**GTU Last year question papers**

**GUJARAT TECHNOLOGICAL UNIVERSITY**

**BE- V****th ****SEMESTER–EXAMINATION – MAY/JUNE – 2012**

**Subject code: 151004 **

**Subject Name: Electronics Communication**

** **

**Instructions:**

**1. Attempt all questions.**

**2. Make suitable assumptions wherever necessary.**

**3. Figures to the right indicate full marks.**

**Q.1 **Answer the following

(i) Draw the block diagram of the basic communication system

(ii) What is modulation and why modulation is required in communication?

(iii)Define : Amplitude modulation Frequency modulation

(iv) Explain briefly : Noise factor Signal to Noise ratio

(v) Define : Figure of merit ( Q- factor )

Resonance frequency

(vi) Why IF is generated at receiver? List the factors influencing the choice of IF for radio receiver.

(vii) Write difference between FM and AM receivers

**Q.2 (a) **Draw and explain block diagram of the Superhetrodyne Receiver with necessary waveforms** **

**(b) **Draw and explain working of parallel tuned circuit. Derive expression for resonance frequency, Q-factor and impedance of parallel tuned circuit.

** ****OR**

**(b) **A high frequency transformer has identical primary and secondary circuits for which Lp = Ls = 150 μH , Cp = Cs = 470 pF and the Q-factor for each circuit alone is 85. The co-efficient of coupling is 0.001.The load resistance is 5000_ and the constant current source feeding the transformer has an internal resistance of 75_. Calculate the transformer impedance at resonance.

** ****Q.3 (a) **What are the natural sources of noise? Explain Shot noise, Partition noise and Flicker noise.

** ****(b) **Two resistance of 20K_ and 50 K_ are at room temperature (290K). For a bandwidth of 100 kHz ,calculate the Thermal noise voltage generated by (i) each resistor (ii) the two resistors in series (iii) the two resistors in parallel

**OR**

**Q.3 (a) **What tracking means in a superhetrodyne receiver? Explain three points tracking in radio receiver.

** ****(b) **What is image frequency? An AM broadcast receiver has an IF of 465 kHz and tuned to 1000 kHz and the RF stage has one tuned circuit with Q of 50. Calculate image frequency and image rejection in decibels.

** ****Q.4 (a) **(i)What advantages are of transform techniques? Write properties of Fourier Transform with their equations. (03)

(ii) Explain briefly :

signal energy and energy spectral density

signal power and power spectral density

** ****(b) **Draw and explain block diagram of double conversion superhetrodyne receiver with RF and IF stage response curve

** ****OR**

**Q.4 (a) **Compare DSB and VSB techniques for amplitude modulation with necessary waveforms and derivations.

** ****(b) **What are the different methods for generation of SSB signals? Explain SSB generation by phase shift method with necessary block diagram and derivations.

** ****Q.5 (a) **Compare and Contrast following :Wideband and narrowband FM Frequency modulation and phase modulation

** ****(b) **Draw only block diagram of Armstrong indirect FM transmitter and discuss the nature of distortion inherent in Armstrong indirect FM generator.

** ****OR**

**Q.5 (a) **(i) Define following for FM : Frequency deviation , Modulation index Deviation ratio , Percentage modulation

(ii) An angle-modulated signal with carrier frequency _c = 2_ × 105 is describe by the equation (05) _EM (t) = 10Cos(_ct + 5 Sin3000t +10 Sin2000_t)

Calculate power of the modulated signal , frequency

deviation , deviation ratio , phase deviation ratio and

estimate the bandwidth of _EM (t)

** **

**(b) **(i) Sketch _EM (t) for modulation indices μ = 0.5, m(t) = BCos _mt. Determine _ and percentage of total power carried by sidebands of the AM waves for tone modulation

(ii) Draw and explain circuit of envelope detector for AM.

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