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2003 Course Optical & Microwave Communication

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Total No. of Questions : 12] P1349 [Total No. of Pages :4 [3764] - 232 B.E. (E & TC) OPTICAL AND MICROWAVE COMMUNICATION (2003 Course) Time : 3 Hours] [Max. Marks:100 Instructions to the candidates: 1) Attempt Q.1 or Q.2, Q.3 or Q.4, Q.5 or Q.6 from section - I & Q.7 or Q.8, Q.9 or Q.10, Q.11 or Q.12 from section - II. 2) Answers to the two sections should be written in separate books. 3) Neat diagrams must be drawn wherever necessary. 4) Figures to the right indicate full marks. 5) Assume suitable data, if necessary. SECTION - I Q1) a) b) c) Compare surface Emitting & Edge Emitting structures of LED. [6] Explain the construction & working of APD with the help of neat diagram. [6] Find core radius necessary for single mode operation at 820 nm of step Index fiber with n1 = 1.480 & n2 = 1.478. What is NA and maximum acceptance angle of fiber? Also calculate corresponding solid angle. [6] OR Q2) a) A double heterojunction In GaAs P LED emitting at a peak wavelength of 1310 nm has radiative & nonradiative recombination times of 30 nsec & 100 nsec resp. The drive current is 40 mA. Determine total recombination life time, internal quantum efficiency & internal power level of the source. [6] b) Explain the three transmission windows along with the attenuation curve for single mode fiber. [6] c) Draw the structure of PIN photodiode & explain its operation in brief. Plot the responsivity curve as a function of wavelength for PIN constructed of silicon. [6] P.T.O. Q3) a) Explain the major causes of attenuation along with their mechanism in an optical fiber. [6] b) Determine the maximum bit rate for RZ & NRZ encoding for the following pulse sperading constants & cable lengths. [4] i) t = 10 ns/m, L = 100 m ii) t = 20 ns/m, L = 1000 m iii) t = 2000 ns/m, L = 2 km c) What is dispersion? Explain intermodal dispersion in detail. [6] OR Q4) a) Explain bending losses in the fiber. [4] b) What are the linear scattering losses in the fiber. [6] c) A multimode SIF has NA of 0.3 and a core R.I of 1.45. The material dispersion parameters for the fiber is 250 ps nm-1 km-1 which makes material dispersion the totally dominating intramodular dispersion mechanism. [6] Estimate i) The total RMS pulse broadening per km when fiber is used with a LED source of RMS spectral width of 50 nm. ii) The corresponding BW-length product for the fiber. Q5) a) A D-IM analog optical fiber link of length 2 km employs an LED which launches mean optical power of -10 dBm into a multimode optical fiber. The fiber cable exhibits a loss of 3.5 dB km-1 with splice losses calculated at 0.7 dB km-1. In addition there is a connector loss at the receiver of 1.6 dB. The pin photodiode receiver has a sensitivity of -25 dBm for an SNR of 50 dB and with a modulation index of 0.5. It is estimated that a safety margin of 4 dB is required. Assuming there is no dispersionequalization penalty : [8] i) Perform an optical power budget for the system operating under the above conditions and ascertain its viability. ii) Estimate any possible increase in link length which may be achieved using an injection laser source which launches mean optical power of 0 dBM into the fiber cable. In this case the safety margin must be increased to 7 dB. What are the applications of optical amplifiers? State advantages of each of them. [8] b) OR [3764]-232 2 Q6) a) Write a short note on 2*2 fiber coupler. [4] b) Explain basic structure of STS - 1 SONET frame. [6] c) Explain in detail the concept of photonic switching. [6] SECTION - II Q7) a) An air filled rectangular waveguide has dimensions of 6 cm 4 cm. The signal frequency is 3GHz. Compute the following for TE10 mode. [6] i) Cut off frequency. ii) Wavelength in the waveguide. iii) Wave impedance in the waveguide. b) Write a short note on waveguide termination. [6] c) Differentiate between TM & TE mode in rectangular waveguide. [6] OR Q8) a) Define scattering matrix & state its properties for a reciprocal & lossless [6] network. b) Explain directional coupler. Define coupling coefficient, directivity & [8] insertion loss. c) Write a short note on - Microwave Isolators. Q9) a) b) [4] Compare two cavity klystron & Reflex klystron with relevant sketches.[8] The parameters of a two cavity klystron amplifier are [8] Vo = 1200V, Io = 30 mA, F = 10 GHz Gap spacing in each cavity (d) = 1 mm Spacing between two cavities (L) = 4cm Effective shunt resistance of each cavity = 40 k . i) Find the input microwave voltage V1 in order to generate maximum output voltage. ii) Determine the voltage gain. iii) Calculate the efficiency of the amplifier. [3764]-232 3 OR Q10)a) A helical slow wave structure has a pitch p of 2 mm & a diameter of 4 cm. Calculate the wave velocity in axial direction of helix. [4] b) Explain how mode jumping is avoided in magnetron. [4] c) Why magnetron is called crossfield devices? Explain how oscillations sustained in magnetron. [8] Q11)a) What are the limitations of conventional tubes at microwave frequencies? Explain how these oscillations can be overcome. [8] b) Explain principle of operation, IV chara. and equivalent circuit of microwave tunnel diode. [8] OR Q12)a) Draw constructional details & equivalent circuit of varactor diode. Explain its operation with any four applications. [8] b) A GaAs Gunn diode has an active region length of 10 m. If the electron drift velocity is 105 m/s, find the natural frequency & the critical voltage. The critical electric field is 3 kV/cm. [6] c) List the different operating modes of Gunn diode. kbkb [3764]-232 4 [2]

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