EC-403 Analog Communication - Jun 2025 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
   aWhat is Gate function? Determine the Fourier transform of Gate function.
   How to answer

   • Define Gate (rectangular) function with mathematical expression and waveform
   • Set up Fourier transform integral for Gate function
   • Evaluate integral to get sinc function form $\tau \mathrm{sinc}(f\tau)$
   • Sketch magnitude spectrum and mention properties
2. 7 Marks
   bExplain Auto and Cross Correlation in brief. Also discuss energy and power spectral density in brief.
   How to answer

   • Define auto-correlation and cross-correlation with formulas
   • Explain properties and physical significance in brief
   • Define energy spectral density and power spectral density
   • Give relation with autocorrelation via Wiener-Khintchine theorem
3. 7 Marks
   aDiscuss the phase discrimination method for generation of SSB-SC signal with suitable diagram.
   How to answer

   • State principle of phase discrimination / phase-shift method for SSB-SC
   • Draw block diagram with two balanced modulators, 90 degree phase shifters and adder
   • Explain operation for LSB/USB selection
   • Mention advantages and limitations
4. 7 Marks
   bAM system is replaced by SSB-SC system. If modulation index is half then calculate the % power saving.
   How to answer

   • Write total power in AM in terms of modulation index $\mu$
   • Write power in SSB-SC (only one sideband)
   • Substitute $\mu/2$ for SSB-SC case as stated
   • Calculate % saving = $(P_{AM}-P_{SSB})/P_{AM} \times 100$
5. 7 Marks
   aExplain the basic needs of modulation with suitable diagram of communication system.
   How to answer

   • Draw basic block diagram of communication system (source, transmitter, channel, receiver)
   • Explain need for modulation: antenna size, multiplexing, narrowbanding
   • Explain need: noise immunity, frequency translation, range extension
   • Summarize with suitable diagram
6. 7 Marks
   bDrive the time domain equation for AM modulation.
   How to answer

   • Define carrier $c(t)$ and message $m(t)$ signals
   • Write general AM expression $s(t)=A_c[1+\mu m(t)]\cos(2\pi f_c t)$
   • Show derivation steps for single-tone case
   • Mention meaning of modulation index
7. 7 Marks
   aDefine Angle modulation. Write the equation of narrow band FM signal.
   How to answer

   • Define angle modulation (FM and PM)
   • Write general angle modulated expression
   • Apply narrowband approximation $\beta \ll 1$
   • Write NBFM equation showing carrier plus sidebands similar to AM
8. 7 Marks
   bWrite short note on: i) Pre emphasis and de-emphasis ii) PLL detector
   How to answer

   • Explain pre-emphasis: boosting high frequencies at transmitter with circuit
   • Explain de-emphasis: attenuation at receiver to improve SNR
   • Describe PLL detector principle with block diagram
   • Mention applications in FM broadcasting
9. 7 Marks
   aDescribe foster-Seeley discrimination method for FM detection.
   How to answer

   • Draw circuit/block diagram of Foster-Seeley discriminator
   • Explain transformer double-tuned operation and phase relations
   • Show S-curve and frequency-to-voltage conversion
   • Mention alignment and limitations
10. 7 Marks
    bDefine Carson's rule. If the angle modulated signal is $S(t) = 10 \cos [2\pi f_ct + 5\sin 8\pi.10^3t]$ then determine the frequency and phase deviation.
    How to answer

    • Define Carson's rule for FM bandwidth $B_T=2(\Delta f+f_m)$
    • Identify phase deviation $\beta=5$ rad from given $S(t)$
    • Determine modulating frequency $f_m=4\times10^3$ Hz and frequency deviation $\Delta f=\beta f_m$
    • Calculate bandwidth using Carson's rule
11. 7 Marks
    aExplain the noise in AM and FM modulation in detail.
    How to answer

    • Write SNR expressions for AM (DSB, SSB) with noise
    • Explain FM noise performance and threshold effect
    • Compare AM vs FM under noise with formulas
    • Conclude FM superiority at high modulation index
12. 7 Marks
    bDefine image signal rejection. Also describe the block diagram of super-heterodyne receiver.
    How to answer

    • Define image frequency and image signal rejection
    • Draw block diagram of super-heterodyne receiver (RF, mixer, IF, detector)
    • Explain function of each block and image rejection
    • Mention advantages of superhet receiver
13. 7 Marks
    aDefine noise temperature. Compare the internal and external noise.
    How to answer

    • Define noise temperature $T_e$ with formula $N=kT_eB$
    • List internal noise sources (thermal, shot, flicker)
    • List external noise sources (atmospheric, extraterrestrial, man-made)
    • Compare internal vs external on basis of origin and control
14. 7 Marks
    bDerive the figure of merit for DSB-SC modulation.
    How to answer

    • Define figure of merit as output SNR / input SNR
    • Write signal and noise power at DSB-SC coherent detector input
    • Derive output SNR expression
    • Show final figure of merit = 1 for DSB-SC
15. 14 Marks
    Write short note on: i) VSB-SC ii) AGC iii) FM transmitter iv) Convolution theorem
    How to answer

    • Explain VSB-SC principle, filter and applications in TV
    • Explain AGC purpose, types and operation
    • Describe FM transmitter block diagram (direct/indirect)
    • State convolution theorem in time and frequency with proof outline