EC-403 Analog 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
   aExplain about COSTAS loop with a neat block diagram for demodulating DSB-SC wave.
   How to answer

   • Define COSTAS loop for DSB-SC coherent demodulation
   • Neat block diagram with I and Q arms, VCO, multipliers, LPFs
   • Explain phase-error voltage generation and VCO locking
   • Locking condition and demodulated output recovery
2. 7 Marks
   bWith the help of circuit diagram explain the operation of square law modulator for AM.
   How to answer

   • Circuit diagram of square-law modulator with nonlinear device
   • Expression for nonlinear current with carrier plus message
   • Role of bandpass filter centred at carrier to select AM term
   • Condition for square-law operation and modulation generation
3. 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 in Bessel series with $\beta=1$
   • List Bessel coefficients J0 to J3 for beta=1
   • Apply ideal bandpass filter of bandwidth $5f_m$ centred at $f_c$
   • Retain passed sidebands and give amplitude spectrum lines
4. 7 Marks
   bExplain the indirect method of generation of FM wave and any one method of demodulating an FM wave.
   How to answer

   • Principle of Armstrong indirect method: NBFM generation then frequency multiplication
   • Block diagram: crystal oscillator, balanced modulator, multipliers, mixer
   • Need for frequency and phase multiplication to get WBFM
   • Any one FM demodulator: slope detector / Foster-Seeley / ratio detector / PLL
5. 7 Marks
   aDraw the block diagram of phase shift discriminator and explain the functionality of each block.
   How to answer

   • Block diagram of phase-shift discriminator / Foster-Seeley
   • Function of primary and secondary tuned circuits with centre-tapped transformer
   • Vector addition producing AM dependent on frequency deviation
   • Envelope detectors and subtractor to get modulating output
6. 7 Marks
   bCalculate the percentage power saving when the carrier and one of the sidebands are suppressed in an AM wave modulated to a depth of 100% and 50%.
   How to answer

   • Total AM power formula $P_t = P_c(1+\mu^2/2)$
   • Power in carrier and one sideband for $\mu=1$ and $\mu=0.5$
   • Power saved = carrier + one sideband power over total
   • Compute percentage for 100% and 50% cases
7. 7 Marks
   aOf all the frequencies that must be rejected by a super heterodyne receiver, why is the image frequency so important? What is the image frequency and how does it arise? If the image-frequency rejection of a receiver is insufficient, what steps could be taken to improve it?
   How to answer

   • Define superheterodyne, LO, IF and image frequency $f_{si}=f_s+2f_{IF}$
   • Explain why image also gives same IF and cannot be filtered after mixer
   • Importance due to strong unwanted station interference
   • Improvement: high IF, RF preselector, double conversion, image-reject mixer
8. 7 Marks
   bDefine the following:

   1. i)Noise figure
   2. ii)Noise temperature
   3. iii)Noise Bandwidth
   4. iv)Noise figure measurement
   How to answer

   • Define noise figure F as input to output SNR ratio
   • Define noise temperature Te and its relation to F
   • Define noise bandwidth of receiver/filter
   • Outline Y-factor / hot-cold method for noise figure measurement
9. 7 Marks
   aDraw the block diagram of FM receiver and explain each block.
   How to answer

   • Block diagram of FM superheterodyne receiver from antenna to audio
   • Function of RF amplifier, mixer, IF limiter stages
   • Role of discriminator/de-emphasis and audio amplifier
   • Need for limiter and AGC in FM reception
10. 7 Marks
    bProve that narrowband FM offers no improvement in SNR over AM.
    How to answer

    • SNR expressions for AM (DSB-FC) and NBFM with small beta
    • Show NBFM sidebands similar to AM with quadrature component
    • Derive output SNR ratio showing unity improvement factor
    • Conclude no SNR improvement for narrowband case
11. 7 Marks
    aFor an FM modulator with a modulating signal $m(t) = V_m \sin(300 \times 10^3 t)$, the carrier Signal $V_c(t) = 8 \sin (6.5 \times 10^6 t)$ and the modulator index $= 2$. Find out the significant side band frequencies and their amplitudes.
    How to answer

    • Compute $f_m$ and $f_c$ from $m(t)$ and $V_c(t)$ and $\beta=2$
    • Use Carson rule / Bessel table for beta=2 to find significant sidebands
    • List sideband frequencies $f_c \pm n f_m$
    • Calculate amplitudes $8\cdot J_n(2)$ for significant n
12. 7 Marks
    bDifferentiate A.M, DSB-SC and SSB-SC.
    How to answer

    • Compare spectra: carrier+2 sidebands vs suppressed carrier vs single sideband
    • Compare bandwidth and power efficiency
    • Compare generation/detection complexity
    • Tabulate applications of each
13. 7 Marks
    aWrite properties of impulse function and convolution theorem (time and frequency).
    How to answer

    • Define impulse $\delta(t)$ with sifting and sampling properties
    • List properties: even, scaling, time-shift
    • State time convolution $\leftrightarrow$ frequency multiplication
    • State frequency convolution $\leftrightarrow$ time multiplication
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 $I_t = I_c\sqrt{1+\mu^2/2}$ with $I_c=8$ A, $I_t=8.93$ A
    • Solve for modulation index $\mu$ / percentage modulation
    • Recompute $I_t$ for $\mu=0.8$
    • State results in Amps and percent
15. 7 Marks
    aWhen a broadcast AM transmitter is 50 percent modulated, its antenna current is 12 A. What will be current when the modulation depth is increased to 0.9?
    How to answer

    • Find unmodulated carrier current $I_c$ from $I_t=12$ A at $\mu=0.5$
    • Apply $I_t = I_c\sqrt{1+\mu^2/2}$
    • Substitute $\mu=0.9$ to get new current
    • Compute value ~12.8-13 A
16. 7 Marks
    bGive the applications of AM-FC and VSB modulation schemes.
    How to answer

    • Define AM-FC (conventional AM) and VSB
    • Applications of AM-FC: broadcast, aviation, emergency
    • Applications of VSB: TV video, telemetry
    • Reason: bandwidth saving with low-frequency preservation