EC-403 Analog Communication - Dec 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
   aDefine power spectral density. Also explain the properties of Fourier transform.
   How to answer

   • Define power spectral density as Fourier transform of autocorrelation / power per unit frequency
   • State relation to total power by integration
   • List and explain Fourier transform properties: linearity, time/frequency shifting, scaling, duality, convolution, modulation, Parseval
2. 7 Marks
   bDetermine the Fourier transform of following:

   1. i)Unit Impulse
   2. ii)Unit step
   3. iii)Signum
   How to answer

   • Write definition of Fourier transform pair
   • Derive transform of unit impulse as 1
   • Derive transform of unit step using signum / duality
   • Derive transform of signum function
3. 7 Marks
   aDescribe the envelope detector method for the generation of AM signal with suitable diagram.
   How to answer

   • State envelope detection principle for AM demodulation
   • Draw circuit: diode, RC load, coupling
   • Explain charging/discharging and time-constant condition
   • Mention diagonal clipping and threshold condition
4. 7 Marks
   bCompare the AM, DSB-SC and SSB-SC modulation technique with their waveforms.
   How to answer

   • Write time-domain expressions for AM, DSB-SC, SSB-SC
   • Sketch carrier, message and modulated waveforms for each
   • Compare bandwidth, power efficiency and carrier content
   • Summarize in table
5. 7 Marks
   aDescribe the direct method for generating the FM signal with suitable diagram.
   How to answer

   • State principle of direct FM: VCO / varactor / reactance modulator
   • Draw block diagram: message + VCO / LC oscillator with variable reactance
   • Explain frequency deviation proportional to message amplitude
6. 7 Marks
   bA $100\text{ MHz}$ carrier is frequency modulated by a sinusoidal signal of amplitude $20\text{V}$ and frequency of $100\text{ KHz}$. If the frequency sensitivity of a modulator is $25\text{ KHz/Volts}$. Determine the modulation index, frequency deviation and bandwidth.
   How to answer

   • Compute frequency deviation $\Delta f = k_f A_m$
   • Compute modulation index $\beta = \Delta f / f_m$
   • Compute bandwidth by Carson rule $BW = 2(\Delta f + f_m)$
7. 7 Marks
   aCompare Narrowband FM and AM modulation with waveform.
   How to answer

   • Write expressions for NBFM and AM signals
   • Sketch spectra/waveforms showing carrier plus sidebands
   • Compare bandwidth, modulation index and vector representation
   • Highlight similarity for small beta
8. 7 Marks
   bWrite short note on: AGC, AVC, Fidelity.
   How to answer

   • Define AGC and its need in receivers
   • Define AVC action
   • Define fidelity as reproduction accuracy vs frequency
   • Give applications / circuits
9. 7 Marks
   aExplain the limitation of TRF receiver in detail.
   How to answer

   • Describe TRF block: RF stages, detector, audio stages
   • Explain need for ganged tuning and high gain at RF
   • List limitations: selectivity variation, instability, bandwidth, tuning difficulty
10. 7 Marks
    bDraw and explain the super heterodyne receiver with its applications.
    How to answer

    • Draw block diagram: RF, mixer, LO, IF, detector, audio
    • Explain heterodyning to fixed IF and each block function
    • List applications: AM/FM broadcast, TV, radar
11. 7 Marks
    aDerive the figure of merit for AM modulation.
    How to answer

    • Define figure of merit as output SNR over input SNR
    • Write AM signal and noise power expressions
    • Derive $FOM = \mu^2/(2+\mu^2)$ for tone modulation
    • Discuss efficiency implication
12. 7 Marks
    bWhat is noise? Explain the classification and sources of noise.
    How to answer

    • Define noise as unwanted random signal
    • Classify external vs internal: atmospheric, industrial, thermal, shot, flicker
    • List sources with brief mechanism
    • Mention effect on communication
13. 7 Marks
    aExplain the synchronous detector for AM modulation.
    How to answer

    • State coherent detection principle needing carrier
    • Draw block: multiplier + LPF with local carrier
    • Explain multiplication and filtering to recover message
    • Mention phase synchronization requirement
14. 7 Marks
    bDescribe the balanced modulator for the generation of AM signal.
    How to answer

    • State need for DSB-SC generation by carrier suppression
    • Draw balanced modulator using two AM modulators / diodes / FETs
    • Explain cancellation of carrier and message to leave sidebands
    • Give output expression
15. 7 Marks
    aExplain the block diagram of FM transmitter.
    How to answer

    • Draw FM transmitter blocks: audio, pre-emphasis, VCO/modulator, multiplier, power amp
    • Explain function of each stage
    • Mention wideband generation via multiplication if needed
16. 7 Marks
    bWrite short note:

    1. i)IF frequency
    2. ii)Noise bandwidth
    3. iii)VSB-SC
    How to answer

    • Define IF frequency and its typical values
    • Define noise bandwidth / equivalent bandwidth
    • Define VSB-SC principle and applications in TV
    • Give concise notes with formulas/diagrams