1

(a) (b) (c)

Vidyalankar T.Y. Diploma : Sem. VI [EJ/ET/EX/EN/DE/ED/EI]

Advanced Communication System [ACS] Time : 3 Hrs.] Prelim Question Paper Solution [Marks : 100

Q.1(a) Attempt any THREE of the following : [16]Q.1(a) (i) Write RADAR range equation and state their factor affecting maximum range

of RADAR. [4]

(A) Rmax =

1/42

t2

min

P Ae S

4 π λ P

[2 marks]

where Rmax = maximum range Ae = capture area of antenna Pt = Transmitted power S = Effective surface area of target = signal wavelength pmin = Minimum receivable power The factor affecting maximum range are as follows [2 marks] (i) Transmitted power (ii) Frequency (f) (iii) Target crosssectional area (s) (iv) Minimum received signal (p min) Q.1(a) (ii) Compare Between waveguide and Two wire transmission line. [4](A) [Any 4 points – 1 marks each]

Waveguide Two wire transmission line 1) It is a hollow metallic pipe which carry

microwave energy. Two wire transmission line. It is conductor which carry electrical energy.

2) Dielectric losses are less. Dielectric losses are more. 3) For analysis, wave theory is considered For analysis, circuit theory is considered 4) It can handle more power It can handle less power 5) Rectangular circular waveguides are available Twisted pair, coaxial cable are available 6) Radiation, copper losses are less Radiation, copper losses are more.

Q.1(a) (iii)State four advantages of geostationary satellite. [4](A) Advantages of geostationary satellite [4 points - 1 marks each] (i) Three satellites gives global coverage with the exception of the polar regions. (ii) It requires limited earth station tracking since the statlite remains fixed in its position with respect. (iii) The communication channel can be either broadcast or point to point. (iv) Continuous communication is possible (v) It is not necessary to rotate dish antenna on earth. Q.1(a) (iv) State splicing technologies used for optical fiber? Explain Fusion splicing in

detail? [4]

(A) i) Fusion Splicing ii) Mechanical Splicing

Fusion Splicing [1 mark]

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2

[2 marks] The fusion splicing of single fiber involves the heating of two prepared fiber ends to their

fusing points with the application of axial pressure between two optical fiber. It is essential that shipped fiber ends are adequately positioned and aligned in order to

achieve good continuity at junction. Flame heating source such as microplasma torches are utilized & widely used heating

source is electric are, which gives consistant, easily controlled, heat under field condition. This heat may weaken the fiber.

Q.1(b) Attempt any One of the following : [6]Q.1(b) (i) What is dominant made? Explain How wave propagates through rectangular

waveguide? [6]

(A) [definition 1 mark, diagram 1 mark, explanation 4 marks] Dominant mode: it is mode for which cut off wavelength maximum. Propagation of wave

through the rectangular waveguide or parallel plane waveguide.

In connection with transmission line, standing waves & reflections are produced if a line is terminated in a short ckt voltage is zero & a current is maximum at this termination.

Rectangular waveguide has two pairs of walls & we shall considering their addition one pair at a time.

In one pair, second short is added to ensure that it doesn’t disturb the existing wave pattern. In above diagram three suitable positions for the second short circuit are indicated. Each of them

is at a point of zero voltage on the line & each is located at a distance from the first short circuit that is a multiple of half wavelength’s.

Electromagnetic wave are reflected due to presence of one wall & will set up reflected pattern,

that reflected pattern will be destroyed unless the second wall is placed three half wavelength away from first wall.

When top & bottom walls are added to our parallel plane waveguide, the result is standard

rectangular waveguide. The addition of these two wall do not affect any result. In practice however their presence is required to confine the wave.

y

b

z

a

y

x

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Prelim Question Paper Solution

3

Q.1(b) (ii) Explain the working principle and construction of PIN diode? Write two applications?

[6]

(A) [working principle 2 marks, construction 2 marks, Two application 2 marks] In Pin diode intrinsic larger is sandwiched between P & N region. GaAs is used in construction of PIN diode.

It has narrow larger of Ptype semiconductor separated from an equally narrow ntype material by thicker region of intrinsic material.

Working : [2 marks] PIN diode acts as ordinary diode of frequency upto 100 MHZ. At this frequency it works as rectifier. At microwave frequencies the diode acts as a variable resistance. When a bias is varied on a PIN diode it’s microwave resistance changes from 5 to 10 k

when bias & It changes from 1 to 10 when bias is positive If diode is mounted across 502 coaxial line, it will not significantly load the line when it is

back biased, so that power flow will be unaffected When diode is forward biased, its resistance becomes very low. So that most of power is

reflected and hardly any transmitted. Thus the diode is acting as a switch.

Application: [2 marks] 1. Used for microwave power switching limiting 2. Used as a pulse modulator.

Q.2 Attempt any Four of the following : [16]Q.2(a) List two advantages and two applications of circular waveguide? [4](A) [2 marks]

(i) Propagates both vertically and horizontally polarized wave in the same waveguide. (ii) Easy to manufacture and join together.

Application: [2 marks] (i) Rotating joints in radars to connect the horn antenna feeding a paroboloid reflector. (ii) For short and medium distance broad band communication.

Q.2(b) Justify magnetron as on oscillator? [4](A) [diagram 2marks, Explanation 2 marks]

Klystren, TWT all are linear tubes. Magnetron are cross field tubes in which magnetic fields are perpendicular to each other. In

cavity magnetron , there is interaction of e with a rotating electromagnetic field of constant angular velocity. These provides oscillations of very high peak power. [2 marks]

Fig. (b)

Cavity

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4

Construction: [2 marks] i) It consist of circular anode into which an even number of resonant cavities has been

machined. The diameter of each cavity is equal to 1/2 wavelength of desired operating frequency. Anode is usually made up of copper. It is connected to high positive d.c. voltage.

ii) In the center of anode, called as interaction chamber, is a circular cathode that emits es when heated.

iii) The direction of e is modified by surrounding the tube with a strong magnetic field usually supplied by c permanent magnets.

iv) 8 anode cavities are tightly coupled to each other, both operation is characterized by a combination of frequency and phrase of oscillation relative to adjacent cavity.

v) Total phase shift around the ring of cavity is 3608

= 45

Working: Magnetron is having radial electric field and axial magnetic field. It magnetic field is absent, e travel straight from cathode to anode. Due to radical electric field force acting on it, as shown ‘a’. If magnetic field is increased slightly, it will exert a force bending the path of e as shown by ‘b’ If the strength of magnetic field is mode sufficiently high if prevents e from reaching anode,

shown by ‘c’ anode current = 0 The magnetic field required to return e bact to cathode just touching the surface of anode is

called as cut off magnetic field. If magnetic field is more than cut off field then e come back to cathode quite faster By property adjusting the anode voltage & the strength of magnetic field the e can be made

to bend such that they rarely reach the anode and cause current flow in circular loops. In making their circular passes in the interaction chamber, the e excite the resonant cavities

into oscillation. Therefore magnetron is an oscillator, not an amplifier

Application: i) Pulsed magnetron commonly used in radarsys ii) Continuous wave magnetron are used for heating purpose in microwave oven. iii) Voltage tunable magnetrons are used in telemetry & in missile application.

Q.2(c) Describe the function of satellite earth station its block diagram. [4](A) [block diagram 2 marks, Function 2 marks] Function : The communication is established to satellite through earth station. The earth station can be

located on the ship at the sea, or on the space craft or actually on earth. The location of earth station is decided depending upon the ease of control of satellite and the function of the satellite. The type of earth station depends upon the a) Function of the station . b) Type of service c) Frequency bands used d) Transmitters e) Reciver and f) Antenna characteristic.

The four major subsystems of any earth station are receiver, antenna, transmits and tracking equipment

Multiplexer Baseband UP converter HighPower Amplifier

Modulator

User interface

Demultiplexer Baseband Demodulator Down

converter

Low NoiseAmplifier

Antenna

Encoder

Decoder

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Prelim Question Paper Solution

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Explanation: The baseband signal is applied to the encoder. Encoder converts the format ready for

modulation. The carrier is modulated by the encoded baseband signal. Ths modulated carrier is then upconverted to the uplink frequency of the satellite. The amplifier then amplifies this signal to high power level, ready for transmission. The signal is then passed through the polarization feed of the antenna. The signal received from the antenna is of different frequency (downlink frequency) and is very small in amplitude.

This signal is amplified by the low noise amplifier. It is then down converted to intermediate frequency by the down converter. This signal is then demodulated and decoded to get baseband signal.

These attitude data are satellites contration in space and tendency to shift, which can be sensed by infrared sensors. Based on those data, controlling torque may be generated by two methods Passive and Active. Passive attitude control is used for spin stabilization where the decrease in spin rate and

shifting back to its correct NS orientation. Active attitude control is used with three axes stabilization, three momentum wheel on these

axes controlling their speed of rotation based on the command signal reached form the controlling earth station.

Q.2(d) Describe Radiation losses in optical fiber cable [4](A) [two types Explaining 1 type 2 marks, 2 type 2 marks] Whenever an optical fiber undergoes a bend of finite radius of curvature, radioactive losses

accurs. There are two types of bends.

(1) Macroscopic bend (2) Microscopic bend

The part of mode which is on the outside of bend is required to travel faster than that on inside so that wavefront perpendicular to direction of propagation is maintained. Hence part of mode in the cladding needs the travel faster than the velocity of light in that medium. As this is not possible, the energy associated with this part of mode. If is last through radiation

Microbends are smallscale fluctuations in radius of curvature of fiber axis. They are caused either by nonuniformities in manufacturing of fiber or by non uniform in manufacturing of fiber or by non uniform lateral pressure created during called of fiber core

Q.2(e) Define with respect to satellite in detail.

(i) Numerical aperture (ii) Acceptance cone [4]

(A) [1 mark each] (i) Numerical aperture

Numerical aperture is measure of light collecting ability of fiber. It is independent of fiber core diameter.

Field distribution

Curved fiber

Power lost

Due to radiation

R

Fig.:

Core Claddity

Microbends

Power loss from higher-order modes Vidyala

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Vidyalankar : T.Y. Diploma ACS

6

By snell’s law: n0 sin1 = n1 sin2

In ABC = 2

π- θ

2

n0 sin1 = 1

πn sin - f

2

n0 sin1 = n1 cos = n1(1sin2)1/2

For total internal reflection 1= a & =c

sin c = 2n

n (from critical angle definition)

n0 sina = 1/22 2n - n1 2 = N.A.

(ii) Acceptance cone

Acceptance cone is defined as the range of angles for rays to be propagated by total internal reflection within fiber core.

Q.3 Attempt any four of the following : [16]Q.3(a) An air field rectangular waveguide of inside dimensions 7 3.5 cm operates in

dominant mode TE1,0. Find cut off frequency, & phase velocity at frequency 3.59H2 [4]

(A) [formula 1 mark & answer : 1 mark] Given: Dimensions 7 3.5 cm a = 7 cm b = 3.5 cm m = 1 n = 0 c = 3 108 m/s

Solution: i) fc =

1/22 2

1 mπ nπ+

a b2π με

=

1/22 2

1 1× π 0 × π+

7 3.52π με

=

1/22 2

1 π 0+

7 3.52π με

=

1/22

1 π

72π με

=

1/22

c π

2π 7

=

1/228

m3×10 π

2π 7

=

1/22103×10 π

2π 7

=

1/22103×10 1

2 7

= 3×10 1

×2 7

10

= 3×10

14

10

= 2.1428 109

fc = 2.1428 GHZ

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Prelim Question Paper Solution

7

Phase velocity =

2

0

c

1 - fc / f =

83×10

1- 2.1428 ×10 / 3.5 10

= 83 10

1 0.3748

= 83 10

0.7906

vp = 379.45 106 m/s

To provide monitoring function optical sensor systems are used in mobiles. & fiber optic communication is reliable for short as well as long distance communicate in minimum radies.

Q.3(b) Explain Antenna scanning method’s used in radar. [4](A) [diagram 2 marks, explanation 2 marks] Radar antennas are often made to scan given area of the surrounding space There are many scanning patterns depending on its application.

1) Horizontal Scanning Scans only in horizontal plane wherever the scanning

in horizontal directional is required example ship is ship securing then horizontal scanning is performed.

Nodding scan is an extension of this, the antenna is

moved rapidly in elevation while it rotates more slowly in azimuth and scanning in both planes is obtained. This system can be used to scan limited sector.

Helical Scanning: In this the elevation of the antenna is raised slowly while it rotates more rapidly in azimuth.

If a limited area of more or les circular shape is to be covered, then spiral scan may be used.

Q.3(c) Give characteristics and classification of fiber. [4](A) [characteristics 2 marks, classification 2 marks] The char of light transmission thro a glass fiber depend on many factors.

i) The composition of fiber ii) The amount & type of light introduced in fibers iii) The diameter & length of fiber.

Composition of fiber determines the refractive index by the process called doping. Other material are introduced into material that after its index number. This process produces a single fiber core with core index n1 & cladding index n2

Another chas. op fiber, which depend on its size, is it’s mode of operation. Mode refers to mathematical & physical description of propagation of energy thro a medium no. of mode supported by single fiber can be 1 to 1,00,000 fibers can provide path for one light rays or may rays i.e. single mode or multimode.

lobe

Axis of rotation

Noddi

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8

Archive Index profile It describes the relation of multiple indices which exist in the core adding of particular fiber.

There are 2 major, indicates i) Step index ii) Graded index i) Step index The step index decribes an abrupt index change from core to cladding. Core & cladding

both are having uniform refractive index. ii) Graded index With graded index, the highest index is at the center. This number gradually decreases

until reaches the index no. of cladding. Three classifications of fiber(Types)

a) Multimode step index fiber b) Multimode graded index fiber. c) Single mode step index fiber.

a) Multimode step index fiber. Has a core diameter from 100 to 970 m. With this large core diameter, there are many

paths thro’ which light can travel multimode. Therefore, the light ray travelling the straight path through the center reaches the end before the other rays, which follow a zigzag path. the difference in the length of time it takes the various rays to exit the fiber is called modal dispersion. This form of a signal distortion which limits the bandwidth of fiber.

b) Multimode graded index fiber. Because light rays travels more slowly than the light nearer the surface. Therefore,

both light rays arrive at the exit point at almost the same time, thus reducing model dispersion. Graded index fiber has core diameter ranging from 5085 m & cladding diameter of 125 m.

c) Single mode step index fiber. It is widely used in wideband communication with this fiber, a light ray can travel

on only one path. Therefore, modal dispersion is zero. The core diameter of this fiber range from 5m 10m.

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Prelim Question Paper Solution

9

Q.3(d) Draw field pattern of circular waveguide. [4](A) [diagram 4 marks] Circular waveguide has different method of mode labeling because of circular cross section.

Now, Here m = no of full wave intensity variations around the circumference. n = no of half wave intensity changes radially out from the center to the wall. TE, 1 mode is the dominant mode of circular wavelength.

Advantages of Circular waveguide:

1. Circular waveguide are easier to manufacture than rectangular ones. 2. They are also easier to join together.

Disadvantages:

1. Its cross section area is much bigger than that of a corresponding rectangular waveguide. 2. In circular waveguide plan of polarization of wave’s travel changes through the waveguide.

Waveguide components Flanges: When waveguide pieces or components are joined together, the coupling is done by means of flanges.

Functions of Flanges 1. To ensure a smooth mechanical junction and suitable electrical characteristics. 2. Low external radiation and low internal reflection

Q.3(e) Illustrate the block diagram of fiber optic communication system. [4](A) [block diagram 2 marks, explanation 2 marks] Information source provide an signal to drive electrical stage which ultimately drives an optical

source to give modulation of light wave carrier.

Optical source which provides the electrical optical conversion may either semiconductor laser or LED.

Optical carrier may be modulated using either analog or digital information signal. But analog modulation is less effective because it requires far higher signal to noise ratio at the receiver.

Laser drive ckt directly modulate the intensity of mod semiconductor laser with encoded digital signal. Hence digital signal launched into FOC.

Avalanche photodiode (APD) followed by front end amplifier & equalizer are wed to provides gain as well as linear signal processing & noise bandwidth reduction. Finally signal obtained is decoded to give original information

Information Source

Electrical Transmit

Optical Source

Optical Fiber cable

Destination Electrical Receive

Optical detector

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Vidyalankar : T.Y. Diploma ACS

10

Q.3(f) Describe working principle of OTDR with its Block diagram? [4](A) Block diagram of OTDR [2 marks]

[2 marks]

The main block of reflector meter are the generator of the testing impulse and detection system of the backscattered signal.

An optical time domain reflectometer is used in fiber optics to measure the time and intensity of light reflected. It is used as a trouble shooting device to find fault, splices and bends in fiber optic cables, with an eye toward identifying light locs.

A light pulse is launched into the fiber in forward direction from laser using either a directional coupler or beam splitter.

Coupler makes possible to couple optical excitation power into the tested fiber and simultaneously to deviate the backscattered power to optical receiver.

The backscattered light is deflected using an avalanche photodiode.

O/P of photodiode drives integrator improves the received signal to noise ratio by giving an nonthematic average over a no. of measurement taken at one point within a fiber.

Then signal is fed to log. amplifier and average measurements for successive points with the fiber are plotted.

Q.4 Attempt any TWO : [16]Q.4(a) Draw block dia of MTI radar and explain its operation? [4](A) [4 marks]

Mixer 2

If amplifier

Phase detector

STALO

COHO

Amplifier 2

Amplifier 1 Delay line Subtractor circuit

Radar display MTI

Fig. 1: Block diagram of MTI Radar

Antenna

fo + fc fo + fcfo fo

fo fc

fc fc

fc

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Prelim Question Paper Solution

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Principle: [4 marks] When it is desired to remove clutter due to stationary target & an MTI rader is employed. The basic principle of MTI radar is to compare a set of received echoes with those received

during previous sweep & cancelling out those whose phase has remain unchanged. Moving targets will give change of phase & are not cancelled. The cullers due to stationery

target both man made & natural are removed from displays this allows easier detection of moving targets.

Block diagram description It shows two mixes mixer. Generates transmitters frequency which is obtained by sum frequency produced by two

OSCr The STALO (stable Local OSCr producing fO) & COHO (coherent OSCr producing fC).

The transmitted frequency drives multi cavity klyston amplifier, which act as an o/p tube This amplifier provides the desired amplification for providing a high power pulse when

modulator switches it ON. This Txr pulse is the o/p via the duplexes. Echo pulse from target is received by MT1 radar antenna. If echo is due to moving target the

echo pulse undergoes a dopples frequency shift. The received echo pulse then pass through mixes 1 of receiver which heterodynes the

received signal of frequency (fO + fC) with o/p of STALO at fO & produces a difference frequency fC at it’s o/p

Two mixes 1 & 2 are identical in all respect except that mixes 2 produces different frequency where as mixes 1 produces a sum frequency.

This different frequency signal is further amplified by an, If amplifier & given to phase sensitive detector.

This detector compares IF signal with reference signal from COHO OSCr. Frequency produced by COHO is same as IF frequency & called as coherent frequency. The detector provides an o/p depending up phase difference between two signal. Since all

the received signal pulses will have a phase different compared with transmitted pulses. Thus phase detector gives o/p for fixed target & moving target. Doppler frequency shift causes this variation in phase difference.

A change of half cycle in Doppler frequency shift would cause an o/p of opposite polarity in the phase detector. The o/p of phase detector therefore will have an o/p that has different magnitude.

Q.4(b) Illustrate the block dia of satellite subsystem [4](A) [4 marks]

Solar panels

Charger and

batteries

Regulators protection and conditioning

DC/DC convertor DC/AC convertor

Receiver Freq. Xlator Xmitter

Antenna subsystem

Communication antenna

Telemetry tracking and

control subsystem

i/ps from

on- board

sensors

DC to all subsystems

DC to AC to special subsystem

Attitude control

subsystem

Propulsion subsystem

Power subsystem

Communication subsystem

Control signals to all subsystems

Telemetry antenna Jet thruster

AKM

Fig.

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Vidyalankar : T.Y. Diploma ACS

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Satellite subsystem consist of i) Power Subsystem ii) Communication iii) Telemetry tracking & command iv) Altitude control v) Antenna subsystem

i) Power Subsystem : Provides power to cell subsystem. All solar panels are pointed towards the sun generous

many kilowatts DC power. DC power is used to charge batteries which provides DC current to components when solar panels are not properly positioned DC to DC converter is used to reuse the level of voltages because some component like TWT requires high DC voltage.

ii) Communication Subsystem : It consist of Receiver frequency x10r and transmitter and are designed to operate at separate

frequencies. Transponders shares common antenna subsystem which receivers up link frequency

amplifies then again translate them in frequency & after amplification retransmits as a downlink frequency.

iii) Telemetry tracking & command : This subsystem is responsible for monitoring on board conditions like temp and battery

voltages. After monitoring it sends data back to earth station for analysis. According to result analysis ground station issues the order to satellite by transmitting a

signal to command subsystem. The command subsystem control the function of satellite like firing and jet thrusters. iv) Altitude control : It provides stabilization in orbit and senses the changes in orientation of satellite with built in

jet thruster it makes the necessary correction to attitude of satellite. Population of satellite: It uses AKM to put the satellite into final orbit.

v) Antenna subsystem : It consist of one or more antennas used for receiving signal from ground station and for

transmitting information back to earth station.

Q.4(c) Draw constructional sketch of TWT and explain its working, state its application. [4](A) [Construction diagram 3 marks, Explanation 3 marks, application 2 marks] It consist of thin conduction helical coil. This coil is placed inside the glass tube. Cathode is

used to emit e beam. A magnetic focusing field is provided to prevent the beam from spreading & guide it.

By using i/p waveguide, RF signal is applied to helix similarly at o/p end o/p waveguide is used. Collector is having high positive voltage, placed at the o/p end of helix. So e

beam get accelerated to the collector. The RF signal comes out from o/p end of helix.

Working:

1. Electron beam passes through the center or axis of helix. The microwave signal to be amplified is applied to the end of helix near the cathode & o/p is taken from end of helix near collector.

2. The purpose of helix is to provide a path for the RF signal that will slow down its propagation. The propagation of RF along the helix is made approximately equal to velocity of e beam from cathode to collector.

Vidyala

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Prelim Question Paper Solution

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3. Cathode emits e beam RF field is weak at the i/p of helix which is due to the i/p signal. The electric field produced by the helix cause e to be speeded up & slowed down. This produces velocity modulation, which causes buncing of e s

4. At a point at which axial field is zero e velocity is unaffected.

5. A point where axial field is positive, e coming against it is accelerated & tries to catch up e which encounter the RF axial field.

6. A point where axial field is positive the e gets velocity modulated. 7. The density modulated e beam is in step with electromagnetic wave travelling down the

helix, the e bunches induces voltage into helix which reinforce the voltage already present there

8. This result is that strength of electromagnetic field on helix increases as wave travel down the tube towards the collector.

9. At the end signal is considerably amplified. Coaxial cables are used to extract energy from the helix.

Application [2 marks] 1. In broadband microwave Rxr as a low noise RF amplifier. 2. As a repeater amplifier in wideband communication cables & coaxial cables. 3. It is used as power 0/8 tube in communication. 4. GaAs semiconductor is having two band valance band & conduction band. Bandgap between

them is called as forbidden gap. i) Central vally : It consist of conduction e it is above 1.43 ev from valance band. ii) Satalite valley: It is at higher energy level than central valley.

At room temperature, when applied electric field intensity is flow then all the e are present in lower valley when electric field intensity is increased more photons are generated they strike e because of collision energy is transferred to e their kinetic energy increases. e shifts in the upward direction. This is called as ‘electrontransfer’

If more electric field is applied e moves towards satellite valley, Mass of e increase with further increases in electric field energy e gets increases but the velocity will reduce, and current will also reduce.

Q.5 Attempt any four of the following : [16]Q.5(a) Compare between LED and LASER [4](A) [4 points 1 mark each]

Parameter LED LASER Principle of operation Spontaneous emission Stimulated emission Spectral width Broad Narrow Data rate Low Very High

Transmission Distance Smaller Greater Temp. Sensitivity Less More Coupling efficiency Very low High Compatible fibers Multimode step index Single mode step index Noise More less

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Vidyalankar : T.Y. Diploma ACS

14

Q.5(b) Explain the function of magic Tee in detail. [4](A) [2 marks]

[2 marks]

A magic tee is a combination of E plane Tee & H plane tee. Tee & H plane tee. The various properties of magic tee are as follows: 1) If 2 waves of equal amplitude & p same phase are fed into P1 & P2 the o/p will zero at P3 &

additive at P4. 2) If a wave fed into P4 i.e. H arm it will be equally divided between P1 & P2 with a equal

amplitude & same phase & will not appear at P3. 3) If a wave is fed into P3 it will produce an o/p of equal magnitude & opposite phase at P1& P2

& o/p at P4 is zero. 4) If a wave fed into P2 & P2 of equal amplitude & out of phase then o/p at the P3 will be

maximum & at P4 will be zero. 5) If a wave is fed into one co linear arm i.e. at P1 or P2 it will not appear at P2 or P1 because

Earm causes phase delay while Harm cause phase advance. Q.5(c) With neat sketch describe the operation of GUNN diode. [4](A) Construction: [2 marks]

[2 marks] The Gunn diode is not a diode, It is a thin piece of Ntype GaAs or InP semiconductor. Working: If a relatively small d.c. voltage is placed across a thin slice of GaAs, such as shown in fig that

forms a special registor. The formed resistance is negative resistance i.e. over some voltage ranges an increase in voltage result

in decrease in current and vice versa, just the opposite of ohm’s law predicts for linear resistance. When it is so biased that, the time it takes for electrons to flow across the material is such that

the current will be 180 out of phase with applied voltage.

Port 3

Port 4 Port 1

Collinear arms

Port 2 E-arm

H-arm

Anode gold wire

n+ substrate

8 m ≈ 5 m

Cathode Heat sink

Gold alloy cathode

Active n layer

Vidyala

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Prelim Question Paper Solution

15

If the Gunn diode is so biased and connected to a cavity that is resonant near the e transit time, it result in oscillation.

The Gunn diode, therefore is used primarily as a microwave oscillation.

Feature: Gunn diode are avaible to oscillate at frequency upto 50GHz In the lower microwave range, power o/p upto several watts are possible The thickness of semiconductor, determines the frequency of oscillation. Application: The highpower Gunn diodes are used in variety of lowpower transmitter applications. Gunn diode oscillators use in police radar, CW Doppler radar, burglar alarms and aircrafts. Q.5(d) Write short note on :

1) Foot print 2) Station Keeping [4]

(A) 1) Foot print [2 marks] The satellite downlink antenna transmits signals (beam) towards the earth in a particular patterns called footprint. The footprint of a stalite is the earth area that the satellite can receive from or transmits to. The earths coverage area increase with beam width & beam width decreases with increases in height.

2) Station Keeping [2 marks] Control signals that are to be generated ground to keep satellite in position is known as station keeping.

Gravitational forces causes orbital drift of satellite, so the orbit of satellite must be periodically adjusted. Most satellite contain small rockets or thus jets for keeping satellite in proper orbit.

Process of firing the rocket under ground control to maintain or adjust orbit is referred as station keeping.

Q.5(e) Calculate N.A and acceptance angle of fiber having refractive index of core = 1.40

and cladding = 136 [4]

(A) [Formula 1 mark, answer 1 mark] Given: n1 = 1.40 n2 = 136

Calculate i) N. A. ii) a

Ans: Formula: N. A. = 2 21 2h h = 2 2

1.40 1.36

= 1.960 1.849 = 0.111 N. A. = 0.33

. Q.6 Attempt any four of the following : [16]Q.6(a) Explain function of : i) Circulator ii) Isolator [4](A) i) Circulator

Symbol of circulator

Vidyala

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Vidyalankar : T.Y. Diploma ACS

16

Circulator is a four port device. It has a particular property that each terminal is connected to next clockwise terminal, i.e. port 1 is connected port 2 & not to the port 3 & port 4.

Circulators are used in parametric amplifiers, tunnel diode amplifier & duplexer in radar. ii) Isolator

It provides very small attenuation for transmission. Isolator make use of 45 twisted rectangular waveguide. It is a two port device. In microwave generators the output amplitude & frequency tends to fluctuate with changes in load.

Fluctuation occurs due o mismatch of generator output to load. That result reflected wave

from load. These reflected waves causes the instability of amplitude & frequency of microwave generator.

If the isolator is inserted between generator & load then the generator is coupled to load with

zero attenuation and reflection.

TE1 , mode

Circular Waveguide The basic behavior of wave in circular waveguide is the same as in rectangular guides. Since circular waveguide have a different geometry and same different cup plication minor

modifications are required in all the parameters and definition evolved evolved for rectangular waveguide. 1) The formula for cut off wavelength is different because of different geometry, and it is

given by

0 = 2πr

kr

Where r = radius (internal) of waveguide kr = constant

Port 1

Resistive Card

Top View

Vidyala

nkar

Prelim Question Paper Solution

17

Q.6(b) Illustrate how telemetry, tracking and command system used in satellite communication? [block dia 2m, explanation 2m]

[4]

(A) [2 marks]

[2 marks] These systems are partly on the satellite and partly at the control earth station. They support the

function of space craft management Telemetry: It collects data from all sensors on the satellite and send to the controlling earth sation. The sighting device is used to maintain space craft attitude are also monitored by telemetry. At

the earth station using computer, telemetry data is monitored and decoded. Tracking:

By using velocity and acceleration sensors an spacecraft the orbital position of satellite can be defect from earth sation.

For accurate and precise result no of earth station can be used.

Command: Command is the set of instruction of controlling earth station for the spacecraft. During the

time of launching, command signal can control firing of moter When the satellite has reached proper orbit solar panel deployment and action of spinner

operation start after getting a command signal. Command signal can also be used to make changes in attitude.

Main & Auxiliary propulsion subsystem : Most satellite contains propulsion subsystem in addition with attitude control subsystem It is used AKM to put the satellite into final orbit. By using rockets that could be change the orbit of a satellite, or remove the satellite form

orbit. This subsystem is also operated by command control subsystem. Modes can exit in to but in TM on such zero subscript mode will not exist because magnetic

field exist on both X and y axis. TE, TM1, TM21 such mode exist. TE10 is the dominant mode for TE TM11 is the dominant mode f0 TM where cut off wavelength has maximum value. Physical dimensions of waveguide decides the dominant mode of propogation.

TrackingSystem

Command Subystem

Computer Controller

Processor

Telemetry Receiver

Command Telemetry

Vidyala

nkar

Vidyalankar : T.Y. Diploma ACS

18

Q.6(c) List display methods used in RADAR & Explain Ascope displays method in detail. [4](A) [List 1 mark, diagram 1 mark, explanation 2 marks]

A Scope display It is a deflection modulation of a cathode ray tube screen. A sweep waveform is applied to

horizontal deflection plate of a CRT. & moves the beam slowly form left to right. The playback period is rapid. In absence of any received signal the displays is simply horizontal

straight line. The demodulated receiver o/p is applied to vertical deflection plates & causes the deflection of a

beam from horizontal line. The horizontal deflection sawtooth waveform is sync.with transmitted pulse so that the width of

CRT screen corresponds to time interval between successive pulse. Displacement from left hand side of CRT corresponds to range of target. The 1st blip due to

transmitted pulse is a reference pulse, then comes various strong blips, dut to reflections from ground near by objects of each which is here called ground clutter. The height of each blip corresponds to strength of returned echoes while its distance form P reference blip is a measure of range equally into P1 & P2 in phase & with the same amplitude.

Q.6(d) Draw schematic of LASER and describe its working principle with transition

process involved in LASER process [4]

(A) [2 marks]

2 3

1

Metal

P-type

n-type

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nkar

Prelim Question Paper Solution

19

[2 marks] This is the construction of semiconductor LASER LASER operates on the principle of stimulate emission. There are basically three steps : i) Absorption ii) spontaneous emission iv) stimulated emission Absorption: under normal condition e stays at the lower energy level i.e. ground state E1. If the photon of (E2E1) energy id incident on this e then e absorbs this energy and goes 10 the higher energy level E2. Spontaneous emission, after absorption process the e present at the higher energy level. If no incident light provided externally there stays at this stage for shorter time (other falls down) to the lower level emitting a photon with energy E2 E1. Stimulated emission: Because of this process, light amplification takes place. Before occurance of spontaneous emission, if the incident photon is allowed to strike to excited e then there is no chance for this atom to go to further higher energy level. Hence atom leaves it’s higher position and then emits two photon while going to lower energy level.

Q.6(e) State advantage of fiber optic commination [4](A) [Any 8 points, 2 points 1 marks]

1) Large Band Width: 1013 to 10 16 H2 2) Small Size and Weitht: Diameter is in m 3) Electrical Isolation: Fiber made from glass & fiber create no arcing, shortcircuit

electrically hazardous area. 4) Immunity to interference & cross talk : Fibers are dielectric waveguide. Hence it is free

from EMI and RFI. Hence immune to interference and cross talk 5) Signal Security: Light doesn’t radiate significant. Hence proved high degree of signal

security. 6) Low transmission loss: Losses are low upto 0.4 dB & low attenuation losses. 7) Aexibility: We can provide bent to fiber upto small radii or twisted without damage. 8) System Reliability : Less repeater are required hence in creasing the reliability and

reduction the cost.

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nkar