## Analog Communication - Jun 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)** Define auto correlation function of the random process x(t), Explain the properties of auto correlation function.(8 marks)
**1 (b)** Define the power spectral density and explain its properties.(7 marks)
**1 (c)** The PDF of random variable is given as $$ f, (x) = \left\{\begin{matrix}K &a \le x \le b \\ 0 & Otherwise
\end{matrix}\right. $$ Where K is constant,

i) Sketch the PDF and determine value of K.

ii) If a=-1 and b=2, calculate p(|x|)?C') for C=1/2.(5 marks)
**2 (a)** Explain the generation of AM wave using square law modulator with relevant equations and spectrum.(8 marks)
**2 (b)** Explain the working of costas receiver for demodulating DSB-SC wave.(6 marks)
**2 (c)** An audio frequency signal 10 sin 2Π 500t is used to amplitude modulate a carrier of 75 sin 2Π×10^{6}t. Assume modulation index as 0.5 Find:

i) Side band frequencies.

ii) Amplitude of each side band.

iii) Band width required

iv) Total power delivered to a load of 100Ω.(6 marks)
**3 (a)** Derive the expression for representing SSB wave containing LSB in time domain.(8 marks)
**3 (b)** Define Hilbert transform. Obtain Hilbert transform of the following:

i) x(t)=A_{c} cos 2Π f_{c}t.

ii) x(t)=A_{c} sin 2Πf_{c}t.(8 marks)
**3 (c)** Explain phase discrimination method for generating SSB wave.(6 marks)
**4 (a)** Derive time domain equation for VSB modulated wave containing a vestigial of the lower side band.(7 marks)
**4 (b)** With a neat block diagram, explain the operation of AM super heterodyne receiver.(6 marks)
**4 (c)** What is FDM? Explain the detailed scheme of FDM.(7 marks)
**5 (a)** With neat circuit diagram, explain direct method of generating FM wave. Also explain feed back scheme for frequency stabilization of a frequency modulator in direct method.(12 marks)
**5 (b)** An angle modulated signal is represented s(t)=10cos[2Π×10^{6}t+5 sin 2000 Πt+10 sin 3000Πt] Volt. Find the following:

i) The power in the modulated signal.

ii) The frequency derivation

iii) The derivation ratio.

iv) The phase derivation.

v) The approximate transmission band width.(8 marks)
**6 (a)** With neat circuit diagram, explain FM demodulation using balanced slope detector.(8 marks)
**6 (b)** Starting from block diagram of pLL obtain its non linear and liner model. Show that output of pLL is scaled version of modulating signal.(12 marks)
**7 (a)** Derive the expression for RMS noise voltage at the output of passive RC low-pass filter.(7 marks)
**7 (b)** Define white noise. Give the plot of PSD and auto correlation function of white noise.(7 marks)
**7 (c)** In a communication receiver, the first stage is a tuned amplifier with an available power gain of 20dB and noise figure of 10dB. The output of the amplifier is given to mixer stage. Whose noise figure is 20dB. Determine the overall noise figure of the system.(6 marks)
**8 (a)** Derive expression for the figure of merit for DSBSC receiver.(10 marks)
**8 (b)** Explain function of pre-emphasis and de-emphasis in FM systems.(10 marks)