(4) Answer : misconfigured the radar display are known as clutter. (ii) Target Cross Section :

Transcription

1 B. TECH. VIII SEM. I MID TERM FEB 2018 SUBJECT: RADAR & TV ENGG. TIME ALLOWED: 2 Hours BRANCH: ECE SUB. CODE: 8EC2A MAX. MARKS: 40 Question 1 a) Define the following radar terms & discuss their variation with Radar Frequency Self Clutter Target Cross Section Ambiguous & Unambiguous Range Back Scattering Duplexer Maximum Range. (4) Answer : (i) Clutter: We know that Radar is Radio Detection & Ranging. The basic principle of Radar is that a high power short duration pulse(electromagnetic wave) of microwave frequency range transmitted from the transmitter. This wave get intercept by target & reflected from the crossprocess this signal section of target called Echo Signal which is received by Receiver,Receiver & observe that is it above Threshold level or not? If it is above the threshold level then,target is Present then Receiver process the signal & calculate time, distance, shape, Doppler shift etc. If the returned echo signal are from stationary target or Unwanted signal then it is Clutter. Therefore in competition with return from the aircraft, there are many sources of unwanted signal. These unwanted signals in radar are defined as noise or clutter. Clutter includes sea return, weather return, ground return, stationary target like tree, surface etc. Reflection from storm clouds with misconfigured the radar display are known as clutter. (ii) Target Cross Section : The Radar cross section of a target is the area of intercepting that amount of power which when scattered in all direction, producer an echo at the radar equal to that from target

2 (iii) Ambiguous and Unambiguous Range: The most common radar waveform is a train of narrow pulses. The distance of target is determined by the calculating the time taken by the pulse travel to the target and return. So the distance is R=cT/2 Let time duration between two transmitting pulses is T. If the Echo signal return from target and the time t which is less than T then it is called first round time echo. If the Echo signal return from target and the time t which is greater than T then it is called second round time echo. So the range beyond which target appears to be second round time echo is call maximum unambiguous range. The range up to which target appears to be first round time echo is called maximum ambiguous range.

3 Where f r is the pulse repetition frequency. (iv) Back Scattering : For plane-wave radiation incident on a scattering object or a scattering medium, the ratio of the intensity scattered in the direction toward the source to the incident irradiance. So defined, the backscattering cross section has units of area per unit solid angle, for example, square meters per steradian. In common usage, synonymous with radar cross section, although this can be confusing because the radar cross section is 4π times the backscattering cross section The frequency of the incident radiation determines: - the penetration depth of the waves for the target imaged; - the relative roughness of the surface considered. Penetration depth tends to be longer with longer wavelengths. If we consider the example of a forest, the radiation will only penetrate the first leaves on top of the trees if using the X-band (λ = 3 cm). The information content of the image is related to the top layer and the crown of the trees. On the other hand, in the case of L-band (λ = 23 cm), the radiation penetrates leaves and small branches; the information content of the image is then related to branches and eventually tree trunks. (iv) Duplexer: A device by which transmitter and receiver share the single antenna. Means when the transmitter section is active then with block the receiver section and transmit the signal into the space. On the receiving time of the echo signal transmitter section is off and receiver section is active. So by using a single antenna and a duplexer we can share this antenna between transmitter and receiver, only one section will be active at a time. Example: TR Tube,Circulator. (v) Maximum Range : The range of the radar beyond which the target is not detectable is called maximum range. It depends upon the following parameter like gain of the transmitting antenna, frequency of the transmitting pulse, area of cross section of target, effective aperture area of antenna. b) Explain block diagram & Working principle of RADAR. Derive the Maximum Range Equation of Radar (4) Answer: We know that Radar is Radio Detection & Ranging. The basic principle of Radar is that a high power short duration pulse(electromagnetic wave) of microwave frequency range

4 transmitted from the transmitter. This wave get intercept by target & reflected from the cross- this signal & section of target called Echo Signal which is received by Receiver,Receiver process observe that is it above Threshold level or not? If it is above the threshold level then,target is Present then Receiver process the signal & calculate time, distance, shape, Doppler shift etc. BLOCK DIAGRAM : Timer which is also called trigger generator or the synchronizer vision generate series of narrow pulses at the pulse repetition frequency. So we can say timer is a unit which is used to make the switch on or off the modulator unit. Modulator is a device when generate the modulated waveform which have to be transmitted into the space. Generally we use the magnetron. This modulated wave is amplified by the high power transmitter amplifier.for 100 to 200 miles, the power should be 1MW with pulse repetition frequency of 100 pulses per second. Normally we use the TWT amplifier on Klystron amplifier. The modulated RF pulse transmitted to the antenna by the transmission line. Fast acting switch called duplexer is used to share the antenna by the transmitter and receiver. TR switch stop the receiving section during transmission. After passage of Transmission signal,the TR Switch again connect to the receiving section. The transmitted wave intercept by the target & and portion of the incident wave reflected by the target towards the radar. This is calling echo signal. ATR switch is used to receive the echo signal

5 & fed to receiver. In absence of ATR switch the portion of the received power dissipated in the transmitted section.tr and ATR combined form Duplexer. RF amplifier consists of low noise parametric amplifier, travelling wave tube amplifier. The mixer and the local oscillator convert the RF amplifier output into the intermediate frequency. This intermediate frequency signal is amplified by the IF amplifier. Reflex klystron is used as a local oscillator. Received frequency is different from the transmitted frequency then there is a Doppler Shift which means target is moving. So the velocity of target is calculated by the Doppler shift. The detector extract the message from the modulated signal and compare it with the threshold level. If the signal is above the threshold level that target is said to be present. Output of the detector is bipolar video which is displayed on the radar display like A scope, B scope, PPI display. RADAR RANGE EQUATION: The simple form of the radar range equation is the DERIVATION: If the power transmitted from the antenna is P t then Power density radiated from the isotropic antenna at R distance will be P t /4 2 Power density radiated from the directive antenna of gain G t at R distance will be P t G t /4 2..Eq-1 The target intercept the portion of the incident wave and reradiated in all direction. Eq-2 Where is Target Cross Section...Eq---3 Eq-4

6 Eq-5 By reciprocity theorem if we use same anteena for transmitter & receiver then Gain of Transmitting & Receiving will be same.so. Now this received power should be equal to at least Minimum Threshold level S min at R=R max So OR Question 1 c) Why we replace CW Radar with FM-CW Radar. Draw the block Diagram of Frequency Modulated CW Radar & explain its application in measurement of Range. (4) Answer : CW Radar is Continuous Radar. It is Doppler Radar.It can measure the Doppler shift and calculate the velocity of target. But it have some disadvantage that it is unable to measure the range related to the relative narrow spectrum. This is overcome by Frequency Modulated CW Radar. This does not appear to be possible in a CW radar since we have no way of determining the time after which a particular part of the transmitted waveform comes back in the form of an echo. This is so since it is impossible to distinguish one part of a continuous signal waveform from another. In pulse radars there is considerable gap between one pulse and the next and so it was easy to associate or identify a pulse with its echo. Recall that even there this identification became difficult when the gap between pulses was small (or the target was at a large distance), giving rise to second-time-around echoes. In CW radars an exactly similar effect, though of a more serious nature, occurs thus making it impossible to identify a part of and echo waveform with its original transmitted waveform. This is the reason why an ordinary radar is incapable of measuring range to an object. A solution to this problem can be obtained by using frequency modulation.

7 A simple way to do this is to vary the transmitted frequency over a certain range. Then the transit time is proportional to the difference in frequency between the echo signal and the transmitter signal (for a stationary target). The greater the transmitter frequency deviation in a given time interval, the more accurate the measurement of the transit time. Radars which use this mode of operation are called frequency modulated continuous wave (FM -CW) radars. Below we will describe how range measurement is done in FM-CW radars. In the frequency modulated continuous wave radar the frequency of the waveform changes with respect to the time. The transmitter pregnancy changes linearly with time. With there is a target at a R distance then the signal will return after T=2R/c. If the echo signal is mixed with the portion of the transmission signal then the beat frequency will be produced. Since the beat (or difference frequency) is caused only by the target s range (as the target is stationary) it is also denoted by fr. Consider the transmitter CW signal at time ta, having

8 frequency fa. This signal hits the stationary target and comes back to the radar at time tb when the frequency of the transmitted signal would have increased to fc. Hence, the increase in the transmitted frequency during the to-and-fro transit time T of a signal is (fc fa) and is the beat frequency. Thus, at any given instant in time the difference between the currently trans-mitted signal frequency and the currently received signal is a measure of the to-and-fro transit time of the transmitted signal. We extract range information from a measurement of f)b as follows: Let the slope of the curve be f0, the rate of change of frequency, or the modulation rate. Note that this is a known quantity since the modulation rate is chosen by the designer at the radar end. Then, fb = fr = f0t = f0 2R/c where, R is the distance to the target and so T = 2R/c The frequency of the beat not is calculated by the frequency counter which is calibrated in the form of the distance (d) Explain the MTI Radar. Describe the necessity of Delay Line Canceller. Describe various type of Delay Lines used in MTI Radar. (4) Answer : we know doppler frequency shift can be used in continuous wave radars to determine relative velocity of a moving target or distinguish moving targets from stationary targets. The doppler frequency shift produced by a moving target may also be used in a pulse radar to determine the relative velocity of a target or to separate desired signals from moving targets and undesired signals from stationary objects (clutter). Though the doppler frequency shift is sometimes used to measure relative velocity of a target using a pulse radar, its most interesting and widespread use has been in identifying small moving targets in the presence of large clutter. Such pulse radars which use the doppler frequency shift to distinguish (or discriminate) between moving and fixed targets are called MTI (Moving Target Indicators) and Pulse Doppler Radars.. For instance the MTI radar operates on low pulse repetition frequencies thus causing ambiguous Doppler measurements (blind speeds) but unambiguous range measurements (no second -timearound echoes). On the other hand the pulse doppler radar operates on high pulse repetition. frequency thus causing unambiguous doppler measurements (no blind speeds) but ambiguous range measurements (second-time-around echoes) MTI RADAR: When it is desired to remove the clutter due to stationary targets an MTI radar is employed.

9 The basic principle of MTI radar is to compare a set of received echoes witrh those received during the previous sweep. Moving targets will give change of phase and are not cancelled. Thus clutter due to stationary targets both manmade and natural is removed from the display and this allows easier detection of moving targets. Pulse modulator generator Narrow triggering pulse. This are going to be transmitted by the transmitter section after passing through the klystron amplifier. The section and the transmitter section are sharing the same antenna with the help of the TR Switch of the duplexer. The coherent reference signal is generated by the COHO which is coherent oscillator. The COHO is stable local oscillator output is mixed with the local oscillator frequency. The local oscillator must be a stable oscillator called STALO. Mixer-2 in block diag generates the transmitter frequency (f0+fc)(f0+fc) which is obtain by the sum of frequency produced by two oscillators the STALO and COHO (coherent oscillator producing fcfc). Echo pulse from the target is received by the MTI radar antenna. If the echo is due to moving target, the echo pulse undergoes Doppler frequency shift. The received echo pulse is then passed through mixer-1 which mix (f0+fc)(f0+fc) with f0f0 and produce a difference frequency fc at its output. The detector output is proportional to phase difference between two signals.

10 Phase difference is constant for all stationary targets but varies for moving targets. Thus Doppler frequency shift is there as per phase difference. This delay line acts as high pass filter to separate Doppler shifted echo signal of moving target from stationary clutter. DELAY LINE CANCELLOR : We know that in MTI radar, the phase of the previous sweep and the present sweep have to be compared for the detection of the moving target. So the task of the delay line canceller which to provide the delay equal to the time. of the transmitting pulse and then subtract it with the present pulse. Delay line canceller is acting as a filter which remove the component of the DC clutter. After The Doppler Effect, the equation of the received pulse will be The Subtractor output is

11 Blind Speed : The target is moving with such velocity that the phase change in the received Echo pulse will be integral multiple of 2π i.e 2nπ. In this case the output of The phase detector is zero which implies that target is stationary but actually that It was moving. So the speed of the target which is not detected by the radar is called blind speed

12 Question 2 a) A pulse Doppler Radar has carrier frequency of 9GHz & PRF of 400Hz. Find its Doppler Blind Frequencies & Radial velocity of Target. (4) Answer : F d = n *400 F d = n *400 where f r =400 Hz Now put n=1,2,.. we get f d = 400,800 Hz respectively =(n*3*10 8 *400)/(2*9*10 9 ) = 12*10 10 /18*10 9 m/sec = 6.5 m/sec for n=1 b) Explain the Radar Display with TV & CRO Screen. How a moving target can be displayed on the Radar Screen. Write all Signal Processing Steps. (4) GENERAL RADAR DISPLAY TYPES There are two types of radar displays in common use today. RAW VIDEO Raw video displays are simply oscilloscopes that display the detected and amplified target return signal (and the receiver noise). Raw video displays require a human operator to interpret the various target noise and clutter signals. On the left hand display of Figure 1, an operator could readily identify three targets and a ghost (a ghost is a phony target that usually fades in and out and could be caused by birds, weather, or odd temporary reflections - also referred to as an angel). Target 3 is a weak return and hidden in the noise - an operator can identify it as a target by the "mouse under the rug" effect of raising the noise base line.

13 SYNTHETIC VIDEO Synthetic video displays use a computer to clean up the display by eliminating noise and clutter and creating it's own precise symbol for each target. On the right hand display target 1 comes and goes because it is barely above the receiver noise level - notice that it is quite clear on the raw video. Target 3 wasn't recognized by the computer because it's to far down in the noise. The computer validated the ghost as a target. The ghost might be a real target with glint or ECM characteristics that were recognized by the computer but not the operator. Figure 1. Radar Display Types SEARCH AND ACQUISITION RADARS They generally use either a PPI or a sector PPI display as shown in Figure 2. PPI displays can be either raw video or synthetic video. PPI scope (plan position indicator). Polar plot of direction and distance. Displays all targets for 360 degrees. Sector PPI scope. Polar plot of direction and distance. Displays all targets within a specific sector. Origin may be offset so that "your" radar position may be off the scope.

14 TRACKING RADARS Usually use some combination of A, B, C, or E scope displays. There are many other types of displays that have been used at one time or another - including meters - but those listed here are the most common in use today. Figure 2. Common Radar Displays A-SCOPE Target signal amplitude vs. range or velocity. Displays all targets along pencil beam for selected range limits. Displays tracking gate. Usually raw video. Some modern radars have raw video a-scopes as an adjunct to synthetic video displays. Must be used with a separate azimuth and elevation display of some sort. Also called a range scope (R-Scope). B-SCOPE Range vs. azimuth or elevation. Displays targets within selected limits. Displays tracking gate. May be raw or synthetic video. Surface radars usually have two. One azimuth/one elevation which can result in confusion with multiple targets. C-SCOPE

15 Azimuth vs. elevation. Displays targets within selected limits of az and el. Displays tracking gate. May display bull's-eye or aim dot. May have range indicator inserted typically as a marker along one side. Usually synthetic video. Pilots eye view and very common in modern fighter aircraft heads up displays for target being tracked. Could be used in any application where radar operator needs an "aiming" or "cross hair" view like a rifle scope. E-SCOPE Elevation vs. Range similar to a B-scope, with elevation replacing. The radar antenna sends pulses while rotating 360 degrees around the radar site at a fixed elevation angle. It can then change angle or repeat at the same angle according to the need. Return echoes from targets are received by the antenna and processed by the receiver and the most direct display of those data is the PPI. It is to be noted that the height of the echoes increases with the distance to the radar, as represented in the adjacent image. This change is not a straight line but a curve as the surface of the Earth is curved and sinks below the radar horizon. For fixed-site installations, north is usually represented at the top of the image. For moving installations, such as small ship and aircraft radars, the top may represent the bow or nose of the ship or aircraft, i.e., its heading (direction of travel) and this is usually represented by a lubber line. Some systems may incorporate the input from a gyrocompass to rotate the display and once again display north as "up". Also, the signal represented is the reflectivity at only one elevation of the antenna, so it is possible to have many PPIs at one time, one for each antenna elevation. OR a). Calculate the Maximum Range when the radar operate at 10GHz & has Peak Power =10MWatt,S minimum = 1000W, Antenna Cross Section =100mm 2,Target Cross Section=10mm 2.Gain of transmitting Antenna is 15dB (8) Answer : P t =1 * 10 6 Watt G= 15 db = 32 Area of cross section = 100/ 10 6 Radar cross section = 10/10 6 Wavelength = 3*10 8 /10 10

16 Question 3 a) Explain the Block Diagram of TACAAN & DME & Explain the Operating Principle (4) Answer: TACAN stands for Tactical Air Navigation. This system was developed to obtain both the range measurement as well as azimuth measurements of the aircraft. Azimuth information is obtained with the help of antenna rotating with the speed of about 900 RPM(Revolutions per minute). TACAN also has two parts interrogator and ground beacon and works similar to the DME. These have been described below. Both DME and TACAN operates in the following four modes viz.x-mode, Y-mode, W-mode and Z-mode. Following table mentions frequencies used by the interrogator and ground beacon in these four modes. Both the DME and TACAN pulse pair is of gaussian shapewith duration of about 3.5µS and separation between two pulses is about 12 µs for X-mode and 36 µs for Y-mode..TACAN is a polar-coordinate type radio air-navigation system that provides an aircrew with distance information, from distance measuring e q u i p m e n t (DME), and bearing (azimuth)information. This information, as shown in figure, is usually provided by two meters.

17 One meter indicates, in nautical miles, the distance of the aircraft from the surface beacon. The other meter indicates he direction of flight, in degrees-of-bearing, to the geographic location of the surface beacon. By using the TACAN equipment installed in the aircraft and TACAN ground equipment installed aboard a particular surface ship or shore station, a pilot can obtain bearing to and distance from that location. The distance measuring concept used in TACAN equipment isan outgrowth of radarranging techniques. Radar-ranging determines distance by measuring the round-trip travel time of pulsed rf energy. The return signal (echo) of the radiated energy depends on the natural reflection of the radio waves. However, TACAN beacon-transponders generate artificial replies instead of depending onnatural reflection.. The airborne equipment generates timed interrogation pulse pairs that the surface TACAN system receives and decodes. After a 50-µsec delay, the transponder responds with a reply. The airborne DME then converts the round-trip time to distance from the TACAN facility. The frequency and identification code provide the geographic location of the transmitting beacon. TACAN PULSE PAIRS : TACAN transponders use twin-pulse decoders to pass only those pulse pairs with the proper spacing. The purpose of this twin-pulse technique is to increase the average power radiated and to reduce the possibility of false signal interference. After the receiver decodes an interrogation, the encoder generates the necessary pulse pair required for the transponder s reply. b) How the range increased in a LORAN System. Write its application & merits over a Simple Radar System. (4) Answer: The LORAN (Long-Range-Navigation ) is a position fixing aid. It operates on a single frequency of 100 Khz and has a long range (greater than 1200 km). LORAN determines location by comparing accurately-synchronized powerful radio pulses originating from different reference transmitter sites. Pulses from nearby transmitters arrive earlier than pulses from distant transmitters since radio signals travel at a constant speed. At least three different LORAN signals must be received to determine latitude and longitude. In practice, the distance to more than the minimum three LORAN signals increases accuracy.

18 The basic principle of LORAN is simple. Each LORAN chain consists of a master station and two, three or more slave stations. The aircraft receiver must be tuned to select a chain (of master and slave stations) by manual or by computer selection. Each chain transmits a sequence of pulses. First the master and then after a fixed coding delay, the slaves Each slave in a chain has a unique coding delay that allows the aircraft to receive its signal before any other slave transmits. Usually the master s signal is received by the slaves and retransmitted after the specified coding delay. The number of pulses (eight or nine) and the coding delay identifies the master and slaves of a given chain. the navigation computer in the aircraft is fed with the position information of the master and slave stations in a chain. The receiver in the aircraft measures the difference between the time of arrival of the pulse from the master station and the slave stations. The time difference is measured using the third RF cycle in each pulse as the reference point (see The locus of points of constant time difference is a hyperbola-like line-of-position on the reference ellipsoid which models the surface of the earth. By using the master and a second slave a second hyperbola is obtained. The two hyperbolas intersect at the aircraft s position and at an ambiguous second point. The ambiguity can be resolved by using another slave and obtaining a third line-of position. However, the use of too many lines-of-position can lead to a possible region of location of the aircraft rather than a single point. this region is called a cocked hat in the marine terminology This can also occur when a number of navigation aids are used (multi -sensor navigation). Due to the hyperbola-like lines of position LORAN is also called a hyperbolic navigation system.

19 LORAN-C does not suffer from LOS since it operates at LF band. Hyperbolic grid NAV provides a more direct route similar to WPT NAV. Signal propagation is based on GND waves; therefore large travel ranges are possible. RADAR VS LORAN : Radar operate at Microwave frequency range while Loran at LF Band. Radar calculate the distance by calculating the time of echo pulse. But in Loran the position is defined by the intersection of the Hyperbolas.So more accurate results. OR a) Explain the Aircraft Landing System (ILS) (8) Answer : The Instrument Landing System (ILS) is an internationally normalized system for navigation of aircrafts upon the final approach for landing. The ILS system is nowadays the primary system for instrumental approach for category I.-III- A conditions of operation minimums and it provides the horizontal as well as the vertical guidance necessary for an accurate landing approach in IFR (Instrument Flight Rules) conditions, thus in conditions of limited or reduced visibility. The accurate landing approach is a procedure of permitted descent with the use of navigational equipment coaxial with the trajectory and given information about the angle of descent. The equipment that provides a pilot instant information about the distance to the point of reach is not a part of the ILS system and therefore is for the discontinuous indication used a set of two or three marker beacons directly integrated into the system. The system of marker beacons can however be complemented for a continuous measurement of distances with the DME system (Distance measuring equipment),

20 The ground part of this UKV distance meter is located co-operatively with the descent beacon that forms the glide slope. It can also be supplemented with a VOR system by which means the integrated navigational-landing complex ILS/VOR/DME is formed. Categories of operation minimums. Analysis Category I A minimal height of resolution at 200 ft (60,96 m), whereas the decision height represents an altitude at which the pilot decides upon the visual contact with the runway if he ll either finish the landing maneuver, or he ll abort and repeat it. The visibility of the runway is at the minimum 1800 ft (548,64 m) The plane has to be equipped apart from the devices for flying in IFR (Instrument Flight Rules) conditions also with the ILS system and a marker beacon receiver. Category II A minimal decision height at 100 ft (30,48 m) The visibility of the runway is at the minimum 1200 ft (365,76 m) The plane has to be equipped with a radio altimeter or an inner marker receiver, an autopilot link, a raindrops remover and also a system for the automatic draught control of the engine can be required. The crew consists of two pilots. Category III A A minimal decision height lower than 100 ft (30,48 m) The visibility of the runway is at the minimum 700 ft (213,36 m) The aircraft has to be equipped with an autopilot with a passive malfunction monitor or a HUD (Head-up display). Category III B A minimal decision height lower than 50 ft (15,24 m) The visibility of the runway is at the minimum 150 ft (45,72 m) A device for alteration of a rolling speed to travel speed. Category III C Zero visibility Basic elements of the ILS system and THEIR brief description The ILS system consists of four subsystems: VHF localizer transmitter UHF glide slope transmitter marker beacons approach lighting system

21 Figure The description and placement of the individual parts of the ILS system Localizer One of the main components of the ILS system is the localizer which handles the guidance in the horizontal plane. The localizer is an antenna system comprised of a VHF transmitter which uses the same frequency range as a VOR transmitter (108,10 111,95 MHz), however the frequencies of the localizer are only placed on odd decimals, with a channel separation of 50 khz. The trasmitter, or antenna, is in the axis of the runway on it s other end, opposite to the direction of approach. A backcourse localizer is also used on some ILS systems. The backcourse is intended for landing purposes and it s secured with a 75 MHz marker beacon or a NDB (Non Directional Beacon) located 3 5 nm (nautical miles), or 5,556 9,26 km before the beginning of the runway. The course is periodically checked to ensure that the aircraft lies in the given tolerance

22 Figure Antennas of the localizer system The transmitted signal: The localizer, or VHF course marker, emits two directional radiation patterns. One comprises of a bearing amplitude-modulated wave with a harmonic signal frequency of 150 Hz and the other one with the same bearing amplitude-modulated wave with a harmonic signal frequency of 90 Hz. These two directional radiation patterns do intersect and thus create a course plane, or a horizontal axis of approach, which basically represents an elongation of the runway s axis (see Fig. 3). For an observer a pilot, who is situated on the approaching side of the runway (therefore in front of the LLZ antenna system) predominates a modulation of 150 Hz on the right side of the course plane and 90 Hz on the left. The intersection of these two regions determines the on-track signal. The width of the navigational ray can span from 3 to 6, however mostly 5 are used. The ray is set to secure a signal approximately 700 ft (213, 36 m) wide on the borderline of the runway. The width of the ray magnifies, so at a distance of 10 nm (18,52 km) from the transmitter is the ray about 1 nm (1,852 km) wide. The range of the localizer can be even 18 nm (33,336 km) in the 10 field from the center of the ray (on-track signal) and 10 nm (18,52km) in the field from the center of the ray, because the main part of the signal is coaxial with the middle of the runway. The localizer is identified by an audio signal added to the navigational signal. The audio signal consists of letter I, following with a two-letter addition, for example: I-OW.

23 Radiation pattern of the localizer s VHF transmitter UHF descent beacon glide slope The transmitted signal: The glide slope, or angle of the descent plane provides the vertical guidance for the pilot during an approach. It s created by a ground UHF transmitter containing an antenna system operating in the range of 329, MHz, with a channel separation of 50 khz. The transmitter (Fig. 4) is located ft (228,6 381 m) from the beginning of the runway and ft (121,92 182,88 m) from it s axis. The observed tolerance is ±0,5. The UHF glide slope is paired with the corresponding frequency of the VHF localizer. The UHF descent beacon draws a glide slope in the area (figure source: ILS_09R_Glideslope.jpg) Like the signal of the localizer, so does the signal of the glide slope consist of two intersected radiation patterns, modulated at 90 and 150 Hz. However unlike the localizer, these signals are arranged on top of each other and emitted along the path of approach, as you can see in Fig. 5. The thickness of the overlaping field is 0,7 over as well as under the optimal glide slope.

24 The radiation pattern of the UKV descent beacon forming the glide slope The signal of the glide slope can be set in the range of 2 4,5 over the horizontal plane of approach. Typically it s a value of 2,5 3, depending of the obstacles along the corridor of approach and the runway s inclination. False signals can be generated along the glide slope. It s happening in multiples of the angle that s formed by the glide slope and the horizontal plane. The first case arises at approximately 6 over the horizontal plane. These false signals are inversive, which means that the directions to climb or descend will be swapped. A false signal at 9 will be oriented the same as the real glide slope. There are no false signals under the glide slope. Onboard equipment Localizer receiver The signal is received on board of an aircraft by an onboard localizer receiver. A simplified block scheme of the onboard receiver of the localizer s signals is displayed in The localizer receiver and the VOR receiver form a single unit. The signal of the localizer launches the vertical indicator called the track bar (TB). Provided that the final approach does occur from south to north, an aircraft flying westward from the runway s axis (Fig. 7) is situated in an area modulated at 90 Hz, therefore the track bar is deflected to the right side. onboard course beacon s signal receiver Block scheme of the

25 A plane flying approximately along the axis of approach, however partially turned away to the left On the contrary, if the plane s positioned east from the runway s axis, the 150 Hz modulated signal causes the track bar to lean out to the right side. In the area of intersection, both signals affect the track bar, which causes to a certain extent a deflection in the direction of the stronger signal. Thus if an aircraft flies roughly in the axis of approach leaned out partially to the right, the track bar is going to deflect a bit to the left. This indicates a necessary correction to the left. In the point where both signals 90 Hz and 150 Hz have the same intensity, the track bar is in the middle. Meaning that the plane is located exactly in the approach axis nearly in the approach axis slighlty leaned out to the right A plane flying

26 When the track bar is used in conjunction with a VOR, a lean out of 10 to one or the other side from the signal causes a full deflection of the indicator. If the same pointer is used as an indicator of the ILS localizer, a full deflection will be induced by a 2,5 diversion from the center of the localizer s beam. Therefore the sensitivity of the TB is roughly four times greater in the function as an indicator of the localizer as at the indication of information from the VOR. A plane flying exactly in the axis of approach In case that a red NAV bat appears in the upper right section of the onboard ILS indicator it represents that the signal is far too weak or out of the receiver s reach and for that reason the pointer s deflection cannot be considered to be accurate. The vertical pointer will return to the neutral position, meaning to the center of the indicator. A momentary display of the NAV bat, short deviations of the TB, or both instances happening at once can occur in the case that an aircraft flies between the receiver s antenna and the transmitter, or some other obstacle gets into their way. of reach of the VKV course beacon s signal A plane situated out glide slope receiver The glide slope s signal is on board of a plane received by means of a UHF antenna. In modern avionics are the controls for this receiver combined with the VOR s controls, so the correct frequency of the glide slope beacon is tuned in automatically at the instant when the localizer s frequency is selected.

27 The glide slope s signal puts the horizontal pointer of the glide slope into operation which intersects the TB,. This indicator has its own GS bat which lights up whenever the glide slope beacon s signal is too weak or the onboard receiver, hence the whole aircraft is out of the signal s reach An example of the displayed GS pointer notifying a diversion from the glide slope, a too weak received signal, or an obstacle on the way. The onboard indicator of the ILS system can be used by a pilot to determine the exact position because it provides vertical as well as horizontal guiding. The case in Fig. 12 portrays both indicators in the middle, which means that the aircraft is located in the point of intersection of the course plane (horizontal) and the glide slope. The event pictured in Fig. 13 indicates that the pilot must descent and correct the flight course to the left in order to aquire the correct course and glide slope level. The case in Fig. 14 shows a necessity to ascend and adjust the flight course to the right. With a 1,4 overlapping of the beams is the area around 1500 ft (457,2 m) wide at a distance of 10 NM (18,52 km), 150 ft (45,72 m) at a distance of 1 NM (1,852 km), and less than one foot (0,3 m) at the instant of touch down. The apparent sensitivity of the instrument increases as the aircraft closes in to the runway. The pilot has to watch the indicator with attention so that he can keep an overlap of both needles of the pointer in the middle of the indicator. Thereby he ll achieve a precise homing all the way to the touch down.

28 Both pointers in the middle the aircraft is located in the point of intersection of the course and descent plane. Figure 13 A case when the aircraft is located right of the runway s axis and too high over the glide slope.

29 Figure A case when the aircraft is located left of the runway s axis and too low under the glide slope. Marker beacons For the purpose of discontinuous addition of navigation data with the value of a momentary distance from the aircraft to the runway s threshold, the following marker beacons are used: Outer Marker (OM) The outer marker is located 3,5 6 NM ( km) from the runway s threshold. Its beam intersects the glide slope s ray at an altitude of approximately 1400 ft ( m) above the runway. It also roughly marks the point at which an aircraft enters the glide slope under normal circumstances, and represents the beginning of the final part of the landing approach. The signal is modulated at a frequency of 400 Hz, made up by a Morse code a group of two dots per second. On the aircraft, the signal is received by a 75 MHz marker receiver. The pilot hears a tone from the loudspeaker or headphones and a blue indicative bulb lights up. Anywhere an outer marker cannot be placed due to the terrain, a DME unit can be used as a part of the ILS to secure the right fixation on the localizer. In some ILS installations the outer marker is substituted by a Non Directional Beacon (NDB). Figure The outer position marker (blue). Middle Marker (MM) The middle marker is used to mark the point of transition from an approach by instruments to a visual one. It s located about 0,5 0,8 NM ( m) from the runway s threshold. When flying over it, the aircraft is at an altitude of ft (60,96 76,2) above it. The audio signal is made up of two dashes or six dots per second. The frequency of the

30 identification tone is 1300 Hz. Passing over the middle marker is visually indicated by a bulb of an amber (yellow) colour. It was removed in some countries, e.g. in Canada. Figure The middle marker (yellow). The inner marker emits an AM wave with a modulated frequency of 3000 Hz. The identification signal has a pattern of series of dots, in frequency of six dots per second. The beacon is located 60m in front of the runway s threshold. The inner marker has to be used for systems of the II. and III. category. Figure The inner marker (white). Question 4 a) Explain the Composite Video Signal. Define Front & Back Porch in Horizontal Sync Pulse Pedestal Height & Noise Level in CVS Pre & Post Equalizing Pulse in Vertical Sync Pulse (4) Answer : Composite video signal consists of a camera signal corresponding to the desired picture information, blanking pulses to make the retrace invisible, and synchronizing pulses to synchronize the transmitter and receiver scanning. A horizontal synchronizing (sync) pulse is needed at the end of each active line period whereas a vertical sync pulse is required after each field is scanned. The amplitude of both horizontal and vertical sync pulses is kept the same to obtain higher efficiency of picture signal transmission but their duration (width) is chosen to be different for separating them at the receiver. Since sync pulses are needed consecutively and not simultaneously with the picture signal, these are sent on a time division basis and thus form a part of the composite video signal. The level of the video signal when the picture detail being transmitted corresponds to the maximum whiteness to be handled, is referred to as peak-white level. This is fixed at 10 to 12.5 percent of the maximum value of the signal while the black level corresponds to approximately 72 percent. The sync pulses are added at 75 percent level called the blanking level. The difference between the black level and blanking level is known as the Pedestal. Thus the picture information may vary between 10 percent to about 75 percent of the composite video signal depending on the relative brightness of the picture at any instant. The darker the picture the higher will be the voltage within those limits. Note that the lowest 10 percent of the voltage range (whiter than white range) is not used to minimize noise effects. This also ensures enough margin for excessive bright spots to be accommodated without causing amplitude distortion at the modulator. D.C. component of the video signal. In addition to continuous amplitude variations for individual picture elements, the video signal has an average value or dc component corresponding to the average brightness of the scene. In the absence of dc component the receiver cannot follow changes in brightness, as the ac camera signal, say for grey picture elements on a black background will then be the same as a signal for white area on a grey back-ground.

31 The pedestal height is the distance between the pedestal level and the average value (dc level) axis of the video signal. This indicates average brightness since it measures how much the average value differs from the black level. Even when the signal loses its dc value when passed through a capacitor-coupled circuit the distance between the pedestal and the dc level stays the same and thus it is convenient to use the pedestal level as the reference level to indicate average brightness of the scene. The blanking pulses. The composite video signal contains blanking pulses to make the retrace lines invisible by raising the signal amplitude slightly above the black level (75 per cent) during the time the scanning circuits produce retraces. As illustrated in Fig., the composite video signal contains horizontal and vertical blanking pulses to blank the corresponding retrace intervals. The repetition rate of horizontal blanking pulses is therefore equal to the line scanning frequency of Hz. Similarly the frequency of the vertical blanking pulses is equal to the field-scanning frequency of 50 Hz. Sync pulse and video signal amplitude ratio.: The overall arrangement of combining the picture signal and sync pulses may be thought of as a kind of voltage division multiplexing where about 65 per cent of the carrier amplitude is occupied by the video signal and the upper 25 per cent by the sync pulses. Thus, as shown in Fig. 3.1, the final radiated signal has a picture to sync signal ratio (P/S) equal to 10/4.

32 Front porch. This is a brief cushioning period of 1.5 µs inserted between the end of the picture detail for that line and the leading edge of the line sync pulse. This interval allows the receiver video circuit to settle down from whatever picture voltage level exists at the end of the picture line to the blanking level before the sync pulse occurs. Back porch. This period of 5.8 µs at the blanking level allows plenty of time for line flyback to be completed. It also permits time for the horizontal time-base circuit to reverse direction of current for the initiation of the scanning of next line. Infact, the relative timings are so set that small black barsback porch. This period of 5.8 µs at the blanking level allows plenty of time for line flyback to be completed. It also permits time for the horizontal time-base circuit to reverse direction of current for the initiation of the scanning of next line. Infact, the relative timings are so set that small black bars. The vertical sync pulse train added after each field is somewhat complex in nature. The reason for this stems from the fact that it has to meet several exacting requirements. Therefore, in order to fully appreciate the various constituents of the pulse train, the vertical sync details are explored step by step while explaining the need for its various components. The basic vertical sync added at the end of both even add odd fields is shown. Its width has to be kept much larger than the horizontal sync pulse, in order to derive a suitable field sync pulse at the receiver to trigger the field sweep oscillator. b) Explain the Image Orthicon Camera Tube. Define the following reason What is Divergence Effect & How we solve it? Why the Electron de accelerated at target during Scanning. What is Image Multiplication (4) Answer : IMAGE ORTHICON This tube makes use of the high photoemissive sensitivity obtainable from photocathodes, image multiplication at the target caused by secondary emission and an electron multiplier. A sectional view of an image orthicon. It has three main sections: image section, scanning section and electron gun-cum-multiplier section.

33 (i) (ii) (iii) (iv) (v) (vi) (vii) Image Section The inside of the glass face plate at the front is coated with a silverantimony coating sensitized with cesium, to serve as photocathode. Light from the scene to be televised is focused on the photocathode surface by a lens system and the optical image thus formed results in the release of electrons from each point on the photocathode in proportion to the incident light intensity. Photocathode surface is semitransparent and the light rays penetrate it to reach its inner surface from where electron emission takes place. Since the number of electrons emitted at any point in the photocathode has a distribution corresponding to the brightness of the optical image, an electron image of the scene or picture gets formed on the target side of the photocoating and extends towards it. The electron image produced at the photocathode is made to move towards the target plate located at a short distance from it. The target plate is made of a very thin sheet of glass and can store the charge received by it. This is maintained at about 400 volts more positive with respect to the photocathode, and the resultant electric field gives the desired acceleration and motion to the emitted electrons towards it. Divergence Effect The electrons, while in motion, have a tendency to repel each other and thin can result in distortion of the information now available as charge image. To prevent this divergence effect an axial magnetic field, generated in this region by the long focus coil is employed. Solution of Divergence Effect : This magnetic field imparts helical motion of increasing pitch and focuses the emitted electrons on the target into a well defined electron image of the original optical image. The image side of the target has a very small deposit of cesium and thus has a high secondary emission ratio. Because of the high velocity attained by the electrons while in motion from photocathode to the target plate, secondary emission results, as the electrons bombard the target surface. These secondary electrons are collected by a wire-mesh screen, which is located in front of the target on the image side and is maintained at a slightly higher potential with respect to the target. The wire-mesh screen has about 300 meshes per cm2 with an open area of 50 to 75 per cent, so that the screen wires do not interfere with the electron image. The secondary electrons leave behind on the target plate surface, a positive charge distribution, corresponding to the light intensity distribution on the original photocathode. Image Multiplication : Because of the high secondary emission ratio, the intensity of the positive charge distribution is four to five times more as compared to the charge liberated by the photocathode. This increase in charge density relative to the charge liberated at the photocathode is known as image multiplication. SCANNING SECTION : The electron gun structure produces a beam of electrons that is accelerated towards the target. As indicated in the figure, positive accelerating potentials of 80 to 330 volts are applied to grid 2, grid 3, and grid 4 which is connected internally to the metalized conductive coating on the inside wall of the tube. The electron beam is focused at the target by magnetic field of the external focus coil and by voltage supplied to grid 4. The alignment coil provides magnetic field that can be varied to adjust the scanning beam s position, if necessary, for correct location. Deflection of electron beam s to scan the entire target plate is accomplished by magnetic fields of vertical and horizontal deflecting coils mounted on yoke external to the tube. These coils are fed from two oscillators, one working at Hz, for horizontal deflection, and the other operating at 50 Hz, for vertical deflection.

34 The target plate is close to zero potential and therefore electrons in the scanning beam can be made to stop their forward motion at its surface and then return towards the gun structure. DEACCELERATING GRID : The grid 4 voltage is adjusted to produce uniform deceleration of electrons for the entire target area. As a result, electrons in the scanning beam are slowed down near the target. This eliminates any possibility of secondary emission from this side of the target plate. SCANNING : If a certain element area on the target plate reaches a potential of, say, 2 volts during the storage time, then as a result of its thinness the scanning beam sees the charge deposited on it, part of which gets diffused to the scanned side and deposits an equal number of negative charges on the opposite side. Thus out of the total electrons in the beam, some get deposited on the target plate, while the remaining stop at its surface and turn back to go towards the first electrode of the electron multiplier. Because of low resistivity across the two sides of the target, the deposited negative charge neutralizes the existing positive charge in less than a frame time. The target can again become charged as a result of the incident picture information, to be scanned during the successive frames. As the target is scanned element by element, if there are no positive charges at certain points, all the electrons in the beam return towards the electron gun and none gets deposited on the target plate. The number of electrons, leaving cathode of the gun, is practically constant, and out of this, some get deposited and remaining electrons, which travel backwards provide signal current that varies in amplitude in accordance with the picture information. Obviously then, the signal current is maximum for black areas on the picture, because absence of light from black areas on the picture does not result in any emission on the photocathode, and there is no secondary emission at the corresponding points on the target, and no electrons are needed from the beam to neutralize them. On the contrary for high light areas, on the picture, there is maximum loss of electrons from the target plate, due to secondary emission, and this results in large deposits of electrons from the beam and this reduces the amplitude of the returning beam current.

35 OR a) Explain the Interlace Scanning. Calculate no. of Lines in TV System if W/H=4/3 & D=6H and resolution angle=1/2 minutes. Justify How the No. of Lines Decided? (8) Answer : Flicker : Although the rate of 24 pictures per second in motion pictures and that of scanning 25 frames per second in television pictures is enough to cause an illusion of continuity, they are not rapid enough to allow the brightness of one picture or frame to blend smoothly into the next through the time when the screen is blanked between successive frames. This results in a definite flicker of light that is very annoying to the observer when the screen is made alternately bright and dark. This problem is solved in motion pictures by showing each picture twice, so that 48 views of the scene are shown per second although there are still the same 24 picture frames per second. As a result of the increased blanking rate, flicker is eliminated. Interlaced scanning. In television pictures an effective rate of 50 vertical scans per second is utilized to reduce flicker. This is accomplished by increasing the downward rate of travel of the scanning electron beam, so that every alternate line gets scanned instead of every successive line. Then, when the beam reaches the bottom of the picture frame, it quickly returns to the top to scan those lines that were missed in the previous scanning. Thus the total number of lines are divided into two groups called fields. Each field is scanned alternately. This method of scanning is known as interlaced scanning and is illustrated in Fig It reduces flicker to an acceptable level since the area of the screen is covered at twice the rate. This is like reading alternate lines of a page from top to bottom once and then going back to read the remaining lines down to the bottom. In the 625 lime monochrome system, for successful interlaced scanning, the 625 lines of each frame or picture are divided into sets of lines and each set is scanned alternately to cover the entire picture area. To achieve this the horizontal sweep oscillator is made to work at a frequency of Hz ( = 15625) to scan the same number of lines per frame (15625/25 = 625 lines), but the vertical sweep circuit is run at a frequency of 50 instead of 25 Hz.

36 Note that since the beam is now deflected from top to bottom in half the time and the horizontal oscillator is still operating at Hz, only half the total lines, i.e., (625/2 = 312.5) get scanned during each vertical sweep. Since the first field ends in a half line and the second field commences at middle of the line on the top of the target plate or screen), the beam is able to scan the remaining alternate lines during its downward journey. In all then, the beam scans 625 lines ( = 625) per frame at the same rate of lines ( = 15625) per second. Therefore, with interlaced scanning the flicker effect is eliminated without increasing the speed of scanning, which in turn does not need any increase in the channel bandwidth. No. of Scanning Lines : The ability of the scanning beam to allow reproduction of electrical signals according to these variations and the capability of the human eye to resolve these distinctly, while viewing the reproduced picture, depends on the total number of lines employed for scanning. It is possible to arrive at some estimates of the number of lines necessary by considering the bar pattern where alternate lines are black and white. If the thickness of the scanning beam is equal to the width of each white and black bar, and the number of scanning lines is chosen equal to the number of bars, the electrical information corresponding to the brightness of each bar will be correctly reproduced during the scanning process. Obviously the greater the number of lines into which the picture is divided in the vertical plane, the better will be the resolution. However, the total number of lines that need be employed is limited by the resolving capability of the human eye at the minimum viewing distance. The maximum number of alternate light and dark elements (lines) which can be resolved by the eye is given by Nv = 1/ αρ where Nv = total number of lines (elements) to be resolved in the vertical direction, α = minimum resolving angle of the eye expressed in radians, and ρ = D/H = viewing-distance/picture height. For the eye this resolution is determined by the structure of the retina, and the brightness level of the picture. it has been determined experimently that with reasonable brightness variations and a minimum viewing distance of four times the picture height (D/H = 4), the angle that any two adjacent elements

37 must subtend at the eye for distinct resolution is approximately one minute (1/60 degree). This is illustrated. Substituting these values of α and ρ we get Nv = 1 /(π/180 *1/ 60 *4) =860 In given numerical (D/H = 6), α=1/120 so Nv = 1 /(π/180 *1/ 120 *6) =1146 Lines Question 5 a) Explain the Plumbicon Camera Tube & show the formation of red halo. (8) Answer : This picture tube has overcome many of the less favourable features of standard vidicon. It has fast response and produces high quality pictures at low light levels. Its smaller size and light weight, together with low-power operating characteristics, makes it an ideal tube for transistorized television cameras. Except for the target, plumbicon is very similar to the standard vidicon. Focus and deflection are both obtained magnetically. Its target operates effectively as a P I N semiconductor diode. The inner surface of the faceplate is coated with a thin transparent conductive. layer of tin oxide (SnO2). This forms a strong N type (N+) layer and serves as the signal plate of the target. On the scanning side of this layer is deposited a photoconductive layer of pure lead monoxide (PbO) which is intrinsic or I type. Finally the pure PbO is doped to form a P type semiconductor on which the scanning beam lands. The details of the target The overall thickness of the target is m. It shows necessary circuit details for developing the video signal. The photoconductive target of the plumbicon functions similar to the photoconductive target in the vidicon, except for the method of discharging each storage element. In the standard vidicon, each element acts as a leaky capacitor, with the leakage resistance decreasing with increasing light intensity. In the plumbicon, however, each element serves as a capacitor in series with a reverse biased light controlled diode. In the signal circuit, the conductive film of tin oxide (SnO2), is connected to the target supply of 40 volts through an external load resistance RL to develop the camera output signal voltage. Light from the scene being televised is focussed through the transparent layer of tin-oxide on the photoconductive lead monoxide. Without light the target prevents any conduction because of absence of any charge carriers and so there is little or no output current. A typical value of dark current is around 4 na ( Amp). The incidence of light on the target results in photo excitation of semiconductor junction between the pure PbO and doped layer. The resultant decrease in resistance causes signal current flow which is proportional to the incident light on each photo element. The overall thickness of the target is 10 to 20 µm.

38 OR a) Explain the Monochrome Picture Tube. Define Why we use Phosphor Coating? Difference between TV CRT & CRO CRT. Why we use Aluminium Coating? Role of Aquadag Coating. (8) Answer: Picture tube convert the electronic signal in to the Video signal. Modern monochrome picture tubes employ electrostatic focussing and electromagnetic deflection. A typical black and white picture tube. The deflection coils are mounted externally in a specially designed yoke that is fixed close to the neck of the tube. The coils when fed simultaneously with vertical and horizontal scanning currents deflect the beam at a fast rate to produce the raster. The composite video signal that is injected either at the grid or cathode of the tube, modulates the electron beam to produce brightness variations of the tube, modulates the electron beam to produce brightness variations on the screen. This results in reconstruction of the picture on the raster, bit by bit, as a function of time. However, the information thus obtained on the screen is perceived by the eye as a complete and continuous scene because of the rapid rate of scanning. Electron Gun The various electrodes that constitute the electron gun are shown in Fig.. The cathode is indirectly heated and consists of a cylinder of nickel that is coated at its end with thoriated tungsten or barium and strontium oxides. These emitting materials have low work-function and when heated permit release of sufficient electrons to form the necessary stream of electrons within the tube. The control grid (Grid No. 1) is maintained at a negative potential with respect to cathode and controls the flow of electrons from the cathode. However, instead of a wiremesh structure, as in a conventional amplifier tube, it is a cylinder with a small circular opening to confine the electron stream to a small area. The grids that follow the control grid are the accelerating or screen grid (Grid No. 2) and the focusing grid (Grid No. 3).

39 These are maintained at different positive potentials with respect to the cathode that vary between V to V. All the elements of the electron gun are connected to the base pins and receive their rated voltages from the tube socket that is wired to the various sections of the receiver. Electrostatic Focussing The electric field due to the positive potential at the accelerating grid (also k nown as 1st anode) extends through the opening of the control grid right to the cathode surface. The orientation of this field is such that besides accelerating the electrons down the tube, it also brings all the electrons in the stream into a tiny spot called the crossover. This is known as the first electrostatic lens action. The resultant convergence of the beam is shown in Fig. The second lens system that consists of the screen grid and focus electrode draws electrons from the crossover point and brings them to a focus at the viewing screen. The focus anode is larger in diameter and is operated at a higher potential than the first anode. The resulting field configuration between the two anodes is such that the electrons leaving the crossover point at various angles are subjected to both convergent and divergent forces as they more along the axis of the tube. This in turn alters the path of the electrons in such a way that they meet at another point on the axis. The electrode voltages are so chosen or the electric field is so varied that the second point where all the electrons get focused is the screen of the picture tube. Electrostatic focusing is preferred over magnetic focusing because it is not affected very much by changes in the line voltage and needs no ion-spot correction. In order to give the electron stream sufficient velocity to reach the screen material with proper energy to cause it to fluoresce, a second anode is included within the tube. This is a conductive coating with colloidal graphite on the inside of the wide bell of the tube. This coating, called aquadag, usually extends from almost half-way into the narrow neck to within 3 cm of the fluorescent screen as shown in Fig. Use of Aqua Dag Coating: It is connected through a specially provided pin at the top or side of the glass bell to a very high potential of over 15 kv. The exact voltage depends on the tube size and is about 18 kv for a 48 cm monochrome tube. The electrons that get accelerated under the influence of the high voltage anode area, attain very high velocities before they hit the screen.

40 Most of these electrons go straight and are not collected by the positive coating because its circular structure provides a symmetrical accelerating field around all sides of the beam. The kinetic energy gained by the electrons while in motion is delivered to the atoms of the phosphor coating when the beam hits the screen. This energy is actually gained by the outer valence electrons of the atoms and they move to higher energy levels. While recturning to their original levels they give out energy in the form of electromagnetic radiation, the frequency of which lies in the spectral region and is thus perceived by the eye as spots of light of varying intensity depending on the strength of the electron beam bombarding the screen. Because of very high velocities of the electrons which hit the screen, secondary emission takes place. If these secondary emitted electrons are not collected, a negative space charge gets formed near the screen which prevents the primary beam from arriving at the screen. The conductive coating being at a very high positive potential collects the secondary emitted electrons and thus serves the dual purpose of increasing the beam velocity and removing unwanted secondary electrons. The path of the electron current flow is thus from cathode to screen, to the conductive coating through the secondary emitted electrons and back to the cathode through the high voltage supply. A typical value of beam current is about 0.6 ma with 20 kv applied at the aquadag coating. BEAM DEFLECTION : CRO use electrostatic deflection and TV use magneto static deflection. Both electric and magnetic fields can be employed for deflecting the electron beam. However, (a) As already stated the electron beam must attain a very high velocity to deliver enough energy to the atoms of the phosphor coating. Because of this the electrons of the beam remain under the influence of the deflecting field for a very short time. This necessitates application of high deflecting fields to achieve the desired deflection. For example with an anode voltage of about 1 kv, as would be the case in most oscilloscopes, some 10 V would be necessary for 1 cm deflection of the beam on the screen, whereas in a picture tube with 15 kv at the final anode, about 7500 V would be necessary to get full deflection on a 50 cm screen. It is very difficult to generate such high voltages at the deflection frequencies. On the other hand with magnetic deflection it is a large current that would be necessary to achieve the same deflection. Since it is more convenient to generate large currents than high voltages, all picture tubes employ electromagnetic deflection. (b) With electrostatic deflection the beam electrons gain energy. Thus larger deflection angles tend to defocus the beam. Further, the deflection plates need to be placed further apart as the deflection angle is made larger, thus requiring higher voltages to produce the same deflection field. Magnetic deflection is free from both these shortcomings and much larger deflection angles can be achieved without defocusing or nonlinearities with consequent saving in tube length and cabinet size. (c) For electrostatic deflection two delicate pairs of deflecting plates, are needed inside the picture tube, whereas for magnetic deflection two pairs of deflecting coils are mounted outside and close to the neck of the tube. Such a provision is economical and somewhat more rugged. Deflection Yoke The physical placement of the two pairs of coils around the neck of the picture tube is illustrated in Fig. 5.3 and the orientation of the magnetic fields produced by them is shown in Fig In combination, the vertical and horizontal deflection coils are called the Yoke. This yoke is fixed outside and close to the neck of the tube just before it begins to flare out.

41 The magnetic field of the coils reacts with the electron beam to cause its deflection. The horizontal deflection coil which sweeps the beam across the face of the tube from left to right is split into two sections and mounted above and below the beam axis. The vertical deflection coil is also split into two sections and placed left and right on the neck in order to pull the beam gradually downward as the horizontal coils sweep the beam across the tube face. Each coil gets its respective sweep input from the associated sweep circuits, and together they form the raster upon which the picture information is traced. This is the maximum angle through which the beam can be deflected without striking the side of the bulb. Typical values of deflection angles are 70, 90, 110 and 114. As shown in Fig. 5.5, it is the total angle that is specified. For instance a deflection angle of 110 means the electron beam can be deflected 55 from the centre. The advantage of a large deflection angle is that for equal picture size the picture tube is shorter and can be installed in a smaller cabinet. However, a large deflection angle requires more power from the deflection circuits. For this reason the tubes are made with a narrow neck to put the deflection yoke closer to the electron beam. A 110 yoke has a smaller hole diameter (about 3 cm) compared with neck diameters for tubes with lesser deflection angles. Use of Phosphor in Screen : The phosphor chemicals are generally light metals such as zinc and cadmium in the form of sulphide, sulphate, and phosphate compounds. This material is processed to produce very fine particles which are then applied on the inside of the glass plate. As already explained the high velocity ellectrons of the beam on hitting the phosphor excite its atoms with the result that the corresponding spot fluoresces and emits light. The phosphorescent characteristics of the chemicals used are such that an afterglow remains on the screen for a short time after the beam moves away from any screen spot. This afterglow is known as persistence. Medium persistence is desirable to increase the average brightness and to reduce flicker. However, the persistence must be less than 1/25 second for picture tube screens so that one frame does not persist into the next and cause blurring of objects in motion.