Subject Code: 12271 Model Answer Important … height b. Rectangular waveguide is a hollow metallic...

MAHARASHTRA STATE BOARD OF TECHNICAL EDUCATION

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(ISO/IEC - 27001 - 2005 Certified)

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WINTER– 14 EXAMINATION

Subject Code: 12271 Model Answer

Important Instructions to examiners:

1) The answers should be examined by key words and not as word-to-word as given in the model

answer scheme.

2) The model answer and the answer written by candidate may vary but the examiner may try to

assess the understanding level of the candidate.

3) The language errors such as grammatical, spelling errors should not be given more Importance

(Not applicable for subject English and Communication Skills.

4) While assessing figures, examiner may give credit for principal components indicated in the

figure. The figures drawn by candidate and model answer may vary. The examiner may give credit

for any equivalent figure drawn.

5) Credits may be given step wise for numerical problems. In some cases, the assumed constant

values may vary and there may be some difference in the candidate’s answers and model answer.

6) In case of some questions credit may be given by judgement on part of examiner of relevant

answer based on candidate’s understanding.

7) For programming language papers, credit may be given to any other program based on

equivalent concept.


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Q.1(A) Attempt any THREE of the following: 12

i. Define cut off frequency, phase velocity, group velocity and guided wavelength of

waveguide.

Ans i. (Four correct definitions – 1 Mark Each)

Cut off frequency of a waveguide:

It is the frequency of the signal above which propagation of waves occur.

𝑓𝑐 =𝑐

2√(

𝑚

𝑎)2

+ (𝑛

𝑏)2

Therefore lowest frequency which can propagate on the waveguide is fcTE10

No energy can propagate in a rectangular waveguide at a frequency below fcTE10. This is absolute

cut off frequency of the waveguide.

Guide wavelength:

It is defined as the distance travelled by the wave in order to undergo a phase shift of 2π radians.

𝜆𝑔 =𝜆

√1 + (𝜆𝜆𝑐)2

where λ = wavelength of the signal

𝜆𝑐= cut off wavelength

Phase velocity:

Phase velocity is defined as the rate at which the wave changes its phase in terms of the guide

wavelength.

The phase velocity is given by

𝑣𝑝 =𝑣𝑐

√1 + (𝜆𝜆𝑐)2

Group velocity:

It is defined as the rate at which the wave propagates through the waveguide and is given by

𝑣𝑔 = 𝑣𝑐√1 + (𝜆

𝜆𝑐)2

Group velocity is always less than speed of light.


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ii. State working principle of PIN diode and state its four applications.

Ans ii. (Working principle – 2 Marks, 4 Applications – 2 Marks)

Zero bias:

At zero bias the diffusion of the holes and electrons across the junction causes space charge

region of thickness inversely proportional to the impurity concentration.

An ideal ‘i’ layer has no depletion region i.e. p layer has a fixed negative charge and n layer

has a fixed positive charge.

Reverse bias:

As reverse bias is applied the space charge regions in the p and n layers will become thicker.

The reverse resistance will be very high and almost constant.

Forward bias:

With forward bias carrier will be injected into the I layer and p and n space charge regions will

become thinner.

So the electrons and holes are injected into the i layer from p and n layers respectively.

This increases the carrier concentration in the I layer above equilibrium.

Thus resistivity decreases as increase in forward bias.

Therefore low resistance is offered in the forward direction.

Applications:

It is used as switch

It is used as a phase shifter.


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It is used as an amplitude modulator.

It is used as a limiter.

iii. List different antenna scanning pattern in radar, explain any one.

Ans iii. (List – 1 Marks, Any one type Diagram & Explanation – 1+2 Marks)

Horizontal scanning

Nodding scanning

Helical scanning

Spiral scanning

Conical scanning

Horizontal scanning: If scanning is required in only plane is called horizontal scanning.

e.g. Ship to ship communication, navigation.

(OR)

Nodding: In this scanning, antenna is moved rapidly assuming in slowly in elevation. It

covers limited area or complete hemisphere.

(OR)

Helical: Elevation is slowly raised while it rotates more rapidly in assuming. Covers

complete hemisphere and it takes place in both plane e.g. tracking of satellite.


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(OR)

Spiral: It is required to scan limited area. When target is to be detected, scanning take place

first with somewhat wide because of width. Whereas tracking is locate at exact position of

target which take place with narrow phase shift beam width.

(OR)

Conical: Conical scanning achieve by mounting parabolic antenna slightly of corner and

rotate above axis of parabola. Speed is low as compare to pulse frequency. Disadvantage is

motion of antenna more complex, due to more than one return pulse require to locate

target.

iv. W.r.t. satellite communication define:

a. Look angle

b. Azimuth angle.

Ans iv. (Correct definitions – 2+2 Marks)

a. Look angle: It is the angle at which the earth station antenna is oriented to

maximize the transmission and reception of EM waves. There are two look angles-

Angle of elevation & Azimuth angle.

b. Azimuth angle: It the angle measured from true north to the sub satellite point in

the horizontal plane.


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Q.1.B. Attempt and ONE of the following 08

i. Explain with the help of neat diagram, the working principle, construction and applications of

multicavity Klystron amplifier.

Ans i. (Diagram-3 Working Principle-2, Construction-2, Any 2 Applications – 1 Marks

Each)

Typically power gain of two cavity klystron is 30dB

To increase the overall gain, connect several two cavity tubes in cascade, feeding the output of

each of the tubes to the input of the following one.

So, to provide high gain multicavity klystron have designed and produced.

Construction:

Here each intermediate cavity act as a buncher which passes electron beam inducing an

enhanced Rf voltage than the previous cavity.

With four cavities, power gain is around 50dB can be easily achieved.

Practically upto seven cavities have been used.

For narrow band operation the cavities can be tuned to the same frequency i.e. synchronous

tuning.

For broadband operation the intermediate cavities are tuned to either side of center frequency

i.e. stagger tuning.

Stagger tuning improves the bandwidth.


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Stagger tuning is employed in UHF klystron for TV transmitter output tubes and in satellite

earth station transmitter as power amplifier at 6GHz.

Applications:

It is used as a medium, high and very high power amplifier in the UHF and microwave ranges.

Used in satellite earth station transmitter as power amplifier

Used in UHF TV transmitter.

ii) Draw block diagram of MTI radar and explain its working

Ans ii. (3M for diagram 2 M for waveform and 3 M for explanation)

Working principle:

The echo pulse from the target is received by MTI radar antenna.

If echo is due to moving target, the echo pulse under goes a Doppler frequency.

The received echo pulses than pass through mixer 1 of the receiver.

Mixer 1 heterodynes the received signal of frequency (Fo+Fc) with the output of the stalo

at Fo.

Mixer 1 produces a difference frequency Fc at its output.

This difference frequency signal is amplified by an IF amplifier.

Amplifies output is given to phase detector.

The detector compares to IF amplifier with reference signal from the COHO oscillator.

The frequency produced by COHO is same as IF frequency so called coherent frequency.

The detector provides an output which depends upon the phase difference between the two

signals.


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Since all received signal pulses will have a phase difference compared with the transmitted

pulse.

The phase detector gives output for both fixed and also moving targets.

\Phase difference is constant for all fixed targets but varies for moving targets.

Doppler frequency shift causes this variation in the phase difference.

A change of half cycle in Doppler shift would cause an output of opposite polarity in the

phase detector output.

The output of phase detector will have an output different in magnitude and polarity from

Successive pulse in case of moving targets.

And for fixed target magnitude and polarity of output will remain the same as shown in

figure.


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Q.2. Attempt any FOUR of the following: 16

a. Define the following terms:

i. Dispersion (2 Marks)

ii. Reflection (2 Marks)

Ans a.

i. Dispersion: Dispersion causes broadening of transmitted light pulse as they travel along

the channel. Dispersion of transmitted light signal causes distortion for both digital and

analog transmission along fiber.

Each pulse broadens and overlaps with its neighbours eventually becoming indistinguishable at

the receiver input. It limits the maximum bandwidth attendable with a particular fiber.

ii. Reflection: When light ray is incident on the interface between two dielectric of differing

refractive indices, refraction occurs and part of light rays are reflected into the medium

with high refractive index.

b. Explain the propagation of waves in rectangular waveguide

Ans b. (propagation diagram – 2 Marks, Explanation – 2 Marks)

Rectangular waveguide is a hollow metallic tube with a rectangular cross section. It has width a

and height b.

Rectangular waveguide is a hollow metallic tube with a rectangular cross section. TEM wave cannot exist in rectangular waveguide. The wave propagates down the wave guide in a zig – zag manner with the Electric

field maximum at the center of the guide and zero at the walls. Two consequences of the zig-zag propagation are 1) The velocity of propagation in a wg must be less than in free space. 2) Waves can no longer be TEM as propagation by reflection requires not only a

normal component but also a component in the direction of propagation.


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c. State the applications of multicavity klystron amplifier & Reflex klystron (Two points each)

Ans c.

Applications multicavity klystron amplifier is: (any two) 2 Marks

It is used as a medium, high and very high power amplifier in the UHF and microwave ranges.

Used in satellite earth station transmitter as power amplifier

Used in UHF TV transmitter.

Applications Reflex klystron is: (any two) 2 Marks

Reflex klystron is most widely used in applications where variable frequency is desired as

In radar receivers

Local oscillator in microwave receivers

Signal source in microwave generator of variable frequency

Portable microwave link

Pump oscillator in parametric amplifier.

d. Derive the radar range equation and write factors influencing maximum range

Ans: (Equation -3M, factors-1M)

The Radar range equation is given by,


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The factors influencing maximum range are as follows :

Transmitted power (Pt): if the radar range is to be doubled we have to increase a transmitted

power by 16 times.

Frequency(f) : increase in frequency increase the range

Target cross sectional area(S). Radar cross sectional area of the target is not a controllable

factor.

Minimum received signal (Pmin): A decrease in minimum receivable power will have the same

effect has raising the transmuting power.

e. Draw corners & twists section of waveguide. State its applications.

Ans e. (Diagram – 2 Marks, Any two Applications – 2 Marks)

i) Corners: Corners are used change the direction of the wave propagating through waveguide.

Twists: Twists are used to convert vertical to horizontal polarization and vice – versa


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Applications (Any two relevant applications) (2 Marks)

ii) RADAR system.

iii) Terrestrial microwave system

iv) Satellite communication system

Q. 3) Attempt any four of the following:

a) Compare waveguide with two wire transmission line (any four points).

Ans: 1M each point (Any other appropriate point should be given marks)

Wave guide

Two wire Transmission line

1.A wave guide is a hallow metallic pipe

design to carry microwave energy from

one place to another

1.Transmission on line is a conductor or

wire designed to carry electrical energy

below microwave range from one place

to another

2.Used for Microwave frequency above

1GHz

2. Used for RF up to 500 in GHz. Upto 18

GHz. For short distance.

3.Power handling capacity is high 3.Power handling capacity is low

4.Wave theory is consider in wave guide

analysis

4. circuit theory

5.Application-Signal transmission in

Radar

5.Signal transmission in TV etc

b) State and explain concept of dominant mode in waveguide.

Ans: (Correct Explanation – 4 Marks)

Dominant mode is that mode for which cut off wavelength (λc) assumes a maximum value.

Of these λc10 has the maximum value since a is the larger dimension. Hence the TE10 mode is

the dominant mode in the rectangular wave guide.


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c) Explain any two splicing technique with neat diagram.

Ans: 1M each-diagram, 1M each- explanation

Different types of splicing are:

1. Fusion splicing or welding

2. Mechanical splicing

3. Elastic tube splicing

FUSION splicing

It is accomplished by applying localized heating i.e by a flame or an electrical arc at an

interference between two butted, pre aligned fiber ends.

The figure shown below:

This technique involves heating of two prepared fiber ends to their fusing point by applying

sufficient axial pressure between the two optical fibers.

For heating most widely source is electric arc.

Following are steps for fusion process

PREFUSION: It is a technique, which involves the rounding of the fiber ends with a low

energy discharge before pressing the fibers together.

By moving movable block, with proper pressure two fibers are pressed together.

Then there will be accomplishment of splice.

(OR)

MECHANICAL splicing

Using rigid alignment tube.

In this method accurately produced rigid alignment tube is used to bond the prepared fiber

ends permanently.

Figure shows the snug tube splicing.


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In snug tune splicing technique uses a glass or ceramic capillary tube with an inner diameter

just large enough to accept the optical fibers.

Transparent adhesive is injected through a transverse bore in capillary to give mechanical

sealing and index matching of the splice.

Average insertion losses as low as 0.1dB have been obtained.

Figure shows the loose tube splice.

In this splice an oversized square section metal is used to accept the prepared fiber ends.

Transparent adhesives are first inserted into the tube followed by the fibers.

The splice is self-aligned, when fibers are curved in a same plane.

Mean splice insertion losses of 0.73dB have been achieved.

o Using V-grooves:

In this technique V-grooves are used to secure the fibers to be joined.

This method utilizes a V-groove into which the two prepared fiber ends are pressed.

The V-groove splice ends through insertion in the groove.

The splice is made permanent by securing the fibers in the V-grooves with epoxy resin.

For single mode fiber splice insertion losses of less than 0.01 Db.


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c) Draw and explain the block diagram of OTDR.

Ans: 2M-diagram 2M explanations

Explanation

A light pulse is launched into the fiber in the forward direction from an injection laser using

either a directional coupler or a beam splitter.

Beam splitter or coupler makes possible to couple the optical power impulse into the tested

fiber and simultaneously to deviate the backscattered power to the optical receiver.

The backscattered light is detected using avalanche photodiode receiver. Output of photodiode

receiver drivers an integrator.


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Integrator improves the received signal to noise ratio by giving an arithmetic receiver over a number of

measurements taken at one point within the fiber. The signal from the integrator is fed through a log

amplifier and average measurements for successive points within the fiber are plotted on as a chart

recorder.(A) Attempt any two of the following: 8 Marks

i) Draw the block diagram and explain the operation of CW Doppler radar.

Ans: 2M-diagram,2M- explanation

In earlier day, radar experiments worked with continuous rather pluses transmission.

Type of radar which employees a continuous transmission is continuous wave (CW)

radar.

CW radar, transmit a constant amplitude continuous microwave, sine wave.

And echo generated by sine wave is also of constant amplitude but with small

amplitude.

CW radar gives better understanding of nature and use of Doppler information contain

echo signals.

It provides a measurement of relative velocity to distance moving target from stationary

object.

When a moving air plane, ship, missile or auto mobile is detected by radar the reflected

signal gives frequency range.

That frequent change is use to determine the speed of the target.

As shown in diagram CW transmitter generates a continuous wave of frequency f0 that

is radiated by antenna.

Some part of energy is intercepted by target and scattered in the direction of radar.

Receiving antenna collects its.

Then receive signals will be shifted in frequency from transmitted freq.f0 target is in

motion by an amount +or- fd.

If the distance between target and radar is decreasing then received signal freq.is greater

than transmitted signal freq, is f0+ fd.

If the distance between target and radar is increasing then received signal freq.is lesser

than transmitted signal frequency is f0-fd.


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So the received signals frequency enters the radar and the heterodyned with portion of

transmitted signal which produce the Doppler beat of frequency.

Beat frequency amplifier eliminated echo from stationary target and amplify the

Doppler echo signals at the level that can operate the indicator.

Indicator may be pair of ear phones or frequency meter.

ii) Explain the advantages and disadvantages of FOC (four points each).

Ans: Advantages:- (any 6 – 3 Marks)

Optical fibers are having very small diameter cores. And also having light weight.

Optical fibers are very tensile.(flexible)

Optical fibers are fabricated from electrical insulator.(glass and plastic polymer)

The light from optical fibers does not radiate and therefore they provide a high degree of signal

security.

Optical fibers are exhibit low attenuation or transmission loss as compared to copper conductor.

Having large bandwidth.

Reliability and easy maintenance.

Disadvantage:- (Any 2 – 1 Marks)

Brittleness and small size makes it difficult to work with.

Difficult to manufacture.

Expensive tools and techniques are required.

It is difficult to lay fiber to cover large area.

Broadcasting not possible.

iii) Explain with neat diagram the working of photodiode.

Ans: 2M-diagram,2M- explanation

P-i-n photo diode In order to allow operation at longer wavelength where the light penetrates

more deeply in to the semiconductor material a wider depletion region is necessary. To achieve

this the n-type material is doped so that it can be considered as intrinsic and to make a low

resistant contact a highly doped n-type (n+)layer is added. In fig. device structure consist of p


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and n regions separated by very lightly doped intrinsic (i) region.

Normally sufficient large reverse bias voltage is applied across the device so that the intrinsic

regions is fully depleted of carriers .When an incident photon has energy greater than or equal

to the band gap energy of semiconductor material, the photon can give up its energy and

excite an electron from valence band to conduction band. This process generates free

electrons and hole pairs which are called as photo carriers. In photo detector photo carriers are

generated in the depletion region where most of incident light is absorbed. The high electric

field present in depletion region causes the carrier to separate and be collected across the

reverse bias junctions. This gives rise to current flow in external circuit.

4. (B) ATTEMPT ANY ONE OF THE FOLLOWING 8

i) Sketch block diagram of satellite earth station and state the function of each block.

Ans: 4M-diagram, 4M- explanation

(OR)


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Explanation (any relevant explanation to be considered)

Transmitter Side

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

modulation.

The carrier is modulated by the encoded baseband signal.

The modulated carrier is then up converted 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.

Receiver Side

The signal received from the antenna is of different frequency (downlink frequency) and is very

weak signal.

This signal is amplified by the low noise amplifier.

It is then down converted to the intermediate frequency by the down converter.

This signal is then demodulated and decoded to get baseband signal.

ii) Draw E-H plane Tee, explain its working and state its application.

Ans: 4M-diagram, 2M- explanation, applications – 2 M


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The magic tee is a combination of E and H plane tees. Arm 3 forms an H-plane tee with arms 1

and 2. Arm 4 forms an E-plane tee with arms 1 and 2. Arms 1 and 2 are sometimes called

the side or collinear arms. Port 3 is called the H-plane port, and is also called the Σ port, sum

port or the P-port (for "parallel"). Port 4 is the E-plane port, and is also called the Δ

port, difference port, or S-port (for "series"). A signal injected into the H-plane port will be

divided equally between ports 1 and 2, and will be in phase. A signal injected into the E-plane

port will also be divided equally between ports 1 and 2, but will be 180 degrees out of

phase. If signals are fed in through ports 1 and 2, they are added at the H-plane port and

subtracted at the E-plane port.

Applications: (any two)

1. It is used for measurement of impedance.

2. It is used as a duplexer.

3. It can be used as a mixer.

4. It can be used as a microwave discriminator & bridge etc.

Q-5 Attempt any FOUR of the following 16 Marks

(a) Define uplink frequency and downlink frequency in satellite communication system?

Ans: (Each definition 2 marks)

Uplink frequency: The uplink frequency can be defined as a frequency used by signal

transmitted from transmitting earth station towards satellite.

Downlink frequency: The downlink frequency can be defined as the frequency of signals

which is retransmitted from satellite to earth station.

Uplink frequencies are generally higher than the down link.

(b) When the mean optical power launched into an 8km length of fiber is 120uW, the mean

optical power at the fiber output is 120uW.Determine the overall signal attenuation or loss in

decibels through the fiber, assuming there are no connectors or splices.

Ans: (Formula – 1 mark, Answer – 3 marks)

Overall signal attenuation or loss in decibels through the fiber assuming there are no connectors

or splices is given by,

Signal attenuation (αdb L) = 10log10Pi/Po

= 10log10 (120x10-6

/120x10-6

)

= 10log101

= 0 dB


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(c) Differentiate between LED and LASER.

Ans. (Any 4 correct points - 1 marks each)

Parameter LED LASER

Principle of operation Spontaneous emission Stimulated emission

Output Beam Non-coherent Coherent

Data rate Low(max. 400Mbps) High(several Gbps)

Coupling efficiency Very low High

Spectral width 20 to 100nm 1 to 5nm

Transmission distance Smaller

Greater

Compatible with Multimode SI/GI fiber

Single mode fiber

Circuit complexity Simple Complex

(d) Sketch and label frequencies of light wave spectrum?

Ans. (Sketch – 2 marks, Labeling- 2 marks)


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(e) State the application of fiber optic communication in industrial and defense field?

Ans. (Each 2 marks) (Note: any relevant applications should be considered)

Application of defense field

1. Mobiles One of the most promising areas of military application for optical fiber communication is within

military mobiles such as aircraft, ships and tanks.

2. Communication links Short distance optical fibre systems may be utilized to connect closely spaces items of electronic

equipment in such areas as operations rooms and computer installations.

Application of industrial field

1. Automotive Fiber optics

Communications and sensing in automobiles is of utmost importance with the continual

increase in onboard safety devices and systems.

2. Fiber optics for data storage

Data storage devices are made with different types of connectivity interfaces, the primary

being optical.

Q-6Attempt any FOUR of the following 16 Marks

a) Draw and explain the block diagram of a fiber optic communication?

Ans. (Block diagram -2 marks, any relevant explanation- 2 marks)

(OR)


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TRANSMITTER TERMINAL

1) Information input Information signal transmitted may be voice, video or computer data.

The information may be an audio conversation, a stream of data from one computer to another, or

several simultaneous television broadcasts.

2) Encoder/Signal shaping circuit Its purpose is to make the transmitted signal compatible to the communication channel by limiting

the effective bandwidth of the transmission.

3) Modulator / Driver In optical communications, intensity modulation (IM) is a form of modulation in which the optical

power output of a source is varied in accordance with some characteristic of the modulating signal

4) Optical Source Then digital pulses are used to drive a powerful light source off and on very rapidly.

In low cost system for shorter distance communication LED is used.

Color of light emitted depends on material used to construct LED.

Another commonly used light source for long distance communication is LASER.

LASER generates single high frequency light beam these are then fed into FOC.

5) Transmission Medium

1) Fiber An optical fiber is flexible and transparent made of very pure glass (silica) not much wider than a

human hair that acts as a waveguide, or "light pipe", to transmit light between the two ends of the

fiber

Repeaters are used in long transmission system for avoiding losses of signal strength

Since the light is greatly attenuated when it travels over long distances at some point it may be too

weak to be recovered reliably.

To overcome this attenuation problem, special relay stations are used to pick up the light beam,

convert it back into electrical pulses that are amplified and then retransmit the pulses on another

light beam.

RECEIVER TERMINAL

1) Optical Detector


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An optical detector is a transducer that converts an optical signal into an electrical signal.

It does this by generating an electrical current proportional to the intensity of incident light.

At the receiving end a light sensitive device/ light detector i.e. Photodiodes generally avalanche

photo diode is used to detect light pulses.

Light detector converts light pulses into electrical signal.

2) Amplifier & Reshaper These electrical pulses are reshaped and amplified back into digital form, then these are fed to

decoder.

3) Decoder/Demodulator A DAC is used where the original voice or video is recovered.

A demodulator is an electronic circuit (or computer program in a software defined radio) that is

used to recover the information content from the modulated carrier wave.

4) Information Output The information out may be an audio conversation, a stream of data from one computer to another,

or several simultaneous television broadcasts depending on the demodulated input.

(b) Describe the construction of fiber optic cable?

Ans. (Construction diag. – 2 marks, explanation – 2marks)

An optical fiber cable is dielectric waveguide made up of silica glass with refractive index of

around 1.5

The optical fiber consist of a transparent core with refractive index n1 and surrounded by

transparent cladding of slightly lower refractive index n2 i.e n1>n2. The light is confined within

the core only.

The cladding layer is thick and it reduces radiation loss into the surrounding air. It enhances the

mechanical strength of fiber it protect core from absorbing surface contaminates.

The fibers are encapsulated in an abrasion resistant plastic material called buffer sheath or jacket.

It provides additional strength to fiber and isolates it from small geometrical irregularities,

distortions or roughness of adjacent surface.


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(c)Explain TE and TM modes in rectangular waveguide? (Each 2 marks)

Mode in waveguide is all possible configuration of electric and magnetic field set up in a

waveguide.

TE mode

.In TE mode electric field remains perpendicular to the direction of travel or propagation In TE

mode Hz ≠ 0 and Ez=0 if Z is the direction of propagation then no electric line is in the direction

of propagation.For rectangular waveguide TE10 mode is dominant mode. It signifies all electric

fields are transverse to the direction of propagation and no longitudinal electric field is present.

The electric field is maximum at the centre of the waveguide and drops off sinusoid ally to zero

intensity at the walls.TEmn mode: m-indicates the number of half wavelength of electric field

intensities along the width of the guide, n- number of half wavelength of electric field along the

height of the guide or no E-field patterns across ‘b’ dimension.

TM mode :

In transverse magnetic (TM) mode, Hz = 0 and Ez ≠ 0

It can be shown that if either m = 0 or n = 0 for TM modes, then `E = `H = 0. This means that no

TM modes with m = 0 or n = 0 are allowable in a rectangular waveguide.


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(d) Explain radar becons and state its applications?

Ans. (Explanation -3 marks and any 2 applications 1 mark)

RA-dar bea-CONS also called as radar responders or transponders. RACONS are

receiver/transmitters with omnidirectional antenna. This device is used as navigation aid,

identifying land marks. Racon responds to a received radar pulse by transmitting an identifiable

mark back to radar set.

All racons operate at frequency range of 9300 to 9500MHz and 2900-3100 Mhz. Racons range is

approximately line of site range Racon range depends upon a number of factors including

mounting height, atmospheric conditions and racon receiver sensitivity setting.

Modern racon have wide band receiver that detects the incoming radar pulse, turns the transmitter

and responds with 25μsecs long signal within 700nsec. Older racons operates in a slow sleep. It

only responds only if the frequency of an incoming signal at moment it arrives tuned to it.

RACON is a device that on receiving radar signal, transmits coded signals in response to help

navigators determine their position

Application:- (any two)

1. To identify aids to navigation both seaborne and land based.

2. To identify land full.

3. To indicate navigable spans under the bridges.

4. To identify offshore oil platforms.

5. To identify and warn of environmentally sensitive areas.

6. To identify centers and turning points.

7. To mark a uncharted hazards.

8. Used to identify itself.

9. Used in airport traffic control

10. Military purposes

(e) Explain footprint and station keeping?

Ans. (Each 2 marks)

Footprint

The area of the Earth covered by the microwave radiation from a satellite dish

(transponder) is called the satellites footprint.

The size of the footprint depends on the location of the satellite in its orbit, the shape and

size of beam produced by its transponder and the distance from the earth.

The footprint also indicates the appropriate area over which an uplink may successfully

access a transponder on the satellite.

Station keeping

The control signals that are to be generated on ground to keep the satellite in position is

known as station keeping.

Once a satellite is in orbit, the forces acting on it tend to keep it in place.

Variety of forces causes orbital drift of a satellite.

So the orbit of the satellite must be periodically adjusted.

Most satellite contains small rockets or thruster jets for that purpose.


(Autonomous)


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Depending on how accurate the orbit must be, these rockets may be fired every several

weeks or once per year.

The process of firing the rockets underground control to maintain or adjust the orbit is

referred to as station keeping.

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