IT-404 Analog & Digital Communication - Jun 2022 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
   aDefine and sketch the following signals: Signum Function, Impulse function, Unit step function
   How to answer

   • Definition of signum, impulse and unit step functions with mathematical expressions
   • Key properties: values for t>0, t<0, t=0
   • Neat amplitude vs time sketches for each signal
   • Relation between them e.g. u(t) and delta(t), sgn(t)
2. 7 Marks
   bExplain the about Linear and non-linear systems with example.
   How to answer

   • Definition of linearity: superposition, homogeneity and additivity
   • Condition for linear vs non-linear system with mathematical test
   • Examples: resistor/linear amplifier vs saturating amplifier, square-law device
   • Sketch of input-output characteristics for both
3. 7 Marks
   aIf $x(t) = u(t) - u(t-1)$. Plot $y(t) = x(2t+3)$.
   How to answer

   • Identify x(t)=u(t)-u(t-1) as rectangular pulse 0 to 1
   • Apply time-shifting, scaling: x(2t+3) transformation steps
   • Find new interval: -1.5 <= t <= -1
   • Plot y(t) amplitude 1 over that interval
4. 7 Marks
   bState and prove the time-scaling property of Fourier transform.
   How to answer

   • Statement: if x(t) <-> X(f) then x(at) <-> (1/|a|)X(f/a)
   • Proof using Fourier integral with substitution tau = a t
   • Separate cases a>0 and a<0 for limits
   • Physical meaning: compression in time expands in frequency
5. 7 Marks
   aExplain the generation of AM signals using square law modulator
   How to answer

   • Block diagram of square-law modulator: adder, non-linear device, bandpass filter
   • Expression v1 = m(t)+Ac cos wc t, v2 = a v1 + b v1^2 expansion
   • Explanation of desired AM term and filtering of unwanted terms
   • Output AM waveform expression
6. 7 Marks
   bWrite short notes on frequency translation and FDM?
   How to answer

   • Definition of frequency translation / up-down conversion principle
   • Need for multiplexing and concept of FDM hierarchy
   • Block diagram of FDM transmitter with modulators and filters
   • Example: telephone / cable TV application
7. 7 Marks
   aA 400W, 1MHz carrier is amplitude-modulated with a sinusoidal signal of 2500Hz. The depth of modulation is 75%. Calculate the sideband frequencies, bandwidth, and power in sidebands and the total power in modulated wave.
   How to answer

   • Formulas: f_LSB = fc-fm, f_USB = fc+fm, BW = 2fm, Pt = Pc(1+mu^2/2)
   • Calculate sideband frequencies ~997.5 kHz and 1002.5 kHz and 5 kHz BW
   • Calculate sideband power and total power from Pc=400 W, mu=0.75
   • Interpretation of power distribution
8. 7 Marks
   bDerive the expression for single tone frequency modulated signal.
   How to answer

   • Start with FM definition: s(t)=Ac cos[wc t + kf integral m(t)]
   • Put single tone m(t)=Am cos wm t to get phase deviation beta
   • Expand using Bessel functions to show sidebands
   • Final expression with carrier and infinite sidebands
9. 7 Marks
   aDerive expressions of Signal to Noise Ratio for an DSB system using coherent demodulation.
   How to answer

   • Block diagram of DSB coherent detector with local carrier and LPF
   • Multiplication of DSB-SC with coherent carrier and trigonometric expansion
   • Filtering to recover message and expressions for signal power
   • Derivation of output SNR expression
10. 7 Marks
    bExplain the Delta Modulation and Adaptive Delta Modulation (ADM) in detail.
    How to answer

    • Principle of delta modulation: 1-bit differential encoding with staircase
    • Block diagrams of modulator/demodulator and slope overload vs granular noise
    • Limitation of fixed step size leading to ADM concept
    • ADM operation with adaptive step size logic and advantages
11. 7 Marks
    aDerive the Signal to Noise Ratios for coherent reception with SSB modulation
    How to answer

    • SSB coherent reception model with bandpass noise
    • Derivation of output signal and noise after multiplication and LPF
    • Expression for input and output SNR
    • Figure of merit =1 conclusion
12. 7 Marks
    bWhat is the maximum data rate that can be transmitted using a QPSK modulation with a roll-off factor of 0.2 for a 36 MHz transponder?
    How to answer

    • Formula: BW = Rs(1+alpha), QPSK carries 2 bits per symbol
    • Given BW=36 MHz, alpha=0.2 to find symbol rate Rs
    • Multiply by 2 to get bit rate Rb = 60 Mbps
    • State assumptions of ideal Nyquist channel
13. 7 Marks
    aIn a digital communication system employing Frequency Shift Keying (FSK), the 0 and 1 bit are represented by sine waves of 10 kHz and 25 kHz respectively. What is the bit interval of these wave forms?
    How to answer

    • FSK orthogonality condition for bit interval T
    • Use frequencies 10 kHz and 25 kHz separation
    • Calculate T from integer cycles difference
    • Resulting bit interval value
14. 7 Marks
    bDraw and explain PPM, PAM and PWM schemes.
    How to answer

    • Definition of PAM, PWM, PPM with sampling principle
    • Waveform diagrams showing amplitude/width/position variation with message
    • Generation/detection block diagrams in brief
    • Comparison of bandwidth and noise performance
15. 7 Marks
    aWrite short notes on the spectrum and bandwidth of FSK. And explain describe FSK modulation.
    How to answer

    • FSK signal expression and two-frequency concept
    • Power spectral density and Carson-like bandwidth formula
    • Spectrum sketch with peaks at f0, f1
    • FSK modulator block diagram explanation
16. 7 Marks
    bDraw and explain the operations of Non-coherent and coherent ASK modulators.
    How to answer

    • Block diagrams of coherent ASK (product detector) and non-coherent (envelope detector)
    • Operation: multiplication with carrier vs rectification + LPF
    • Waveforms for binary 1/0 transmission
    • Comparison of complexity and performance