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Posted By: jothi vignesh Member Level: Gold Posted Date: 07 Jul 2008
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2005 Anna University B.E DIGITAL COMMUNICATIONS Question paper
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Code: A-20
Time: 3 Hours Subject: DIGITAL COMMUNICATIONS
Max. Marks: 100
NOTE: There are 11 Questions in all.
Question 1 is compulsory and carries 16 marks. Answer to Q. 1. must be written in the space provided for it in the answer book supplied and nowhere else.
Answer any THREE Questions each from Part I and Part II. Each of these questions carries 14 marks.
Any required data not explicitly given, may be suitably assumed and stated.
Q.1 Choose the correct or best alternative in the following: (2x8)
a. The TEom modes of a circular waveguide have a unique characteristic in that the attenuation of these fields continues to
(A) increase at higher frequencies.
(B) decrease at higher frequencies.
(C) constant over the operating band.
(D) decrease at lower part of frequency based and then increases.
b. A Smith chart is used
(A) To calculate Current. Voltage.
(C) To calculate Power.
(B) To calculate
(D) To calculate Impedance.
c.
A Dominant mode in a rectangular wave guide is a
(A Non propagating mode.
(B) A propagating mode with lowest cut off frequency.
(C) Propagating mode with circular polarization.
(D) Hybrid mode.
d. A Microstrip is formed by
(A) One conductor between two ground planes and dielectric.
(B) One conductor and one ground plane and dielectric.
(C) One conductor surrounded by a circular ground plane and dielectric.
(D) Two conductors with one ground plane and dielectric.
e. A matched termination is a
(A) A tapered Capacitive load
(C) A tapered Resistive load. (B) A tapered Inductive load.
(D) Tuned circuit load.
f. PIN diode is used for
(A) Switching.
(C) Oscillation.
(B) Amplification.
(D) Attenuation.
g.
VSWR stands for
(A) Very Short Waves Receiver.
(C) Voltage Standing Wave Ratio.
(B) Voltage Standing Wave Range.
(D) Variable Source Wave guide
Resonator.
h.
Repeators are circuits
(A) To provide attenuation.
(C) To provide oscillation. (B) To provide feedback.
(D) To provide amplification.
________________________________________________________
PART I
Answer any THREE Questions. Each question carries 14 marks.
_________________________________________________________
Q.2 Derive the relevant equations for determining the characteristic impedance of a transmission line.
An open wire transmission line has and . The signal frequency is 4 GHz. Calculate
the characteristic impedance of the line.
the propagation constant of the wave along the line
(8)
Q. 3
A lossless transmission line has a normalised load admittance . The observed standing wave ratio on the line is 4.0. It is proposed to use a short circuited stub to match the load to the line.
Determine the electrical distance from the load to the place where the stub should be located.
Find the length of the stub in terms of wave length.
(14)
Q.4 Derive the relevant equations to compute the Wave impedance of TE mode in the waveguide. (7)
An air filled rectangular waveguide with dimensions of 2.0 cm x1.0 cm is operating at a frequency of 11 Ghz. Find the following :
(i) possible modes (ii) cut-off frequencies
(iii) guide wavelengths.
(7)
Q.5 Explain with the help of a neat figure, the microwave set up to measure the frequency of a wave in a waveguide. (7)
A slotted line is used to measure the frequency and it was found that the distance between nulls is 1.85 cm. Given the guide dimension as 3 cm x 1.5 cm, calculate the value of the frequency
Q.6
Explain two properties of Scattering matrix.
Find the Scattering matrix of a Magic Tee using the above principles.
(4)
(10)
________________________________________________________
PART II
Answer any THREE Questions. Each question carries 14 marks.
_________________________________________________________
Q.7 Explain the working of Travelling Wave Tube with neat diagrams.
(7)
In a helix-type travelling wave tube operating at 5 x 108 Hz, the pitch to the circumference ratio of the helix is 1 : 10 and the beam density is such that the plasma, frequency is . What accelerating voltage is required for the electron gun in order that the condition shall be fulfilled? Where , and are the phase constant of the transmission line model of the slow wave structure, the beam and in plasma respectively.
(7)
Q. 8 A standard x - band waveguide with inside dimension 2cm x 1 cm is connected to another waveguide with dimension 1.6cm x 0.8 cm. The latter guide is terminated in a matched load. Calculate VSWR that would be caused by this discontinuity at 10 GHz. (Reactance associated with the geometrical discontinuity may be ignored.)
Explain with neat diagram, the principle of operation of Magnetron tube.
(7)
(7)
Q.9
Explain the working of
(i) Isolator (ii) Power splitter with neat figures.
(14)
Q.10 A LOS communication link operating at 50 MHz has the transmitting antenna at a height of 20 m. The distance to the receiving site is 15 Km, use the flat earth interference to calculate the height of the receiving antenna for maximum received signal
(7)
In a communication link the antenna heights are 40 and 60 m. The distance between terminals is 30 Km. Assume standard refraction occurs. Find the path gain factor for this system when Assume a reflection coefficient of -1.
(7)
Q.11 Write short notes on any FOUR of the following :
(i) Microstripline.
(ii) Stripline.
(iii) Phase shifter.
(iv) E & H plane Tee.
(v) Varactor diode.
(vi) Troposcatter Propagation (31/2 x 4 = 14)
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