# GTU Last year question papers

GUJARAT TECHNOLOGICAL UNIVERSITY

BE- Vth 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.

(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

(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.