P.B.COLLEGE OF ENGINEERING, CHENNAI
Department of ECE

MODEL EXAM (October 2008)
                  
Sub: EC1301-Communication Theory
Marks: 100 										Duration: 3 HRS

Part A (10x2=20)

1. Define Carson’s BW Rule for FM
2. What are the merits of indirect FM?
3. Draw the phasor diagram of AM
4. What are the applications of phase locked loop?
5. Define thermal noise. Give the expression for the thermal noise voltage across a resistor.
6. Define Kraft Inequality.
7. What is threshold effect of FM?
8. What is the purpose of pre emphasis and de emphasis in Fm?
9. Define Entropy.
10. What is a prefix code?                                                

Part B(5x16=80)

11. (a)(i) Draw the circuit diagram of Balanced modulator and explain generation of DSBSC wave(8)
(ii) Explain SSB generation using modified phase shift method (8) 
(or)
(b) (i) Draw the circuit diagram of a ring modulator and explain its operation . (8)
     (ii) Discuss coherent detection of DSBSC modulated wave with a block diagram of the detector. (8)

12. (a) (i) Derive the expression for the frequency modulated signal. Explain what is meant by 
      narrow band FM and wideband FM using the expression.
(ii) Discuss the indirect method of generating a wideband FM signal.
(Or)
(b) (i) Explain FM stereo multiplexing with block diagram (8)
     (ii) Draw the circuit diagram of balanced slope detector and explain (8)
13. (a) (i) What is meant by noise equivalent bandwidth? Illustrate it with a diagram. (8)
      (ii) What is a narrow band noise? Discuss the properties of the narrowband noise.
(Or) 
(b) ( i) Derive the expression for output SNR for a DSBSC receiver using coherent detection .(8)
      (ii) Write short notes on noise in SSB receivers. (8)

14. (a) (i) Explain the noise in AM receiver using its noisy model block diagram. (8)
 (ii) What are pre emphasis and de emphasis in FM? Draw suitable circuits and Explain. (8) 
(Or)
(b) (i) Explain the capture effect and FM threshold effect. (6)
(ii) Draw the block diagram of FM demodulator and explain the effect of noise in detail and compare the performance of AM and FM in the presence of noise (10)

15 (a) (i) What is entropy? Explain the important properties of entropy.
    (ii) Five symbols of the alphabet of a discrete memory less source and their probabilities are   
    {x1,x2,x3,x4,x5} and {0.2,0.3,0.1,0.25,0.15} compute Shannon and Huffman codes. 
(Or)
(b)	A binary symmetric channel cascaded as is given below

	
(a) Derive a Resultant Channel matrix   
b) Draw the Resultant channel graph 
c)  Calculate output probabilities when p(x1) and p(x2) = 0.5 and 0.5  
(d) Calculate p(x,y), H(X) ,H(Y) and H(Z)