • Block diagram stages: RF amp, mixer, local oscillator, IF amp, detector, AGC
   • Function of each block and image rejection concept
   • Definitions of sensitivity and selectivity
   • Typical waveforms/frequency translation
4. 7 Marks
   bExplain modulation index of amplitude modulated waveform. Calculate modulation index if carrier varies between 4V and 1V after amplitude modulated signal.
   How to answer

   • Definition of modulation index mu = Vm/Vc = (Vmax-Vmin)/(Vmax+Vmin)
   • Explanation of under/over modulation
   • Substitute Vmax=4V, Vmin=1V
   • Calculate mu = 0.6 and percentage
5. 7 Marks
   aExplain Foster Seeley Discriminator with the schematic diagram and phase diagrams.
   How to answer

   • Circuit schematic of Foster-Seeley discriminator with transformer and diodes
   • Phase relationship between primary and secondary voltages
   • Phasor/phase diagrams for f=fc, f>fc, f<fc
   • S-curve output vs frequency
6. 7 Marks
   bExplain balanced slope detector with the help of schematic diagram and its characteristic diagram.
   How to answer

   • Schematic with two tuned slope circuits and balanced connection
   • Individual and combined characteristic curves
   • Operation above and below resonance
   • Advantages/limitations for FM demodulation
7. 7 Marks
   aExplain various pulse modulation techniques such as PAM, PWM and PPM with suitable wave forms.
   How to answer

   • Principle of PAM, PWM, PPM with sampling
   • Waveforms showing amplitude/width/position variation with message
   • Generation and detection outline
   • Comparison of bandwidth and noise performance
8. 7 Marks
   bDifferentiate sampling theorems used for low pass and band pass signals.
   How to answer

   • Low-pass sampling theorem: fs >= 2fm
   • Band-pass sampling theorem and minimum rate condition
   • Differences in aliasing, guard band and reconstruction
   • Examples of application
9. 7 Marks
   aExplain why DBPSK signal is more reliable with the help of suitable example. Also explain the decoding of the incoming symbols into binary bits with DBPSK receiver.
   How to answer

   • Concept of differential encoding and phase comparison
   • Example bit sequence showing error propagation avoidance
   • Block diagram of DBPSK receiver/differential decoder
   • Decoding rule from phase difference to binary bits
10. 7 Marks
    bOutline the possible reasons for higher probability of errors in QPSK and minimum probability of error in offset QPSK.
    How to answer

    • QPSK constellation and 90-degree transitions causing envelope variation
    • Higher error due to simultaneous I/Q switching
    • Offset QPSK staggering by half symbol
    • Reduced phase jumps and minimum error reasoning
11. 7 Marks
    aClassify the signals and explain each with the suitable wave form diagrams and their expressions.
    How to answer

    • Classification basis: continuous/discrete, periodic, energy/power, deterministic/random
    • Expression for each signal type
    • Waveform diagrams for each category
    • Examples and significance
12. 7 Marks
    bExplain AM detector with the help of its circuit diagram and waveforms.
    How to answer

    • Envelope detector circuit with diode, RC load
    • Operation during positive half cycles and discharge
    • Input and output waveforms
    • Diagonal clipping and component selection
13. 7 Marks
    aExplain frequency and phase modulation with proper expression and waveform diagrams.
    How to answer

    • FM expression with frequency deviation and modulation index
    • PM expression with phase deviation
    • Waveforms showing frequency vs phase variation with message
    • Relation between FM and PM
14. 7 Marks
    bExplain time division multiplexing with suitable block diagrams and also state its applications.
    How to answer

    • Principle of TDM with commutator/decommutator
    • Transmitter-receiver block diagrams with synchronization
    • Frame structure and guard times
    • Applications in telephony and digital systems
15. 7 Marks
    aExplain QAM modulation and demodulation with the help of suitable waveforms and block diagrams.
    How to answer

    • QAM principle combining ASK and PSK
    • Modulator block diagram with I/Q channels
    • Demodulator with coherent detection
    • Constellation and waveforms
16. 7 Marks
    bCompare and contrast the bandwidth requirements of binary ASK, FSK and PSK signals. Also explain the generation and recovery methods of the ASK, FSK and PSK modulated signals.
    How to answer

    • Bandwidth formulas for ASK, FSK, PSK in terms of bit rate
    • Comparison table of spectral efficiency
    • Generation methods: switching/multiplier/VCO
    • Coherent/non-coherent recovery methods