THEORY EXAMINATION (SEM–VI) 2016-17 ANALOG AND DIGITAL COMMUNICATION

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=Ic2​2(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​=Ic​1+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​=21​e−2N0​Eb​​ 

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=Blog⁡2(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+5sin⁡6π103t)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=Lavg​H​ 

5. Radio receiver performance parameters

Sensitivity                                                                             Selectivity

Fidelity                                                                                 Noise figure

Superheterodyne advantages:

Better selectivity                                                                  Constant bandwidth

High gain                                                                             Improved sensitivity