About this previous year paper
ANALOG AND DIGITAL COMMUNICATION (EEC609)
Time: 3 Hours Max Marks: 100
SECTION – A (Short Answer Questions)
(10 × 2 = 20 Marks)
(a) Modulation
Modulation is the process of varying one or more parameters (amplitude, frequency, or phase) of a high-frequency carrier signal in accordance with the message signal.
(b) Two advantages of Digital communication over Analog
High noise immunity
Easy error detection and correction
(c) Figure of merit
Figure of merit is a measure of receiver performance, defined as the ratio of output SNR to input SNR.
(d) Frequency deviation
Frequency deviation is the maximum change in carrier frequency from its unmodulated value due to modulation.
(e) Power content of AM signal
Given:
Carrier power Pc=100 kWP_c = 100 \, \text{kW}Pc=100kW
Modulation index m=0.6m = 0.6m=0.6
Total power:
Pt=Pc(1+m22)P_t = P_c\left(1 + \frac{m^2}{2}\right)Pt=Pc(1+2m2) Pt=100(1+0.362)=118 kWP_t = 100\left(1 + \frac{0.36}{2}\right) = 118 \, \text{kW}Pt=100(1+20.36)=118kW
(f) Why FSK is preferred over ASK
FSK is preferred because it is less affected by noise compared to ASK and provides better error performance.
(g) Information
Information is a measure of uncertainty reduction when a message is received.
(h) Nyquist sampling criterion
A signal must be sampled at least at twice the highest frequency component to avoid aliasing.
(i) Depth of modulation
Depth of modulation is the ratio of modulating signal amplitude to carrier amplitude.
(j) Entropy
Entropy is the average information content of a source, measured in bits.
SECTION – B (Long Answer Questions)
(Attempt any FIVE – 5 × 10 = 50 Marks)
2(a) Delta Modulation and its errors
Delta modulation transmits only the difference between successive samples.
Errors:
Slope overload distortion: When input changes rapidly
Granular noise: When input varies slowly
2(b) Square law modulator
Square law modulator uses a nonlinear device where output current is proportional to the square of input voltage. After filtering, an AM signal is obtained.
2(c) Numerical – Modulation index
Given:
Carrier current Ic=10AI_c = 10AIc=10A
Modulated current It=10.63AI_t = 10.63AIt=10.63A
m=2(It2−Ic2)Ic2m = \sqrt{\frac{2(I_t^2 - I_c^2)}{I_c^2}}m=Ic22(It2−Ic2) m=0.5=50%m = 0.5 = 50\%m=0.5=50%
For m=0.8m = 0.8m=0.8:
It=Ic1+m22=11.4AI_t = I_c\sqrt{1 + \frac{m^2}{2}} = 11.4AIt=Ic1+2m2=11.4A
2(d) Generation of FSK and probability of error
FSK uses different frequencies to represent binary 0 and 1.
Probability of error:
Pe=12e−Eb2N0P_e = \frac{1}{2}e^{-\frac{E_b}{2N_0}}Pe=21e−2N0Eb
2(e) Noise terms
Thermal noise: Due to random motion of electrons Shot noise: Due to discrete charge flow
Noise figure: Measure of noise degradation SNR: Ratio of signal power to noise power
Equivalent noise temperature: Represents noise power in terms of temperature
2(f) Channel capacity
Channel capacity: C=Blog2(1+SN)C = B \log_2(1 + \frac{S}{N})C=Blog2(1+NS)
Where B = bandwidth, S = signal power, N = noise power.
2(g) Generation of SSB-SC
SSB-SC is generated using: Filter method
Phase shift method
It transmits only one sideband, saving bandwidth and power.
2(h) Comparison TDM vs FDM
TDM uses time slots FDM uses frequency bands
FM vs PM
FM varies frequency M varies phase
SECTION – C (Very Long Answer Questions)
(Attempt any TWO – 2 × 15 = 30 Marks)
3. Indirect FM generation & numerical
Indirect FM uses phase modulation followed by frequency multiplication.
Given signal:
10cos(2π106t+5sin6π103t)10\cos(2\pi 10^6 t + 5\sin 6\pi 10^3 t)10cos(2π106t+5sin6π103t)
Frequency deviation:
Δf=βfm=5×3kHz=15kHz\Delta f = \beta f_m = 5 \times 3kHz = 15kHzΔf=βfm=5×3kHz=15kHz
Bandwidth (Carson’s rule):
BW=2(Δf+fm)=36kHzBW = 2(\Delta f + f_m) = 36kHzBW=2(Δf+fm)=36kHz
4. Huffman coding
Given probabilities: 0.25, 0.25, 0.125, 0.125, 0.125, 0.0625, 0.0625
Huffman tree is constructed by combining lowest probabilities.
Average code length and efficiency are calculated as: η=HLavg\eta = \frac{H}{L_{avg}}η=LavgH
5. Radio receiver performance parameters
Sensitivity Selectivity
Fidelity Noise figure
Superheterodyne advantages:
Better selectivity Constant bandwidth
High gain Improved sensitivity