Picture Tube
Picture Tube: The Picture Tube is the costliest and most important part of a television receiver. The picture tube is a cathode ray tube similar to that used in a Cathode Ray Oscilloscopes. It consists of an evacuated glass bulb (funnel shaped) which has an electron gun and phosphor screen. The inside of the screen is coated with a luminescent material that produces light when electrons strike the screen. The narrow neck contains the electron gun that produces the beam of electrons. A black and white TV receiver has one electron gun and a screen with continuous phosphor coating that produces a black and white picture. A color picture tube has three electron guns with screen formed withvertical stripes of red, green and blue phosphors.
The neck of the picture tube contains the electron gun assembly. The election gun assembly consists of filament, cathode, control grid, accelerating grid (for 1st anode) and a focusing anode. These electrodes are generally made of nickel or nickel alloy, mounted on ceramic insulator or supports inside the glass neck. The control grid is kept at a negative potential with respect to cathode and controls the flow of electrons from the cathode. The accelerating and focus anodes are given positive voltage. The final anode is kept at very high potential. The inside of the screen has a fluorescent material coating
.
Working: When cathode is heated up it emits electrons. The control grid controls the number ofelectrons and the accelerating anode provides a high speed to the electrons. The focusing anode focuses the electrons in the form of a beam so that this beam of electrons is sharply focused at the screen. When this beam strikes the screen, a dot of bright light is formed there. These electrons can be deflected from the centre of the screen to any point of the screen. The deflection of electrons is achieved by deflection yoke placed on the neck of the CRT.
Two pairs of deflection coils are placed at the neck of the picture tube. One pair of coils is placed at the neck of the picture tube. One pair of coils placed above and below the electron beam produces horizontal deflection. The pair of coils placed left and right of the beam produces vertical deflection. The deflection of beam is possible only when required current is given to horizontal and vertical pairs of coils. As soon as current is given to these coils, a magnetic field is produced which causes the movement of electrons in horizontal and vertical directions. These movements of electrons form the complete scanning raster.
Monochrome TV Receiver
Monochrome TV Receiver:
Monochrome TV Receiver has the following blocks.
1. TV Receiving Aerial: the TV signal radiated by the transmitter has to be intercepted. For this an antenna with high gain, Broad Band, highly directional is used. Yagi-uda multi-element array is preferred. Impedance of the aerial should match the impedance of the transmission line. The aerial also selects the required signal and rejects unwanted signals.
2. Tuner: Also called RF Tuner/ Front end. Signals from Aerial are amplified, down converted to intermediate frequency. Band and channel selection is also carried out here. This is a separate, sealed and riveted unit mounted away from other circuits. Unit’s body is grounded to avoid interference by other strong signals. Delayed AGC is used at RF amplifier.
3. VIF Amplifier: The output of RF Tuner has two intermediate frequencies i.e. video IF (38.9 MHz) and sound IF (33.5 MHz), occupying a band width of 7 MHz. Most of the amplification and selection is done here. This is also a separate, enclosed unit. Keyed AGC is used at the base of 1st IF Amplifier.
4. Video Detector: The output of VIF section is around 2 to 5VPP. This is fed to video detector to separate various signals i.e., video, AGC, inter carrier SIF. A special diode driven into saturation has many harmonics at its output. Using LPF, signals above 5 MHz are filtered out. OA 79, Ge, heavily doped, point contact diode is used for detection.
5. Video Output Amplifier: Output of video detector which is around 2 to 5 VPP is amplified by a single stage broad band, voltage amplifier to give an output of 80 VPP. Wave Trap is used to prevent 5.5 MHz inter – carrier SIF (38.9 – 33.4 MHz = 5.5 MHz) and disturb the picture. Output of this amplifier is fed to picture tube.
6. Picture Tube: This is a special type of CRT, having widescreen with wide deflection angle. Electromagnetic deflection and electro-static Focusing is used. H and V deflection coils moulded intoa single unit ‘Yoke’ is mounted on the neck. Final anode is applied with Extra high Tension (EHT) which is around 12 to 18 kV. Screen is formed by long persistence P4 phosphor, which is a mixture of cadmium tungstate and zinc sulphide. The light radiated is Yellowish white. With proper output from syn section, picture is displayed on the screen.
7. Sync Section: Output of video detector which is composite video signal has H and V sync pulses either at top or bottom edges. By feeding video signal to sync separator, sync pulses can be separated. We can use clipper circuit when the video signal is with negative sync. Further, by using LPF, V- sync can be separated. Using HPF, H sync pulses can be separated. These pulses are used to synchronize the H and V saw-tooth oscillators. Output of these oscillators are amplified and used to drive the deflection coils on the picture tube.
8. Inter Carrier SIF Section: Output of detector has 5.5 MHz, which is the result of heterodyning between VIF and SIF (38.9 MHz – 33.4 MHz). This has frequency modulated Audio Signal. By using highly selective tuned circuit (sound-take-off coil), 5.5 MHz is separated. This is amplified and frequency demodulated to recover Audio signal.
9. Audio Section: Output from SIF section which is weak AF signal is amplified both in terms of voltage and power to drive the loud speaker. Finally, we get sound output.
10. Power Supply: Various levels of DC voltages are required for the operation of TV receiver. So, 230 V, 1 φ AC is rectified, filtered and regulated to provide ripple free steady voltage to various stages. However due to many advantages switch mode power supplied (SMPS) are widely used now-a-days. EHT and BHT required at picture tube is obtained from Auxiliary power supply using Line output transformer (LOT/EHT).
Monochrome TV Transmitter
Monochrome TV Transmitter: Figure shows the simplified block diagram of a television transmitter. The video signals obtained from camera tube are applied to a number of video amplifier stages. First stage is located in camera housing to increase weak signal voltage to such a level as to be transmitted over a coaxial cable to the succeeding amplifier stages.
Synchronizing generator produces sets of pulses to operate the system at appropriate timings. This unit includes wave generating and shaping circuits. Eg: Multivibrator circuit, blocking oscillator circuit and clipping circuits etc. The repetition rates of the pulse trains are controlled by frequency stabilized master oscillator.
The horizontal synchronizing pulses are applied to horizontal saw-tooth generator; verticalsynchronizing pulses are applied to vertical deflection saw-tooth generator; two sets of blanking pulses are applied to control grid of camera tube to blank it during vertical and horizontal retrace; and a pulse train consisting all above pulse groups is applied to video-amplifier channel for transmission to receiver.
The carrier frequency generated from a crystal controlled oscillator is passed through a number of frequency multiplier and amplifier stages. This results in a production of a carrier wave of desired frequency and energy content. The level of image signals, together with synchronizing and blanking pulses, is raised to modulate this carrier frequency. A high level grid modulation is usually employed.
The carrier when amplitude modulated with video signal (BW = 5 MHz) generates two sidebands and the total bandwidth, required for TV channel would be 10 MHz which is too large. Therefore vestigial sideband transmission in which one sideband – (say upper) is transmitted in full along with reduced second sideband is used. For this purpose, output of the final RF amplifier is applied to a vestigial sideband filter which suppresses the undesired portion of the lower sideband of the modulated wave.
The modulated RF energy is carried from transmitter to the transmitting antenna by means of a co-axial transmission line. The antenna elevation is kept high for large transmission area.
An FM transmitter is used for the purpose of audio signal transmission. The carrier frequency used in audio modulation is 5.5 MHz above that which is used in audio modulation. Both, sound and picture signals are transmitted by the same antenna by using a diplexer called picture – sound diplexer.
Positive and Negative Modulation
Positive and Negative Modulation:
We use AM for video signal. Generally, the amplitude of the carrier increases with increase in amplitude of the signal and vice versa. This is called “positive modulation”. AM is effected by noise.
Generally noise, when occurs tends to increases the amplitude of modulated signal. This increase causes bright blobs (spots) on the screen, when ever noise occurs. To over come this, negative modulation is followed.
In negative modulation, the instantaneous amplitude of the carrier decreases with increase in instantaneous amplitude of the signal. The resulting modulated wave has white level at lower side, black levels at higher level. So, whenever, noise occurs, due to increase in amplitude, the level goes towards less brightness level i.e., gray or black. Thus, the effect of noise is minimized.
In addition to reduced effect of noise in negative modulation, there are many other advantages.
Advantages of negative modulation over positive modulation:
(a) RF noise pulses will not cause adverse effect on picture.
(b) Greater peak transmitter power can be possible.
(c) AGC circuit will have stable level as reference.
Thus, negative modulation is preferred in TV broadcasting. Output frame camera tube (video signal) when it reaches the modulator, the polarity reversal is done so that black level is at 72.5 % and white level is at 12.5 %. Thus, negative modulation takes place. However, strong noise pulses may cause false triggering of deflection oscillator being mistaken as sync pulses.
Differences between Positive and Negative Modulation:
No:
|
Positive Modulation
|
Negative Modulation
|
1
|
Affected by Noise.
|
Effect of Noise is minimized.
|
2
|
Bright blobs (spots) appear on the screen due to noise.
|
No bright spots appear due to noise.
|
3
|
The instantaneous amplitude of carrier increases with that of modulating signal.
|
The instantaneous amplitude of carrier decreases with that of modulating signal.
|
4
|
RF noise pulses cause adverse affects on picture.
|
RF noise pulses will not cause any adverse affects on picture.
|
5
|
Peak Transmitter power is less.
|
Peak Transmitter power is more.
|
6
|
AGC circuit does not have stable reference level.
|
AGC circuit has stable reference level.
|
Composite Video Signal
Composite Video Signal: The composite video signal is the video signal into which blanking and synchronizing pulses are inserted at proper timings.
Composite video signal consists of a camera signal corresponds to desired picture information, blanking pulses to make the retraces invisible and synchronizing pulses to synchronize the transmitter and receiver scanning.
Video signal of the composite video signal varies in between 10 % to 72 % levels. The 10 % level of the signal corresponds the peak white level while the 72 % level of the signal corresponds to peak dark level. Gray shades are represented by the signals with the amplitude level varying in between 10 % and 72 % levels.
Blanking pulses are placed from 72 % level to 75 % level. Sync pulses are placed from 75 % level to 100 % level (both vertical & horizontal during the blanking periods).
Peak White level: 10 to 12 % level is called peak white level because when video signal is having this amplitude a peak white spot is produced on the picture tube.
Peak Black level: 72% level is called black level. This level produces blackness on the raster.
Blanking level: 75 % level is called blanking level because the blanking pulses are inserted at this level.
Pedestal Height: The difference between blanking level and average brightness level is called pedestal height.
DC level or average brightness level: This level corresponds to the average value of the complete frame.
Blanking, Retrace, Synchronizing and Equalizing Pulses
Blanking, Retrace, Synchronizing and Equalizing Pulses:
In TV, ‘blanking’ means ‘going to black’ as part of the video signal, the blanking voltage is at the black level. Video voltage at the black level cuts off the beam currents in the picture tube to black out the light from screen. The purpose of providing the blanking pulses is to make invisible the retraces of the scanning process. The horizontal blanking pulse at the frequency of 15625 Hz, blanks out the retrace from right to left for each line. The vertical blanking pulses at 50 Hz blank out the retrace from bottom to top for each field.
The time period of blanking pulses is 16% of the each horizontal line, i.e., 16 % of 64 µs = 10.2 µs. In other words, retrace from left to right must be completed in 10.2 µs.
The time period of vertical blank pulses is 8% of each vertical field. It comes equal to 8% of 1/50 s = 0.0016 s. In other words, the vertical retrace must be completed within 1.6 ms.
A blanking pulse comes first to put the video signal at black level, then a sync pulse comes to start the retrace. This sequence applies to blanking, horizontal and vertical retraces.
To produce a true and undistorted picture, it is necessary that the scanning process at the transmitter camera tube should be quite in step with that at the receiver picture tube. Thus the timing pulses generated by the synchronizing generator to trigger the saw tooth generator for vertical and horizontal plates are not only applied to the transmitter camera tube system but also transmitted to the receiver along with the image signals. At the receiver, these triggering pulses are separated from the signal components, which are then differentiated (horizontal synchronizing pulses) to trigger saw-tooth wave generators for the application of saw-tooth voltage to horizontal and vertical deflection plates of picture tube respectively.
Frequency of scanning, synchronizing and blanking pulses:
The sync pulses and blanking pulses have the same frequency as that of scanning. The values are shown in the table given below.
No:
|
Particles
|
Frequency in Hz
|
1
|
Horizontal Scanning Pulses
|
15625
|
2
|
Horizontal Sync Pulses
|
15625
|
3
|
Horizontal blanking Pulses
|
15625
|
4
|
Vertical Scanning Pulses
|
50
|
5
|
Vertical Sync Pulses
|
50
|
6
|
Vertical blanking Pulses
|
50
|
7
|
Equalizing Pulses
|
31250
|
TV Standard and Frequency Requirements
TV Standard and Frequency Requirements:
(a).American Standards: Figure (a). The band of frequencies assigned to a station for transmission of the signal is called a channel. Each TV station has a 6 MHz channel within specific bands for commercial broadcasting.
(i) Video Modulation: The 6 MHz band width is needed for picture carrier signal. The carrier is amplitude modulated by the video signal.
(ii) Chrominance Modulation: For color broadcasting 3.58 MHz chrominance signal has the color information. The color signal (C signal) is combined with luminance signal (Y signal) to form one video signal that modulates picture carrier wave for the transmission.
(iii) Sound Signal: In 6 MHz channel, sound carrier signal for the picture is also included. The sound carrier is a frequency modulated signal by audio frequencies between 50 Hz to 15 kHz.
(iv) Carrier frequencies: The figure (a) shows how different carrier signals fit into the standard 6 MHz channel. The picture carrier frequency is always 1.25 MHz above lower end of the channel. At opposite end, the sound carrier frequency is 0.25 MHz below the high end.
 |
| figure (a) Broadcasting Channel (American Standards) |
(b).Indian Standards: The total channel width used in India is 7 MHz. The spacing between picture and sound is 5.5 MHz and between picture and color signal is 4.43 MHz. The figure (b) shows the complete spectrum.
 |
| figure (b) Broadcasting Channel (Indian Standards) |
Calculation of Approximate Bandwidth in 625 Line Systems:
Calculation of Approximate Bandwidth in 625 Line Systems:
We know that for transmission, TV picture is divided into a number of horizontal lines and each line is scanned by the electron beam of the camera tube. Each line may be further considered as consisting of a large number of picture elements, or in other words the whole picture may be thought of as consisting of a large number of picture elements.
But there is some limitation to the faithful reproduction of picture elements which arises on account of the fact that any change in light intensity of the image in vertical direction occurring in a distance less than the width of the line cannot be reproduced and thus if there is to be as much detail in the horizontal direction as in the vertical, the picture elements must be of the same size as the distance between the lines. The most detailed pattern that can be transmitted is chequer board pattern of black and white squares. With an aspect ratio (width/height) 4/3 and 625 lines every 1/25 sec., the number of elements to be scanned is given by n = 625 X (4/3 X 625) X 25.
Therefore, the time for reversal from black to white is the case is,
1/n = 3/ (625 X 4 X 625 X 25) sec.
And the time for complete cycle, black-white-black is thus.
T = (3 X 2)/ (625 X 4 X 625 X 25) sec.
And hence the frequency band covered by TV Video signal is,
Band Width (BW) = 1/T = (625 X 4 X 625 X 25)/ (3 X 2) = 6.5 MHz.
In actual practice, it is found that equal horizontal and vertical details can be obtained provided a bandwidth about 70 to 80 % of 6.5 MHz is transmitted. Therefore, in Indian TV System video bandwidth is kept is MHz.
Sequential and Interlaced Scanning
Sequential and Interlaced scanning: The division of picture into many horizontal lines called scanning. Scanning can be compared with that of reading a page of a book. We start at the top, read all the words in the first line from left to right, and then return rapidly to the left to read the next line, and so on, until we reach the bottom line of the page. Similarly, a camera tube scans the horizontal lines one by one.
The television picture is scanned in a sequential series of horizontal lines, one under the other as shown in figure. This scanning makes it possible for one video signal to include all the elements for the entire picture. At one instant of time, the video signal can show only one variation. In order to have one video signal for all the variations of light and shade, all the picture details are scanned in a sequential order of time.
The scanning makes reproduction of a television picture different from that of a photographic print. In a photograph, the entire picture is reproduced at one time. In television, the picture is reassembled line after line and frame after frame. This time factor explains why a television picture can appear with the line structure form apart in diagonal segments and the frames rolling up or down the screen.
 |
| Horizontal Linear Scanning |
The TV picture is scanned in the same way as you would read a text page to cover all the words in one line and all the lines on the page starting at the top left in figure all the picture elements are scanned in successive order, from left to right and from top to bottom, one line at a time. This method is called horizontal linear scanning. It is used in the camera tube at the transmitter to divide the image into picture elements and in the picture tube at the receiver to reassemble the reproduced image.
The sequence for scanning all the picture elements is as follows:
1. The electron beam sweeps across one horizontal line, covering all the picture elements in that line.
2. At the end of each line, the beam returns very quickly to the left side to begin scanning the next horizontal line. The return time is called retrace, or flyback. No picture information is scanned during retrace because both the camera tube and the picture tube are blanked out for this period. Thus the retraces must be very rapid, since they are wasted time in terms of picture information.
3. When the beam has returned to the left side, its vertical position is lowered so that the beam will scan the next line down and not repeat the same line. This is accomplished by the vertical scanning motion of the beam, which is provided in addition to horizontal scanning.
As a result of the vertical scanning, all the horizontal lines slope downward slightly from top to bottom. When the beam is at the bottom, vertical retrace returns the beam to the top to start the scanning sequence again.
To obtain the maximum amount of the picture detail and to avoid flicker, interlaced scanning is used. In interlaced scanning, each picture is scanned twice by the camera. The camera tube first scans the odd lines 1, 3, 5 and so on and skips the even lines 2, 4, 6 etc., unit it completes one field from top to bottom. After the first field is over, the camera tube scans even lines 2, 4, 6 and so on and skips odd lines completely its second field. These two fields are interlaced together from the complete picture as shown in figure (a) and (b). Half i.e., 312 ½ lines are scanned during odd line scanning and 312 ½ lines are scanned during even line scanning.
Since two fields are scanned for each frame, the repetition rate of the fields becomes 50 per second. Since 50 Hz is the AC mains line frequency, vertical scanning frequency of 50 Hz simplifies the design of the TV receiver and TV transmitter power supply filters. Also, if interlaced scanning is not used, then there is a large gap between the first line and the 625th line with the result that bottom of the picture looks brighter as compared to the top. Interlaced scanning is best suited for TV Transmission.
 |
| Interlaced Scanning |
Advantages of Interlaced Scanning:
1. Avoids flicker
2. It is better than sequential scanning.
3. Conserves bandwidth.
Differences between Progressive and Interlaced Scanning:
No:
|
Progressive Scanning
|
Interlaced Scanning
|
01
|
In this every successive line is being scanned.
|
In this the electron beam first scans odd lines from top to bottom and then it scans the lines those are skipped in the previous scanning.
|
02
|
The effective no: of pictures scanned per second are 25 frames/sec.
|
The effective no: of fields scanned per second are 50 frames/sec.
|
03
|
Flicker problem will occur.
|
Flicker problem is avoided.
|
04
|
Total no: of lines scanned at a time from top to bottom are 625 lines.
|
Total no: of lines scanned at a time from top to bottom are 312 ½ lines.
|
TV Camera Tubes
TV Camera Tubes: Camera is the first and basic equipment in a TV. The input to a camera is the light from the picture or scene to be televised and output obtained from camera is the electrical pulses corresponding to the information contained in picture.
The TV Camera is just analogous to human eye. The basic principle of all TV cameras is based on the fact the each picture of all TV Cameras is based on the fact the each picture may be assumed to be composed of small elements with different light intensity. The camera picks up each element and by transducing action convertsit into “electrical signal” proportional to its brightness there is a photosensitive layer called target or image plate in each camera which performs this job. At the same time simultaneous, pick up of this information is also necessary for this purpose. There is an electron gun (which produces an electron beam) which scans the image plate at a fast speed. Thus opto-electric conversion as well as pick-up of the signal takes place simultaneously and at a fast speed.
The image-orthicon, vidicon and plumbicon are some important electronic scan camera tubes which find wide applications these days.
1. Image Orthicon:
It is a sensitive tube and is capable of handling a wide range of light values and contrast. In a single envelope, it includes three sections:
(a). Image Section: This section includes:
1. a photo sensitive surface, called photo cathode, operated at a very large negative potential.
2. a target plate which is a thin plate of glass of low resistivity. Thickness is less than 0.0002 in.
3. a screen located very close to target plate and has about 500,000 openings per square inch.
When the optical image is focused on the photo cathode, photoelectrons, in proportion to the amount of light impinging, are emitted. Most of the photoelectrons pass through the screen and hit the target plate.
As the photoelectrons are accelerated to several hundred electron volts, they liberate several secondary electrons from the target plate surface, and are then collected by the nearby-screen which is at a small positive potential. The emission of secondary electrons from target plate leaves a distribution of positive charge on its surface. The low resistivity of target plate resists the lateral charge flow on its surface and thus the image charge pattern, formed on the plate, is truly restored as such. Since the plate is thin, this charge pattern also appears on the other side (away from screen) of the plate.
(b). Scanning Section: The otherside of the pattern is now scanned by a beam of low velocity electrons generated by an electron gun. The beam is deflected on the plate in vertical and horizontal directions and enables the electron beam to scan the whole plate. This beam gives up the number of electrons required to neutralize the positive charge at that point and thus the returning electron beam varies in magnitude in accordance with the brightness variation of the image. It should be noted here that since the target portion affected by the white portion of the image will be positively charged and hence the electron beam has to give up large number of electrons to neutralize the positive charge at that point, i.e., the intensity of returning electron beam is much reduced and the video signal developed across the output resistor for this part will be small. It, therefore, concludes that the brightest part of image are transmitted as the signals of low amplitude which is very advantageous in avoiding the effect of strong noise at the receiver.
(c). Electron Multiplier Section: An electron multiplier is located within the pick-up tube for amplifying the electron density variation in the returning beam.
Merits and Demerits:
1.It has high Sensitivity
2. The S/N ratio is better and its typical value is 30 dB.
3. It’s spectral response is close to eyes.
4. The ratio of signal current to illumination os gamma and it varies from unity at low light to 0.5 at high light levels.
5. It produces no lag.
6. Size of image orthicon is bulky in nature.
7. It’s operation is elaborate.
8. It is very costly camera tube and life time of this camera tube is nearly 1500 to 6000 hrs.
2. Vidicon:
This camera tube based on the photo conductive properties of semiconductors i.e., decrease in resistance with the amount of incident light. The tube is shown in figure. It consists of
(a). Signal Plate: Which is a conducting metallic film very thin so as to be transparent. The side of this film facing cathode is coated with a very thin layer of photoconductive material (amorphous selenium). This side is scanned by electron beam. The optical image is focused on the other side of this film.
(b).Scanning System: The electron beam for scanning is formed by the combination of cathode, control grid-1, accelerating grid-2 and anode grid-3. The focusing coil produces an axial field which focuses the beam on the film. Vertical and horizontal deflection of the beam, so as to the scan the whole film, is accomplished by passing saw-tooth current waves through deflecting coils which thus produce transverse horizontal and vertical magnetic fields respectively. The alignment coils are for initial adjustment of the direction of electron beam.
Operation: When the scanning beam passes over the photo conductive material of the signal plate, it deposits electrons so that the potential of this side of plate is reduced to that of the cathode. But the otherside of the film (plate) is still at its original potential. Consequently a potential difference across a given pointon the photoconductive material is created. It is approximately 30 V. Before the next scanning (which may be done after an interval of 1/50 or 1/25 sec.) the charge leaks through photoconductive material at a rate determined by the conductivity of the material which, in turn depends upon the amount of incident light.
White portions of the object will project more light on the film and make it more conductive. This charge leaked to photoconductive side of the film will vary according to illumination of the object. As a result, potential at every point on the photoconductive side will vary. Now the electron beam again starts scanning the photoconductive side of the film but this time the charge deposited by the beam in order to reduce its potential towards zero (cathode potential) will vary with time. Therefore current through RL (and hence the output voltage) will follow the changes in potential difference between two surfaces of the film and hence follows the variations of light intensity of successive points in the optical image.
Advantages:
1. Low cost.
2. Simple Adjustment.
3. Sensitivity is large.
4. Resolution of the order of 350 lines can be achieved under practical conditions.
Disadvantages:
1. Owing to the fact that the resistance of the photoconductive film does not change instantaneously with change of light intensity, different levels of light intensity are adjusted with slight time slag.
2. The response characteristic is non-linear.
3. Plumbicon:
The construction of a plumbicon camera tube is similar to that of a standard vidicon except for the target material. The plumbicon has a new type of photo-conductive target, i.e., lead oxide of the form PbO. The figure below shows the constructional features of a plumbicon camera.
Operation: The operation of a plumbicon camera tube can be best explained from the diagram. Initially, when there is no light input, the PIN diode is reverse biased due to a positive potential appearing on SnO2 coating (n-type) and p-type stabilized at a potential slightly below the cathode due to negatively charged scanning beam. This results in a very small output current which is almost negligible. This is the greatest advantage of a plumbicon camera tube especially when used with color systems. The photo electronic conversion is almost similar to that of a standard vidicon except for the method of discharging each storage element. In standard vidicon each element acted as a leaky capacitor with leakage resistance decreasing with more light. Here when light falls on the target, the diode becomes forward biased upon the extent depending upon light intensity. The forward bias on each diode results from the photo excitation of the pure PbO and doped PbO junction. Thus the target behaves as a capacitor in series with PIN diode.
Merits and Demerits:
1. In plumbicons, the uniluminated or the dark current is negligible and also it is temperature independent.
2. It has got high sensitivity and a high signal to noise ratio.
3. Resolution is good but not as good as that of a vidicon.
4. Operational gamma is unity.
5. It is compact and exhibits simplicity of operation.
6. It is free of spurious signals.
7. Susceptibility to damage by over loads is not as severe as it is in vidicons.
8. There are some forms of PbO which have spectral limitations.
Comparison of various camera tubes:
Parameter
|
Image Orthicon
|
Vidicon
|
Plumbicon
| |
01
|
Photo electric Conversion
|
Photo emission
|
Photo conduction
|
Photo conduction
|
02
|
Sensitivity
|
High
|
Good
|
Better
|
03
|
Illumination (lumen)
|
750 - 1000
|
1500 - 2000
|
750 - 1000
|
04
|
S/N Ratio
|
30 dB
|
50 dB
|
50 dB
|
05
|
Speed
|
No lag
|
Severe lag
|
Reduced lag
|
06
|
Resolution at 400 lines (5MHz)
|
30 – 50 %
|
55 %
|
40 – 50 %
|
07
|
Special response
|
Close to eye
|
Good
|
Poor in Red
|
08
|
Gamma
|
1.0 near black.0.5 near high lights
|
0.4 to 0.9
|
0.9 to 1.0
|
09
|
Operation
|
Elaborate
|
Simple and Quick
|
Simple and Quick
|
10
|
Sizes
|
Length- 15 to 20 inches. Diameter 3 to 4 inches
|
Length- 5 to 8 inches. Diameter 0.6 to 1.6 inches
|
Length- 8 inches. Diameter 1.2 inches
|
11
|
Cost
|
Costliest
|
Cheapest
|
Costlier
|
12
|
Life (hours)
|
1500 – 6000 hours
|
5000 – 20000 hours
|
2000 – 20000 hours
|
posted by azhar ct
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