# Mumbai University Previous year question papers

## Microwave devices and Circuits

NB:-

Question No.1 is compulsory.

Attempt any four questions from the remaining six questions.

Figures to the right indicate full marks.

List at least four advantages of fiber optic communication over other systems of communication.

State and explain the symmetry property of S-matrix.

What do you mean by VSWR ? What is its significance?

Explain why TEM waves cannot propagate in waveguides.

(a) Obtain an expression for the modulated velocity of the electron beam in klystron amplifier in terms \of ‘t,he.beam coupling coefficient Bi and the average transit angle 8g. A two,- cavity klystron amplifier has the following parameters:

Vo= 800 V, 10 = 20 mA, f = 5 GHz,Gap spacing in either cavity d = 1mm. Spacing between the two cavities L = 4 em Find :-

(i) The electron velocity just leaving the cathode

(ii) The gap transit angle

(iii) The beam coupling coefficient

(iv) The dc transit angle betweenthe cavities.

(b) Describe the operating principles of Tunnel diode, Gunn diode and LSA diode.

Describe the operation of IMPATT diode. Compare it with TRAPATT diode.

Explain the microwave circulators with S matrix.

An air filled rectangular waveguide of inside dimensions 7 x 3.5 cm operates  in the dominant TE10mode. Find the cut-off frequency. What is the change in the cut-off frequency if the waveguide is now filled with a dielectric material with Er = 2.5.

(a) A step index fiber with a large core diameter has an acceptable angle in air of 22° and a relative

refractive index difference of 3%. Estimate the numerical aperture and the critical angle at the core cladding interface for the fiber.

An optical fi~er is made of glass core of radius 50 11mwith a refr<;1ctiveindex of 1.55 and a cladding with a refractive index of 1.51. If the launching of the’ light ray takes place from air, find the following for the fiber;

(i) Numerical Aperture

(ii) Normalized

Frequency V at 0.8 mm (Hi) The number of modes that fiber can support at 0.8 um.

(b) Explain material dispession.

A continuous 12 km long optical fiber link has a loss of 1.5 dB/km.

(i) What is the minimum optical power that must be launched into the fiber to maintain an optical power level of 0.3 mW at the receiving end.

(ii) What is the required input power if the fiber has a loss of 2.5 dB/km.

(a) What are the advantages and drawbacks of LEOS in comparison with the injection lasers for use as a source in optical fiber communication.

Define quantum efficiency and .responsivity of a photo detector. Derive an expression for the responsivity of an intrinsic photo detector in terms of the quantum efficiency of the device and the wavelength of the incident radiation.

What is a heterojunction structure? How is it different from a homojunction? 4