EC-403 Analog Communication - Nov 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
   aFind the Fourier Transform of the following signals.

   1. i)Sinusoidal
   2. ii)Co-sinusoidal
   3. iii)Signum
   4. iv)Step
   How to answer

   • Define Fourier Transform pair
   • Derive/write transform of sine and cosine as impulses
   • Derive/write transform of signum function $2/(j\omega)$
   • Derive/write transform of unit step with impulse plus $1/j\omega$ term
2. 7 Marks
   bExplain the significance of Auto correlation and Cross Correlation.
   How to answer

   • Define auto-correlation for energy/power signals
   • Define cross-correlation between two signals
   • Explain significance in detection, synchronization and PSD via Wiener-Khinchin
   • Give examples in radar, communication receivers
3. 7 Marks
   aExplain the need for modulation.
   How to answer

   • Define modulation and baseband limitations
   • Explain antenna size, narrowbanding and multiplexing needs
   • Explain long-distance transmission and noise considerations
   • Summarize with practical examples
4. 7 Marks
   bDiscuss the generation and demodulation of AM signals.
   How to answer

   • Explain AM generation methods e.g. square-law, switching, balanced modulator
   • Draw block diagrams of modulator
   • Explain envelope detector / synchronous demodulation
   • Mention conditions for distortion-free detection
5. 7 Marks
   aWhat is the difference between FM and PM? Explain the significance of pre-emphasis and de-emphasis.
   How to answer

   • Define FM and PM expressions and instantaneous frequency/phase relation
   • Compare modulation index, bandwidth dependence
   • Define pre-emphasis and de-emphasis purpose
   • Explain SNR improvement in FM broadcasting
6. 7 Marks
   bExplain the PLL method of detection of FM signals.
   How to answer

   • Draw PLL blocks: phase detector, LPF, VCO
   • Explain lock and tracking of FM instantaneous frequency
   • Explain how VCO control voltage recovers message
   • Mention advantages and applications
7. 7 Marks
   aDraw a neat block diagram of a super-heterodyne receiver and explain its operation.
   How to answer

   • Draw neat labelled block diagram: RF amp, mixer, local oscillator, IF, detector, AGC
   • Explain frequency translation to fixed IF
   • Explain tuning, amplification and tracking operation
   • Mention advantages over TRF
8. 7 Marks
   bWrite a short note on the following

   1. i)Image signal rejection
   2. ii)Selectivity
   How to answer

   • Define image signal and how it arises in superhet
   • Explain methods of image rejection
   • Define selectivity and its dependence on IF and Q
   • Explain trade-offs and role of RF/IF filters
9. 7 Marks
   aExplain the different sources of noise.
   How to answer

   • Classify external vs internal noise sources
   • Explain thermal, shot, flicker, burst, atmospheric, man-made noise
   • Give formulas e.g. Johnson noise where relevant
   • Summarize effect on communication systems
10. 7 Marks
    bExplain the terms:

    1. i)Noise figure
    2. ii)Noise temperature
    3. iii)Noise bandwidth
    How to answer

    • Define noise figure/Factor with formula $F=SNR_{in}/SNR_{out}$
    • Define noise temperature $T_e=(F-1)T_0$
    • Define noise bandwidth / equivalent bandwidth
    • Explain measurement significance
11. 7 Marks
    aExplain briefly about AGC and AFC.
    How to answer

    • Define AGC purpose and working with block diagram
    • Define AFC purpose and working
    • Compare control parameters: gain vs frequency
    • Mention applications in receivers
12. 7 Marks
    bOn an FM system, the modulating frequency is 1 kHz, its amplitude is 3V and deviation is 4 kHz.

    1. i)If the modulating voltage is now increased to 5V, what is the new deviation?
    2. ii)If the modulating voltage and frequency is increased to 10V and 400 Hz respectively, find the new deviation and modulation index.
    How to answer

    • Use $\Delta f \propto A_m$ to find new deviation for 5V
    • Use $\Delta f$ scaling for 10V case
    • Compute modulation index $\beta=\Delta f/f_m$ for new $f_m=400$ Hz
    • State results with units kHz
13. 7 Marks
    aA carrier wave $4 \sin (2\pi \times 500 \times 10^3t)$ volts is amplitude modulated by an audio wave $[0.2 \sin(3(2\pi \times 500t) +0.1 \sin 5(2\pi \times 500t) ]$ volts. Determine the upper and lower side bands and sketch the complete spectrum of the modulated wave. Estimate the total power in the sideband.
    How to answer

    • Write AM expression and identify $f_c=500$ kHz, $f_{m1}=1.5$ kHz, $f_{m2}=2.5$ kHz
    • Calculate modulation indices and USB/LSB frequencies
    • Sketch spectrum with carrier and sidebands labelled
    • Estimate sideband power $P_{SB}=P_c(\mu_1^2+\mu_2^2)/2$
14. 7 Marks
    bDiscuss why FM is more immune to noise than AM systems?
    How to answer

    • Explain AM carries information in amplitude affected by noise
    • Explain FM limiter and constant envelope property
    • Explain capture effect and amplitude noise rejection
    • Conclude with SNR comparison
15. 7 Marks
    aExplain about the COSTAS Loop with a neat diagram for the demodulation of DSB-SC waves.
    How to answer

    • Draw COSTAS loop: I/Q multipliers, LPFs, VCO, 90-degree shifter
    • Explain in-phase and quadrature paths multiplication
    • Explain error signal generation and VCO locking for coherent carrier
    • Explain DSB-SC demodulated output recovery
16. 7 Marks
    bDiscuss briefly about Narrow band FM and Wide band FM
    How to answer

    • Define NBFM with $\beta << 1$ and spectrum like AM
    • Define WBFM with $\beta >> 1$ and wide Bessel sidebands
    • Compare bandwidth via Carson rule and generation methods
    • Mention applications of each