# Mumbai University Previous year question papers

## Microwave devices and Circuits

N.S.:

Question No.1 is compulsory.

Attempt any four question out of remaining six questions.

Assume any suitable data wherever required but justify the same.

1. Attempt any four from the following :- 20

(a) Comparethe opticalfibercommunicationwith microwavecommunicationsystem.

(b) Describe in brief direct band gap and indirect bandgap semiconductors.

(c) WhyTM01 orTM10 mode is not possible in rectangular waveguide ? Calculate the cutoff frequency for TE10′ TE11 and TM11modes for a rectangular guide having dimension 4cm x 2cm.

(d) What is dispersion in optical fibers? How does it affect the performance of the fiber optic link.

(e) State and explain the symmetry property of S-matrix.

2.

(a) Obtain an expression for the modulated velocity of the electron beam in klystron amplifier in terms of the beam coupling co-efficient Pi and the average transit angle ego

(b) A two cavity rlystron amplifier operating at 5GHz has the following parameters:

Vo = 800 V, 10= 20 mA

Gap spacing d = 1mm,.spacing between two cavities L = 4cm. Find –

(i) the electron velocity just leaving the Cathode

(ii) the gap transit angle

(iii) the beam coupling co-efficient

(iv) the dc transit angle between the cavities.

3. (a) Describe different types of modes that are supported by an optical fiber. What is the significance of cutt off no. of fiber?

(b) Calculate the cross sectional area of a multimode step index fiber with a V no., . V = 100 and a N.A. = 0.3 This fiber will., be used in a data link with a 0.82 /-lm LED (Take n1 = 1.458).

(c) . With the help of a neat schematic, explain the Modified Chemical Vapour Deposition (MCVD) Technique of fabricating an optical fiber. >

4. (a) With the help of suitable diagrams, explain the mechasnim of operation of an 8 cavity magnetron operating in the dominant 1t mode. How are the various modes separated in a magnetron?

(b) A 400 kW cylindrical magnetron operating at x-band has th~ following set of parameters:- anode Voltage Vde  =32 kV,Beam current Ide=84 A Radius fo cathode cylinder (~c) =6cmRadius of anode cylinder (~a) =12 cm Magneticfluxdensity (B)=0.01 Wb/m2 Calculate: (i) The cyclotron angular frequency

(ii) The cut off magnetic flux density for a fixed Vde

iii) The cut off voltage for a fixed Bo and

(iv) The efficiency (,,)

5. (a) Definequantum efficiencyand responsivity of a photodetctor. Derive an expression forthe responsivity of an intrinsicphotodetectors interms of the quantum efficiency of the device and the wavelength of the incident radiation.

(b) 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.

6.(a) Differentiatebetween :-

(i) TWTAand Klystron

(ii) IMPATTdiode and Gunn diode

(b) Distinguish between spontaneous emission and stimulated emission. How stimulated emussion is achieved in LASER.

7. Writeshort notes on any four :-

(a) Cavity Resonator

(b) Multiplexingtechniques in fiber optic system

(c) Measurement of microwave power