Microwave & Radar Engineering - Dec 2012

Electronics & Telecomm. (Semester 7)

TOTAL MARKS: 80
TOTAL TIME: 3 HOURS (1) Question 1 is compulsory.
(2) Attempt any three from the remaining questions.
(3) Assume data if required.
(4) Figures to the right indicate full marks. 1 (a) State and explain Lorentz Reciprocity theorem.(5 marks) 1 (b) Enumerate and explain the advantage and application of Microwaves.(5 marks) 1 (c) Differentiate between Waveguide and Transmission Lines.(5 marks) 1 (d) A 50Ω transmission line is matched to a 10V source that feeds a load Z_L=100Ω. If the line is 2.3λ long and has an attenuation constant α=0.5dB/λ. Find the power that are delivered by the source lost in the line and delivered to the load.(5 marks) 2 (a) Explain the working and derive S matrix for a two hole directional coupler.(10 marks) 2 (b) Derive the expression for velocity modulation process for two cavity klystrons.(10 marks) 3 (a) Derive expression for phase velocity, cut off frequency, cut off wavelength and field equation for circular waveguide.(10 marks) 3 (b) A lossless line of characteristic impedance R₀=50 Ω is to be matched to a load Z_L=50[2+j(2+√3)]Ω by means of a lossless short-circuited stub. The characteristic impedance of the stub is 100 Ω. Find the stub position and length so that a match is obtained.(10 marks) 4 (a) Explain the working of a negative resistance Parametric amplifier and explain its application .
(10 marks) 4 (b) An n GeP Ga Asn GaAs heterojunction transistor at 300^oK has the following Parameter ?
Donor density in n Ge region: N_d=5x10¹⁸ cm^-3
Acceptor density in P GaAs region: N_a=6x10¹⁶ cm^-3
Hole life time: τ_p=6x10^-6 sec
Bias voltage at emitter junction: V_E=1
Cross section: A=2x10^-2 cm^-2
Compute :-
(i) The built in voltage in the P GaAs side
(ii) The hole mobility
(iii) The hole diffusion constant
(iv) The minority hole density in the nGe region
(v) The minority electron density in the P GaAs region
(vi) The diffusion length
(vii) The emitter junction current
(10 marks) 5 (a) A reflection klystron operates under following condition:V_O=600V, L=1mm, R_sh=15KΩ,e/m=1.759x10¹¹, f_r=9GHz.
The tube is oscillating at f_r at the peak of the n=2 mode or 1(3/4) mode.Assume that the transit time through the gap and beam loading can be neglected.
(i) Find the value of repeller voltage V_r
(ii) Find the direct current necessary to give a Microwave gap voltage of 200V
(iii) What is electric efficiency under this condition.(10 marks) 5 (b) Explain different mode of Gunn diodes .(6 marks) 5 (c) Explain measurement of dielectric constant .(6 marks) 6 (a) Design a composite low pass filter by image parameter method with following specification-
R_O=50 Ω, f_c=50MHz, f_∞=52MHz(10 marks) 6 (c) Compare advantage and disadvantage of a GaAs MOSFET with Si MOSFET(4 marks) 6(b) What are different limitation of conventional tube .(6 marks)

Write short notes on:

7 (a) Microwave filters .(5 marks) 7 (b) Magic tee .(5 marks) 7 (c) TWT .(5 marks) 7 (d) Show that TM₀₁ and TM₁₀ modes in a rectangular waveguide do not exist.(5 marks)