Sample paper of Radar and TV engineering
SAMPLE PAPER OF RADAR AND TV ENGINEERING
TIME: 3 HRS.
NOTE: All the questions are compulsory from the section-a and any 5 from the section-b.
SHORT QUESTIONS: (EACH 2 MARKS)
1. What are the basic factors with which the tv system must deal for successful transmission and reception of picture?
ans: Gross structure
no of scanning lines
2. What is vertical scanning?
ans: It is the movement of electron beam spot in the vertical direction.
3. What is raster?
ans: Raster is the scan pattern in which an area is scanned from side lines from top to bottom.
4. What is scanning?
ans: Scanning is the process by which an electron beam spot a mode to move across a rectangular area so as to cover it completely.
5. What is horizontal scanning?
ans: The movement of electron beam spot from left to right and back so as to start a new line in same direction is termed as horizontal scanning.
6. What is video signal?
ans: The time varying electrical signal voltage obtain from the tv camera tube circuit representing the special distribution of brightness of the image is called video signal.
Long questions: (each 10 marks)
1. Explain the proper generation of composite video signal?
2. Draw the NTSC Decoder and encoder.
3. What are the uses of radar ?
ans: Detection and Search Radar : Radar is basically used in detection purpose in military and coastal survelliance which help the people in detecting the harmful objects coming towards them.
Missile guidance system: Nowadays, these radar has become quiet popular in missile guidance as it helps the people in knowing where to launch the missile or where to land it etc.
Air Traffic control: In foreign countries, the traffic is being controlled through the use of radar.
Weather Forecasting: This is quiet helpful in weather forecasting as it helps in detecting the temperature of climate or when will the rainfall occurs .
4. What is Doppler effect?
ans: The sudden change in pitch of a car horn as a car passes by (source motion) or in the pitch of a boom box on the sidewalk as you drive by in your car (observer motion) was first explained in 1842 by Christian Doppler. His Doppler Effect is the shift in frequency and wavelength of waves which results from a source moving with respect to the medium, a receiver moving with respect to the medium, or even a moving medium.
Although first discovered for sound waves, the Doppler effect holds true for all types of waves including light (and other electromagnetic waves). The Doppler effect for light waves is usually described in terms of colors rather than frequency. A red shift occurs when the source and observer are moving away from each other, and a blue shift occurs when the source and observer are moving towards each other. The red shift of light from remote galaxies is proof that the universe is expanding.
5. What are the factors that affect radar performance?
ans: The performance of a radar system can be judged by the following:
(1) the maximum range at which it can see a target of a specified size.
(2) the accuracy of its measurement of target location in range and angle.
(3) its ability to distinguish one target from another.
(4) its ability to detect the desired target echo when masked by large clutter echoes, unintentional interfering signals from other “friendly” transmitters, or intentional radiation from hostile jamming (if a military radar).
(5) its ability to recognize the type of target.
(6) its availability reliability, and maintainability
RADAR AND TELEVISION ENGINEERING Interview viva Questions and for semester exams
- RADAR AND TELEVISION ENGINEERING
No.1(i)why flicker is not removed by progressive scanning?
Ans .flicker can be removed by doubling the speed of progressive scanning,but doubling the speed will double the band-width,which is not desirable.
(ii).why is retraced blanked?
Ans. retraced is blanked because if the retrace is visible on the screen,it will cause distortion in the picture.
(iii).what is kell factor(k)?
Ans. The kell factor (k) or the resolution factor gives the effective number of horizontal scanning lines can be scanned because of practical limitation.the value of kell factor lies between 0.65 to 0.75.
(iv).what is importance of back porch?
Ans. Back porch absorbs ringing oscillations due to fly-back. In the absence of the back porch,these oscillations would have distorted the video signal.
(v).which principles are used for optical electrical conversion?
Ans. Two basic principles are used for optical electrical conversion.They are based on photo electric effects. The two principles are: (a) photo emission. (b) photo conduction.
(vi). List 4 merits of digital TV receivers.
Ans.Reduced ghosts. Reduced of 50 HZ flicker. High resolution pictures. Slow motion action. (vii).write short notes on video disc system.
Ans: The video disc is essential a gramophone record with pictures on it.The first video disc was the laser optical and second video disc system is based on the capacitance principle .
(viii). Why do blind speeds occur?
Ans. Blind speeds occur because of the sampled nature of the pulse radar wave form.Thus it is sampling that is the cause of ambiguities or aliasing in the measurement of the Doppler frequency . (ix). What do you mean by longitudinal video recording?
Ans: A method in which video signals are recorded on at least several tracks along the length of the tape.
(x).What are the two types of video disc system?
Ans. Laser or optical disc system. capacitance disc system.
No(.2) Write the vertical sync and blanking pulse standard.
Ans. A vertical sync waveform is inserted in the composite video signal at the end of each field of 312.5 lines .Each vertical sync consist of (a)pre-equalizing pulses(b)field sync pulses and (c) Field Blanking Period (VB): During this period, video signal is suppressed and field retrace is completed .In CCIR B-system, blanking period VB equals 20 H. Thus VB=20H=1280µS.Since there are two fields per picture frame, the no of active lines is (625-40=585 lines. Field Sync Pulses: The vertical pulses may be distinguished from the horizontal sync pulses by their larger duration, being about 2.5H long in CCIR B system .These serrations are so timed that the leading edges of horizontal sync pulses existed. The extra half line pulses coinciding with horizontal sync pulses. It becomes necessary at the middle of horizontal scanning lines in case of odd field and at the end of a horizontal scanning line in case of an even field. EQUALISING PULSES Solves the shortcoming occurring on account of half line discrepancy. Five narrow pulses of 2.5 line period are added on either side of the vertical sync pulses known as pre-equalizing and post equalizing pulses The effect of these pulses is to shift the half line discrepancy away from both the beginning and end of the vertical sync pulses: PRE-EQUALISING PULSES: 2.3µS duration Result in the discharge of the capacitor to zero voltage in both the fields POST-EQUALISING PULSES Necessary for a fast discharge of the capacitor to ensure triggering of the vertical oscillator at proper time. With the insertion of equalizing pulses:-the voltage rise and fall profile is the same for both the field sequences. The vertical oscillator is triggered at the proper instants exactly at an interval of 1/50th of a second.
No(.3.) What is the function of TV camera tube ?what are the major problem occurring during the conversion of optical to electrical signal in TV camera?
- Heart of a TV camera is a camera tube.
- Camera tube –converts optical information into electrical signal
- Amplitude proportional to brightness.
- Optical image is focused by a lens assembly to a rectangular glass face –plate.
- Transparent conductive coating at the inner side of the glass face plate.
- On which is laid a thin layer of photo conductive material having a very high resistance when no light falls on it.
- Resistance decreases when the intensity increases .
- Electron beam –used to pick up the picture information available on the target plate in terms of varying resistance.
- Beam is formed by an electron gun
- Deflection coils are fed separately from two oscillators-continuously generates saw tooth wave forms having different desired frequency
- Uses magnetic deflection
- Deflection by first coil-horizontal motion of beams and then brings quickly to left side to commence the trace of next line.
- Deflection by second coil-Vertical motion and its quick retrace back to the top to start the process allover again.
- Encounters different resistance across the target plate.
- Result in a flow of current which varies in magnitude during scanning.
- We get the true information of the scene
- Scanning converts the in antenna formation existing in space and time coordinates into time variation only –called a video signal.
- Video signal is amplified-amplitude modulated with channel picture carrier frequency.
The major problems during this conversion are:
- Poor sensitivity.
- Poor resolution.
- Higher noise level .
- Improper spectral response .
- Poor contrast range.
- Difficulties in processing the output signal
No.(4)Derive a mathematical expression for radar equation. The radar range equation relates the range of the radar to the characteristics of the transmitter, receiver antenna ,target & the environment.
- It is used as a tool to help in specifying radar sub system specifications in the design phase of a program.
- The various factor that determine the range of a radar set is
Assume : Rt= Rr =R=Range (distance to target) PT = Transmitter power(peak power of radar transmitter)watt. GT=Antenna power gain(w.r.t.isotropic)i.e max power gain of transmitting antenna relative to isotropic radiators. S=Target cross section =RCS=Radar cross section m2. PR=Received power,i. e power absorbed by receiving antenna relative to isotropic radiators. λ=Wave length Ae= aperture area of receiving antennai.e effective antenna aperature. Ao=Capture area of receiving antenna . _If the transmitter delivers PT watts into an isotropic Antenna,then the power density(watt/m2)at a distance ‘R’ from the radar is PT/4πR2 watts/m2……………………………(1) Therefore,power density from a directional antenna= PTGT/4πR2 watt/m2 ………………………..(2)
- The target intercepts a portion of the incident power which depends on its cross sectional area(‘s’).
- Therefore intercepted power at target is
- =PTGTσ / (4ΠR2)2…………………………………………………………………….(4)
Power delivered to the receivers is =PrGTσ/ (4ΠR2)2.Ae…………………………………………. (5)
- Ae called power of the echo signal delivered to the receivers is
Ae= G.λ2/4π………………………………………………………… (6) Also Ae= AA.na……………………………………………………… (7)
- As same antenna is used for reception & transmission then
|GR = GT…(8)|
Maxm Range, Rmax=(PT.AE2.σ/4π.λ2.Smin)1/4……………………………(10) Rmax= (PTG2T.λ2σ/(4π)3.Smin)1/4 ……………………………………………(11) RMAX= (PTGT.Gr λ2. σ./(4π)3.Smin)1/4 ………………………………………..(12) where GT=GR The equations (10)(11)(12)are different forms of radar range eqution.
No(.5) Explain how the image orthicon develops video signal when light from any scene is focused on its face plate.
Ans.It was first produced in the year 1945 and because of its superior performance its soon replaced all earlier tv camera tube like image dissector,iconoscope and orthicon.It has the following qualities : (i)It has high photo graphic sensitivity . (ii)It provides a very dependable service. (iii)It provides an excellent response over a wide. range of illumination level varying from bright sunlight to dark shadow. As a result of this ,image orthicon remained in popular TV studio use for a long period but it is now being replaced by plumb icon. Principle of working : A lens system focuses light from the scene on to atranslucent photo cathode . Electrons gate emitted from the various points on photocathode surface in proportion to the illumination .These photo electrons travel to a thin glass target causing secondary emission from it .These secondary electrons get collected by a fine mesh placed close to the glass target plate as shown fig.This causes electron deficiency distribution or positive charge distribution on the photo cathode. This positive charge distribution originally on the front surface of the target plate leaks through thin plate to the back surface. A low velocity scanning beam from an electron at the other end of the image orthicon tube scans the back surface of the target plate following a predetermined scanning pattern . theelectroned is slowed down to a non zero velocity as it approaches the target.From a dark element ,the scanning electron beam is returned unaffected .However ,at light element(positively charge element),the scanning beam is deprived of some of its electrons to netralise the positive charge so that the returning electrons to neutralize the positive charge so that the returning electron beam contains lesser electron .This varying density returning beam is fed to an electron multiplier.
No(.6). What is NTSC system and what are the limitation of NTSC? NTSC system (NATIONAL TELEVISION SYSTEM COMMITTEE)
- NTSC system is compatible with 525 lineAmerican system.
- In order to maintain compatibility two new colour difference signals are generated & they arerepresented as I&Q instead of transmitting (R-Y)&(B-Y)directly.
- The signal I&Q are prepared from (R-Y)&(B-Y) using the following relations e.g. Inphase component,
I=0.74(R-Y)-0.27(B-Y)……………………..(1) & Quadrature phase component, Q=0.48(R-Y)+0.41(B-Y)…………………..(2)
- In the NTSC system the luminance signal Y is used for brightness or monochrome transmisssion.
- Since eye is capable of resolving finer details in the region around I,it is allowed to have a maximum bandwidth of 1.5 MHZ.The bandwidth of ‘Q’ signal is restricted to 0.5 MHZ.
The NTSC system has following limitations i.e
- It is sensitive to transmission path difference which is introduce phase errors.These errors change colors .
- When over the program change over takesplace beetween local television network & video tape recorders ‘the phase error has introduced in chroma signal.
- The phase angle of chrominance signal is also affected by levelof signal.
- Cross talk between demodulator output causes distortion.
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