## Analog Communication - Dec 2014

### Electronics & Communication (Semester 5)

TOTAL MARKS: 100

TOTAL TIME: 3 HOURS
(1) Question 1 is compulsory.

(2) Attempt any **four** from the remaining questions.

(3) Assume data wherever required.

(4) Figures to the right indicate full marks.
**1(a)** Show that the area under PDF curve is always equal to unity.(5 marks)
**1(b)** explain cross correlation functions. Give the properties of cross correlation function.(7 marks)
**1(c)** A three digit message is transmitted over a noisy channel having a probability of error $$P(E)=\frac{2}{5}$$ per digit. Find out the corresponding CDF.(8 marks)
**2(a)** Explain the generation of AM wave using square-law modulator along with relevant diagrams and analysis.(7 marks)
**2(b)** With a neat block digram.explain the balanced modulator method of generating DSB - SC wave.(7 marks)
**2(c)** Consider a message signal m(t) = 20 cos (2\pi t)volts and a carrier signal is c(t)= 50 cos $$(100\pi t )$$ volts.

i) Sketch to scale resulting AM wave for 75% modulation.

ii) Find the power delivered across a load of 100$$\Omega $$ due to this AM wave.(6 marks)
**3(a)** Define : i) Hilbert transform; ii) Pre - envelope.(8 marks)
**3(b)** With a neat digram,explain how SSB wave is generated using phase discriminator method with only USB and rejecting the LSB.(8 marks)
**3(c)** Find the Hilbert transform of a signal $$x(t)=cos2\pi f_{1}t\ sin \ 2\pi \ f_{2}t\ where f_{2}$$ is message signal and x(t) = cos 2?f_{1}t sin 2?_{2} Where _{2} is message signal and f_{1} is carrier signal.(4 marks)
**4(a)** Explain with relevant equations the working of envelope detection of USB wave plus carrier.(6 marks)
**4(b)** With a neat block diagram,explain the working of FDM technique.(7 marks)
**4(b)** With the block digram of super heterodyne receiver and specify the importance of IF value in the receiver.List the advantages of such receivers.(7 marks)
**5(a)** Derive the equation for FM wave. Define modulation index,maximum deviation and band width of a FM signal.(8 marks)
**5(b)** Explain FM generation using direct method.(7 marks)
**5(c)** An FM wave is defined by s(t) 10 cos (2-sin(6?t)),Find the instantaneous frequency of s (t).(5 marks)
**6(a)** Explain the working of a balanced slope detector used for FM demodulation.(7 marks)
**6(b)** Explain FM demodulation using zero crossing detectors.(5 marks)
**6(c)** With a neat block diagram, explain the working of FM stereo multiplexing and demultiplexing.(8 marks)
**7(a)** What is noise equivalent bandwidth? Derive an expression for the same.(8 marks)
**7(b)** Derive Friis formula for amplifiers connected in cascade.(8 marks)
**7(c)** An amplifier with 10db nose figure and 4db power gain is cascaded. With a second amplifier has a 10db power gain and 10db noise figure. What is the overall noise figure and power gain?(4 marks)
**8(a)** Derive the equation for output signal to noise ration for a DSB - SC system.(7 marks)
**8(b)** Explain the working of pre-emphasis and de-emphasis in FM.(6 marks)
**8(c)** Show that the signal to noise ratio of the output for a single tone FM is given by $$SNR_{0}=\frac{3E^{2}_{c}(mf)^{2}}{4\ No\ W}$$ Where the symbols have usual notations.(7 marks)