IT-404 Analog & Digital Communication - Jun 2025 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
   aDifferentiate between energy and power signals. Determine whether a rectangular pulse of finite amplitude is an energy or power signal.
   How to answer

   • Define energy signal (finite energy, zero average power) and power signal (finite average power, infinite energy) with formulas
   • Comparison table: energy, power, duration, examples
   • Expression for rectangular pulse of finite amplitude and duration
   • Evaluate energy integral and average power to conclude it is energy signal
2. 7 Marks
   bFind the Fourier Transform of the following signals:

   1. i)Unit step
   2. ii)Gate function
   How to answer

   • Definition of Fourier Transform formula
   • Fourier Transform of unit step using signum function / duality
   • Fourier Transform of gate function as sinc function
   • Mention properties used and final expressions
3. 7 Marks
   aClassify systems as linear/non-linear, time-variant/invariant, causal/non-causal and stable/unstable with examples.
   How to answer

   • Criteria for linearity, time-invariance, causality, stability with definitions
   • Test procedure / conditions for each property
   • Classify with justification for each category
   • Give one example for each type
4. 7 Marks
   bDiscuss square-law and switching modulators for AM generation with circuit diagrams and waveforms.
   How to answer

   • Principle of square-law modulator using non-linear device and bandpass filter
   • Circuit diagram and waveforms / spectra at each stage
   • Principle of switching modulator using diode switch and operation
   • Circuit diagram and waveforms for switching modulator
5. 7 Marks
   aExplain the design and operation of a TRF receiver. What are its limitations compared to a superheterodyne receiver?
   How to answer

   • Block diagram and operation of TRF receiver (RF stages, detector, audio stages)
   • Concept of tuning all RF stages to signal frequency
   • Limitations: instability, poor selectivity, bandwidth variation
   • Comparison with superheterodyne principle and advantages
6. 7 Marks
   bDescribe DSB-SC and SSB-SC systems with block diagrams.
   How to answer

   • Principle of DSB-SC with balanced modulator block diagram and spectrum
   • Principle of SSB-SC generation (filter/phase-shift method) with block diagram
   • Time and frequency domain expressions
   • Comparison of bandwidth and power saving
7. 7 Marks
   aExplain the phenomenon of pre-emphasis and de-emphasis in FM systems. Derive transfer functions for both.
   How to answer

   • Need for pre-emphasis/de-emphasis due to noise in FM high frequencies
   • Pre-emphasis high-pass characteristic and transfer function
   • De-emphasis low-pass characteristic and transfer function
   • Improvement in output SNR with curves
8. 7 Marks
   bExplain direct and indirect methods of FM generation.
   How to answer

   • Direct method: VCO/reactance modulator principle and block diagram
   • Indirect method: Armstrong method with narrowband FM plus multiplication
   • Comparison of frequency stability
   • Merits/demerits of each method
9. 7 Marks
   aExplain the function of a ratio detector with circuit diagram.
   How to answer

   • Circuit diagram of ratio detector with transformer, diodes, RC network
   • Phasor addition for frequency-to-voltage conversion
   • Amplitude limiting action and advantage over Foster-Seeley
   • Output characteristics
10. 7 Marks
    bExplain the working of PCM system with block diagram. What is quantization error? Explain.
    How to answer

    • Block diagram of PCM: sampler, quantizer, encoder and receiver
    • Sampling, quantization and encoding steps
    • Definition and origin of quantization error / noise
    • Effect on SNR and ways to reduce
11. 7 Marks
    aState and prove the sampling theorem for low-pass signals. Discuss the consequences of undersampling (aliasing).
    How to answer

    • Statement of low-pass sampling theorem with Nyquist rate
    • Proof using impulse sampling and spectrum replication
    • Reconstruction with ideal low-pass filter
    • Undersampling effect: aliasing, spectral overlap and consequences
12. 7 Marks
    bDescribe Delta Modulation (DM) and Adaptive Delta Modulation (ADM) with merits and demerits.
    How to answer

    • Principle of Delta Modulation with block diagram and staircase waveform
    • Principle of Adaptive Delta Modulation with variable step
    • Merits: simplicity, low bit rate
    • Demerits: slope overload, granular noise and how ADM improves
13. 7 Marks
    aDescribe the concept and working of DPSK. How does it differ from BPSK in terms of synchronization?
    How to answer

    • Concept of DPSK with differential encoding rule
    • Transmitter and receiver block diagrams and operation
    • No need for coherent carrier: non-coherent detection
    • Difference from BPSK in carrier synchronization requirement
14. 7 Marks
    bWith block diagrams, explain the generation and detection of Quadrature Amplitude Modulation (QAM). Compare with QPSK.
    How to answer

    • Block diagrams of QAM transmitter (I/Q modulators) and receiver
    • Signal constellation and mathematical expression
    • Generation and coherent detection steps
    • Comparison with QPSK in bits per symbol, bandwidth, error performance
15. 14 Marks
    Write short notes on (any two):

    1. a)VSB-SC systems
    2. b)M-ary PSK
    3. c)Sensitivity, selectivity and fidelity of receivers
    4. d)Fourier Transform and its properties
    How to answer

    • Brief principle, block diagram / spectrum for each chosen topic
    • Key mathematical expression or characteristics
    • Advantages, limitations and applications
    • Neat labelled diagram where applicable