EC-403 Analog Communication - Jun 2024 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
   aState and prove the time scaling properties of Fourier Transform.
   How to answer

   • State time-scaling property: $x(at) \leftrightarrow \frac{1}{|a|}X(\omega/a)$
   • Write Fourier Transform integral definition
   • Substitute $\tau = at$ and handle $a>0$ and $a<0$ cases
   • Show amplitude scaling by $1/|a|$ and frequency expansion/compression
2. 7 Marks
   bObtain the Fourier Transform of the signal given below: $X_{1}(t) = +1$ for $-2 < t < 0$ $X_{2}(t) = -1$ for $0 < t < 2$
   How to answer

   • Combine $X_1(t)$ and $X_2(t)$ into single $x(t)$ definition
   • Write $X(\omega)=\int_{-2}^{0}1\cdot e^{-j\omega t}dt + \int_{0}^{2}(-1)e^{-j\omega t}dt$
   • Evaluate integrals to get closed-form expression
   • Simplify to magnitude-phase / sine form
3. 7 Marks
   aDefine and sketch the following signals: Signum Function, Impulse Function, Unit Step Function, Sine and Cosine Function.
   How to answer

   • Define signum, impulse, unit step, sine and cosine functions mathematically
   • Give key properties / values for $t>0$, $t=0$, $t<0$
   • Sketch amplitude vs time for each of the five signals
4. 7 Marks
   bWhat is the need for modulation? Explain the main advantages of modulation.
   How to answer

   • Define modulation: impressing message on high-frequency carrier
   • Explain needs: antenna size, multiplexing, narrowband, propagation
   • List main advantages: reduced antenna height, interference reduction, frequency allocation
5. 7 Marks
   aWith the help of circuit diagram explain the operation of square law modulator for AM
   How to answer

   • Draw square-law device + bandpass filter block/circuit diagram
   • Express non-linear output $y = a x + b x^2$ with $x = m(t)+c(t)$
   • Show filter selects $f_c$ term to get AM $A_c[1+\mu m(t)]\cos\omega_c t$
6. 7 Marks
   bExplain the generation of double sideband suppressed carrier (DSB-SC) modulation. Write the necessary equations.
   How to answer

   • Write DSB-SC expression $s(t)=m(t)\cdot c(t)$
   • Explain balanced modulator / multiplier generation method
   • Derive spectrum with two sidebands and suppressed carrier $\pm f_c \pm f_m$
7. 7 Marks
   aWhat is the difference between direct and indirect methods of FM generation? Explain the working of a balanced frequency discriminator with the help of circuit diagram.
   How to answer

   • Compare direct (VCO/Reactance) vs indirect (Armstrong phase) FM generation
   • Draw balanced frequency discriminator with two tuned circuits and diodes
   • Explain frequency-to-voltage conversion and difference output
8. 7 Marks
   bWhat is significance of Pre-emphasis and De-emphasis? Explain with neat sketch
   How to answer

   • Define pre-emphasis (boost high frequencies at transmitter) and de-emphasis (attenuate at receiver)
   • Explain SNR improvement for FM high-frequency noise
   • Sketch pre-emphasis/de-emphasis frequency response curves
9. 7 Marks
   aAn FM wave with modulation index $\beta = 1$ is transmitted through an ideal band pass filter with mid band frequency $f_{c}$ and bandwidth is $5f_{m}$, where $f_{c}$ is the carrier frequency and $f_{m}$ is the frequency of the sinusoidal modulating wave. Determine the amplitude spectrum of the filter output.
   How to answer

   • Write FM wave as Bessel series with $\beta=1$: $J_0$ to $J_n$ values
   • Apply ideal bandpass filter of bandwidth $5f_m$ centred at $f_c$
   • List surviving components $f_c, f_c\pm f_m, f_c\pm 2f_m$ with amplitudes
10. 7 Marks
    bDraw the block diagram of Superheterodyne receiver and explain the function of each block.
    How to answer

    • Draw superheterodyne blocks: RF amp, mixer, LO, IF, detector, audio
    • Explain frequency translation to fixed IF $455\,kHz$
    • State advantages: selectivity, sensitivity, tuning
11. 7 Marks
    aDiscuss the factors influencing the choice of intermediate frequency (IF) for a radio Receiver.
    How to answer

    • Define IF and its role in selectivity/image rejection
    • Discuss trade-offs: high IF for image rejection vs low IF for selectivity
    • List factors: tuning range, tracking, gain, stability, cost
12. 7 Marks
    bWith the aid of the block diagram explain TRF receiver. List out the advantages and disadvantages of TRF receiver.
    How to answer

    • Draw TRF blocks: RF stages, detector, audio stages tuned to RF
    • Explain ganged tuning operation
    • List advantages (simplicity) and disadvantages (instability, poor selectivity, bandwidth variation)
13. 7 Marks
    aWrite properties of impulse function and convolution theorem (time and frequency).
    How to answer

    • Define impulse $\delta(t)$ with sifting, sampling, scaling properties
    • State time convolution $x_1*x_2 \leftrightarrow X_1X_2$
    • State frequency convolution $x_1x_2 \leftrightarrow X_1*X_2/2\pi$
14. 7 Marks
    bThe antenna current of an AM transmitter is 8 Amps, when only the carrier is sent, but it increases to 8.93 A, when the carrier is modulated by a single sine wave. Find percentage modulation. Determine the antenna current when the percent modulation changes to 0.8.
    How to answer

    • Use $I_t = I_c\sqrt{1+\mu^2/2}$ to solve for $\mu$ from $8$ and $8.93\,A$
    • Calculate percentage modulation $\mu\times 100\%$
    • Calculate new $I_t$ for $\mu=0.8$ using same formula
15. 14 Marks
    Write a short note on any two

    1. i)Convolution Theorem
    2. ii)Vestigial Side Band (VSB)
    3. iii)Effect of Noise on AM and FM Receiver
    4. iv)AVC
    How to answer

    • Select any two topics from i)-iv)
    • Give definition/principle with relevant expressions/diagrams
    • State applications/significance and conclude