Thông tin thiết kế mạch P6 doc

Assumingcomplete collection of the electrons, the current in the resistor R shown in Figure 6.1will be proportional to the light intensity and so will the voltage across R.A primitive vi

One of the simplest devices which can measure the brightness of light is thephototube It consists of a cathode which is coated with a material which gives offelectrons when light is shone on it and an anode which can collect the emittedelectrons when a suitable voltage is applied to it The cathode and anode areenclosed in an evacuated glass envelope The number of electrons emitted by thecathode is proportional to the intensity of the light impinging on it Assumingcomplete collection of the electrons, the current in the resistor R shown in Figure 6.1will be proportional to the light intensity and so will the voltage across R.

A primitive video signal can be generated by using a 3
3 matrix made up ofphototubes as shown in Figure 6.2

For simplicity we assume that the tree is black and its background is white Asuitable lens focuses the image of the tree onto the matrix of phototubes It is clearthat the voltage output from phototubes (1,1), (1,3), (3,1) and (3,3) will be high; allothers will be low The voltages so obtained can be transmitted and used to controlthe brightness of a corresponding 3
3 matrix of lights at a distant point giving avague idea of what the tree looks like! The picture detail can be improved byincreasing the number of elements in the matrix so that each element corresponds tothe smallest area possible The assumption of a black tree on a white background is

no longer necessary since, with increasing detail, different shades of grey can beaccommodated

The information may be sent along individual wires linking the phototube to thelight matrix (parallel transmission) but this would be very expensive and impractical

161Copyright # 2002 John Wiley & Sons, Inc ISBNs: 0-471-41542-1 (Hardback); 0-471-22153-8 (Electronic)

for any system other than the simple one described here A better system would beone in which the voltage from each phototube is scanned in some given order and thevoltage and position of each phototube are sent on a single wire to the receiving endfor reconstruction (serial transmission) The price to be paid for reducing the number

of wires is the increased complexity introduced by the scanner and a system forcoding and decoding the voltage and position information at the transmitter andreceiver, respectively

Figure 6.3 shows the basic components of a television transmitter A system oflenses focus the image onto a camera tube which collects and codes the informationabout the brightness and position of each element of the matrix forming the picture

by scanning the matrix A pulse generator supplies pulses to the camera to controlthe scanning process The output from the camera goes to a video amplifier foramplification and the addition of extra pulses to be used at the receiver for decodingpurposes

162 THE TELEVISION TRANSMITTER

A microphone picks up the sound associated with the picture and afteramplification the signal is fed to the audio terminal of a frequency modulator Thecarrier signal supplied to the modulator is a 4.5 MHz signal, generated by a crystal-controlled oscillator at a lower frequency and multiplied by an appropriate factor.The FM signal carrying the audio information is added to the video signal Theoutput of the video amplifier consisting of the video signal, receiver control pulses,and the frequency modulated signal is fed to the amplitude modulator The carrier ofthe amplitude modulator is supplied by a second crystal oscillator and associatedmultiplier which produce a signal of frequency which lies within the band 54–

88 MHz (VHF) The radiofrequency power amplifier boosts the power to the legallydetermined value and the vestigial sideband filter removes most of the lowersideband signal before it goes to the antenna for radiation

6.3.1 Camera Tube

6.3.1.1 Iconoscope The first practical video camera tube invented by theAmerican scientist Vladimir Zworykin [4,5] is best viewed as a progression fromthe primitive phototube arrangement discussed earlier In Zworykin’s iconoscope he

replaced the matrix of phototubes with what he called the mosaic This was made up

of a very large number of droplets of photosensitive material on one side of a sheet

of mica The other side of the mica sheet was covered with a very thin layer ofgraphite – the signal plate A cross-section of the iconoscope is shown in Figure 6.4.The mosaic and the signal plate constitute a large number of tiny capacitors whichshare one common plate When the image is projected onto the mosaic, eachindividual droplet of photosensitive material emits electrons proportional to the lightintensity These electrons are collected by an anode placed close to the mosaic Themosaic is now a picture ‘‘painted with electric charge’’ The transformation of thecharge into a voltage output is carried out by the electron gun and the associatedcircuitry

The electron gun produces a very narrow beam of electrons focussed on themosaic The arrival of these new electrons have the effect of ‘‘discharging’’ the tinycapacitor on which they fall Since the number of electrons required to discharge thecapacitor is proportional to the charge induced by the light intensity, the electron guncurrent is a function of the charge present and hence of the light intensity Theelectron gun current is then proportional to the voltage which appears across theresistor R

In order to transform the picture into a video signal, it is necessary to make thebeam of electrons sweep across the mosaic in a series of orderly lines The electrongun has two deflection systems for scanning the picture The first moves the electronbeam at a constant speed along a horizontal straight line and returns it very quickly

to the start ready for the next sweep (horizontal trace) The second system controls

formed on the inner wall of the tube and neck Reprinted with permission from H Pender and

K McIlwain (Eds) Electrical Engineers Handbook, 4th Ed., Wiley, 1967, pp 15–21.

164 THE TELEVISION TRANSMITTER

the vertical position of the beam and ensures that each line is swept before returningthe beam to the top of the picture ready for the next frame (vertical trace).Two major disadvantages of the iconoscope are the high light intensity required toobtain acceptable quality images and the production of secondary electrons from thephotosensitive material during the scanning process The secondary electrons causenoise (false information) in the video signal In due course, the iconoscope wasreplaced by the image orthicon, another invention of Zworykin.

6.3.1.2 The Image Orthicon In the image orthicon [6] the two functions of themosaic in the iconoscope, which are (a) the production of the image in terms ofcharge and (b) the target for the electron beam scan, are separated Figure 6.5 shows

a cross-section through the image orthicon

The camera lens focusses the image onto the photocathode Every small segment

of the photocathode emits electrons proportional to the amount of light falling on it.The electrons are attracted to the target by the positive voltage applied to it Thefocus coil creates a magnetic field which ensures that the electrons travel to the target

in straight parallel paths The target is a very thin sheet of glass which has a highconductivity through its thickness but low conductivity across points on its surface.The electrons striking the target dislodge secondary electrons from the photocathodeside of it and are immediately captured by the very fine wire mesh, called the targetscreen, which has a relatively low positive voltage on it This leaves a positive charge

on the photocathode side of the target Due to the high conductivity through thethickness of the target an identical pattern of positive charge is produced on the otherside of it

The electron gun positioned at the other end of the evacuated tube produces anarrow stream of electrons which are accelerated towards the target by the highpositive voltage on the accelerator anode The accelerator anode is a graphitecoating on the inside of the neck of the tube If the electrons were allowed to strikethe target without any control of their speed, it is clear that they will producesecondary electrons To control the speed of the electrons on arrival at the target, asecond ring of graphite coating, called the decelerator grid, is provided By adjustingthe voltages on the accelerator anode and the decelerator grid, the speed of theelectron stream at the target can be controlled to ensure that no secondary electronsare emitted The electron stream striking an elemental area of the target uses up some

of the electrons to neutralize the positive charge left there by the image Theremaining electrons are attracted in a backward direction by the accelerating anodebut they take a different path By placing an electron collector in the appropriateposition in the neck of the tube, the returning electrons can be collected andamplified by an electron multiplier The output of the electron multiplier is convertedinto a voltage inversely proportional to the amount of light impinging on theelemental area of the photocathode By using a scanning system in conjunction withthe electron gun (deflection coil), the complete image can be produced at the output

in the form of a varying voltage which is a function of the brightness of all theelements of the image projected on the photocathode

6.3.1.3 Vidicon The vidicon is not as sensitive as the image orthicon but it isgenerally smaller in size and therefore well adapted to field applications such assurveillance, where high resolution is not critical Its principle of operation isdifferent from both the image orthicon and the earlier iconoscope since it relies on achange of photoconductivity as a function of light intensity An electron scanningsystem is used to extract the video information.

6.3.2 Scanning System

In Section 6.3.1, the role of the electron beam scanning system in the production ofthe video signal was discussed A series of pulses are generated which control theinitiation of the horizontal sweep of the electron beam across the target (horizontaltrace) from the left-hand side to the right A blanking pulse is used to cut off thebeam while it is returned to the left-hand side ready for the next sweep (harmonicretrace) During this period, the vertical trace circuit moves the beam down just theright distance for the second line to be swept When the whole frame has beenscanned, a reset pulse returns the beam to the top left-hand corner ready to repeat theprocess The beam is blanked during the reset All the control pulses are added to thevideo signal and used at the receiver to synchronize the receiver to the transmitter.All the pulses are derived from the 60 Hz power supply or from a 31.5 kHz crystalcontrolled oscillator and divided down to the appropriate frequency

Black-and-white television in North America uses 525 horizontal lines of scan tocover the image If the lines were scanned sequentially, the persistence of thephosphor on the first line would have faded before the last line would have beenscanned This would lead to an annoying flicker on the picture tube, especially athigh levels of brightness The scan is therefore interlaced, that is, all odd numberedlines are scanned first from 1 to 525 and then the beam is returned to line 2 and alleven numbered lines are then scanned So each frame has 525=2 lines and they arescanned 60 times per second This means that there are 262:5
60 lines scanned persecond The horizontal scan therefore operates at 15.75 kHz It must be pointed outthat not all the 525 lines are used for picture production; some are used for verticalretrace and other controls as well as equalization and synchronizing purposes.The period for the horizontal scan is 63 ms and it is divided up into 56 ms for thetrace and 7 ms for the retrace The beam deflection system is therefore required tomove the beam from the left of the picture to the right in 56 ms and return it in 7 ms.The mechanism for deflecting the electron beam relies on the fact that, when acurrent-carrying conductor is placed in a magnetic field, the conductor experiences aforce mutually orthogonal to the field and the direction of the current Motion willoccur if the conductor is free to move In the camera tube, the electron gun and itsassociated components produce a stream of electrons Each of these electrons carries

a charge and a moving charge is a current The electron beam therefore experiences aforce on it and, since it is free to move, it will be deflected from its original path.Figure 6.6(b) illustrates the situation

Consider the current in the conductor (the electron beam) to be flowing into theplane of the page The ‘‘right-hand rule’’ gives the direction of the magnetic flux Bi

associated with the current in the conductor as concentric circles in a clockwisedirection Assuming the conductor is placed in a magnetic flux Bf which is in thevertical upward direction, the two fluxes will interact to produce the distorted fieldshown in Figure 6.6(b) The flux on the left of the conductor is strengthened whilethat on the right is weakened; the conductor therefore moves to the right Whentrying to determine the direction of electron deflection, it is necessary to rememberthat electron flow is opposite to current flow.

The deflection of the electron beam therefore depends on designing a circuit tosupply the appropriate current to the horizontal deflection coil so that it produces amagnetic field which is a linear ramp with respect to time The vertical deflectionsystem also has to be supplied with a current which produces a linear ramp magneticfield and its operation has to be synchronized to the horizontal trace so that it repeatsthe process after 262.5 cycles

The video signal, the control pulses, and the output of the FM modulator arecombined in the video amplifier A typical composite video signal is shown in Figure6.7 Note the following:

(1) Only horizontal synchronizing pulses are shown ( 5 ms); vertical nization pulses ( 190 ms) are distinguished from the horizontal ones by thedifference in their width

synchro-(2) The blanking pulse starts before and ends after the horizontal synchronizationpulse

(3) The video signal contains the radio-frequency carrier of 4.5 MHz frequencymodulated by the audio signal

two magnetic fields generates a force mutually at right angles to the conductor and magnetic field.

168 THE TELEVISION TRANSMITTER

The spectrum of the composite signal at the output of the video amplifier isshown in Figure 6.8.

6.3.3 Audio Frequency and FM Circuits

Various audio frequency amplifiers were discussed in Section 2.7 The design of thecrystal oscillator was discussed in Section 2.4.7 and the frequency multiplier inSection 2.5 Design details for the FM modulator can be found in Section 4.4

transmission of the audio signal.

pulses.

6.3.4 Video Amplifier

6.3.4.1 Calculation of Bandwidth Ideally, a video signal consists of cies from zero (dc) to some high frequency The dc response is required when largeareas of black, white or other intermediate shades have to be transmitted The limit

frequen-of the high-frequency response is determined by the resolution required for a singlevertical black line on a white background or vice versa The minimum visiblehorizontal line has a height (width) equal to the width of the horizontal trace Tokeep the same resolution for the horizontal as for the vertical line, the video signalmust go from white to black in

f ¼1

This means that all the video circuits in the system must have bandwidths equal to orgreater than 6.25 MHz Note that the bandwidths of the sub-circuits must be greaterthan 6.25 MHz because cascading reduces the system bandwidth compared to that ofthe individual sub-circuit

transition from black to white level in 0.11 ms.

170 THE TELEVISION TRANSMITTER

6.3.4.2 Video Amplifier Design Because the video signal contains cies from zero to some high frequency, a video amplifier must be designed to have aflat response from dc to the high frequency in question Ideally, all circuits in thevideo chain must be directly coupled In practice, an amplifier with a bandwidthfrom approximately 30 Hz to approximately 4.0 MHz is considered to be a videoamplifier.

frequen-Consider a FET video amplifier [1] in which the FET is modelled in the source mode as shown in Figure 6.10

common-Since rdRL and jXCdsj rd, both rd and Cds can be neglected as shown inFigure 6.11

in which

Cg¼ ½Cgsþ ð1 þ gmRLÞCgd: ð6:3:8ÞConsider the two-stage video amplifier shown in Figure 6.13

172 THE TELEVISION TRANSMITTER

The resistor R1 is used to bias the gate of Q1 and it is usually very large (MOs).The time-constant CgR1 is therefore very large compared to CgRD To a firstapproximation, the time-constant CgRD determines the high-frequency response ofthe amplifier The 3 dB frequency is

the second stage connected across the output.