Microwave Devices & Circuits - May 2012

Electronics Engineering (Semester 6)

TOTAL MARKS: 100
TOTAL TIME: 3 HOURS (1) Question 1 is compulsory.
(2) Attempt any four from the remaining questions.
(3) Assume data wherever required.
(4) Figures to the right indicate full marks. 1 (a) What are O-type and M-type devices in microwave?(5 marks) 1 (b) With the help of sketch distinguish between IMPATT and TRAPATT diode.(5 marks) 1 (c) What are slow wave structures? For what purpose are the slow wave structures used in microwave devices?(5 marks) 1 (d) What do you understand by the following terms:
1) Cut-off frequency
(2) Degenerate Mode
(3)Phase velocity
(4) Group velocity
(5) Dominant Mode. (5 marks) 2 (a) Derive the field equations for TE modes in rectangular waveguide. (10 marks) 2 (b) Prove that TE10 is the dominant mode of propagation in a rectangular waveguide.(5 marks) 2 (c) A TE11 mode is propagating through a circular waveguide. The radius of the guide is 5 cm, and the guide contains an air dielectric.
(i) Determine the cutoff frequency.
(ii) Determine the wavelength λg in the guide for an operating frequency of 3GHz.
(iii) Determine the wave impedance Zg in the guide.(5 marks) 3 (a) Draw a neat diagram of two cavity Klystron amplifier and explain the bunching process with Applegate diagram. Derive the equation of velocity modulation.(10 marks) 3 (b) A Reflex Klystron operates under the following conditions:
Vo = 600V, L = 1mm
Rsh = 15KΩ fr = 9GHz
The tube is oscillating at fr at the peak of the n = 2 mode. Assume that the transit time through the gap and beam loading can be neglected.
(i) Find the value of the repeller voltage Vr.
(ii) Find the direct current necessary to give a microwave gap voltage of 200V.
(iii) What is the electronic efficiency under this condtion? (6 marks) 3 (c) Explain excitation of modes in rectangular waveguides.(4 marks) 4 (a) What are cross field devices? Explain the working of cylindrical magnetron oscillator and ? mode of operation. (10 marks) 4 (b) An X-band pulsed cylindrical magnetron has the following parameters:
Anode voltage: Vo=26 kV
Beam current: Io=27A
Flux density: Bo=0.336Wb/m²
Radius of cathode cylinder: a=5 cm
Radius of vane edge to centre: b=10 cm
Compute
(i) The cyclotron angular frequency
(ii) The cut off voltage for a fixed B and
(iii) The cut off magnetic flux density for a fixed Vo. (10 marks) 5 (a) Explain Gunn Effect with the two valley model of Gunn diode. What are the criteria that a semiconductor must satisfy in order to exhibit negative resistance?(10 marks) 5 (b) Explain the working of microwave circulators.(5 marks) 5 (c) Why is Hybrid Tee called as the magic Tee? Derive the S-matrix of the E-H plane Tee.(5 marks) 6 (a) Draw and explain two-hole directional coupler. Define coupling factor, directivity and isolation. Derive S matrix for the same. (10 marks) 6 (b) For a rectangular waveguide, which propagates in the dominant mode at a frequency of 5 GHz, find 'a' and 'b'? The cutoff frequency of which is 0.8 of the signal frequency and the ration of the guide height to width is 2. Also determine:-
1. Phase velocity.
2. Group velocity.
3. Guided wavelength.(10 marks)

Write short notes on any four:-

7 (a) Measurement of VSWR in microwave.(5 marks) 7 (b) Travelling Wave Tube.(5 marks) 7 (c) Microwave transistors.(5 marks) 7 (d) Cavity resonators.(5 marks) 7 (e) Limitations of conventional tubes at microwave frequencies.(5 marks)