THE CONSTRUCTION .AND ANALYSI3 OF A LIONOSCO a by FREDERICK GEORGE HERRING B. 3., Uaivorsity of A t3ubm1tte re in Ncla.L3shire, 1V48 THMIS t1 m ew lfillnent of the Its for the deers° AASTER OF SCIENCE Depti.rtment of .leetriel .;.a6.ineering KANSAS STATE COLLEGE OF AGRICULTURE AUD A2.I.?LIED SCIENCE 1949 r t, cAtitak,,,,w01- ......................... BCPPLY M; VOLTA 1,04a THE ..................... CIRCUlT3 DEFLECTION VERTICAL THE Circuit Damper horizontal The Generator 3avitooth and Amplifier l'ulao Horizontal The ......... 42 0 CIRCUITS D14deLECTIDN HORIaaNTAE THE * It * tt 41 36 ilock.airomeats f1r4,72n el Keyed ubled 12indn . Theory Fundamental 9 **** * * ,L5 *** 24 4#0400 eirelowo.4****00*********** 3 ********** 1 ....................... 16 ...................... ompeneation *4.00* ANALYSIS **SC 4 ************** * * Low High-requency .0 A,iDt'LIFIERS VIDEO =SCRIPT GENERAL Deflection Beam tern La The lectrode Consic3eratirao Design * tia Characteri 2hysical ............. Description cenera1 **************** * Stace Output Yequenoy Compenst,,tion a ....a...a. * **** Blanking CIRCUITS CA2ERA 07 N #** 11 . 640 **** ................. .......... LJ CLAAPER T. BRIGHTNESS PICTURE ................................. e Pulse **# *** TUBE MONOSCOPE THE * * . -:11 CT DO' INTRO THZ TAGz zal au21-qx 004,5410411,0 T 2ATT3i,L4 ANALY3I3 0,01*************05100 US, S or THS 40NOZ;002Z 41,50051,0***55515.00410,54* **4. TT," ACANOWLEDUENT 1.4100000.000055, REFLRENOE3 *501004.0004,0004. I,. 61 i4404P0000 74 INTRO U TION The past decade has witnessed the evolution of television from the research laboratory to a proved medium of visual com- munication. This evolution has been achieved through bined efforts of scientists and, the com- engineers, whose objective has been the realization of television's full possibilities. To accomplish this objective, and to facilitate the edvuseement of the art, the development of test equipment for television devices and circuits arose as a logical and aeoessary undertaking. Accordingly television test equipment has been designed to satisfy the various needs of the industry. One such piece of test equipment is the monoscope camera, a static-image signal-generator. It is the purpose of this camera to produce a fixed picture and blanking signal output, which may be combined with synchronizing pulses to provide a composite video signal. This composite signal conform to 1 R. M. . staneards. The monoscope camera is, therefore, a oom- plete television camera which may be used interchanably, in a standard television system, with the studio cameras. The monoscope was designed primarily for the purpose of aligning and checking television equipment such as receivers, transmitters, monitors or other associated circuits. The picture signal from the camera makes it possible to observe contrast, system resolution and to detect nonlinearily in the deflections circuits. It excels the looneecope for this eadio eAnufacturers associations this purpose since it is inexpensive to menufacture and requires no auxiliary source of lighting. strong, equal to 10 time The signal is exceptionally that obtained by a eonventional iocr- os ape withel foot candle of plate illumination. It is apperent that the eeeision to conetruet a monoecope camera was well justified. The conetructien ef this camera has been completed, ena it is now installed as an integral unit of 143.,'LeVis television system, preen This paper s, therefore, the construction and design considerations of the camera and a theoreticel analysis of the monoseope tube and the circuits fund- emeneel to its operation. THL 1ONO3COPZ General Dee° TUBi: le The monoscope is a special type of cathode ray tube resem- bling the iconoacope in general principle, but differing in that the video signal is derived from a printed pattern electrode enclosed within the tube. eceoraingly, it cannot be olaeuified as a pier up tube since it is unable to develop a video signal which repreeen,s action, but is necessarily restrieted to one particular still picture or pattern. The limitations of such a tube are adearent, and if a comperisen of the monoscope and the iconoseope is to be made, then their eharacteristies ehould be evaluated in the light of cope :as intended. the purpose for which the monos- This, primarilye is the testing of tele- vision system performance, and the ability of the moneeeope to generate a strong video siz.na1, rich in hear content, that is dependent of light conditions end free of spurious signals, is identically oommensurable with its purpose as a piece of test equipment. There are in gendral two basic types of static-imae signal- generating tubes. The first of these employs a change ee primary beam current to develop an output sienal. Briefly this type of tube has a signal plate of some designed configuration cut from a conducting surface. ehen the plate is scanned signal our rent is obtained each time the beam strikes the plate. As a result this video signal will produce a picture of the plate when applied to the grid, of a kinescope. The second class of tube able combinatione: on In general, be made in a number of work- two materials having Ceie:ferent secoudary emission ratios are used to make the signal plate. As the plate is scanned, one magnitude of ecoondary emission ourrent is obtained from one material and another magnitude is obtained from the second. The difference in the magnitudes of those secondary emission currents determine the amount of video signal. Therefore, if two materials hich have a numerical difference in secondary eeieeion ratios greater than one are used, it is possible to develop more video current than eould be possible if only the primary current of the beam were util- ized. to Beoauee of this advfintace, a number of combinations of materials having different secondary emission ratios were investigated by the tube manufactures. The end result was a technique of preparing sienal plates which peemitted the accurate reproduotion of all types of subject material (2). hysical Characteristics The statio-image signal-generating monoscope eamera is an 4:C.ii type arl, tube employed in the It hhs an overall length of 13 inches, a maximum bulb diameter of b inches and a phys- ical appearance characteristic of magnetic deflection type eathode ray tubes. The base is a long-shell medium six-pin base that fits a standard six contaut socket. The electron gun con- nectiens are made to the base, while the collector and pattern leads are brought out two recessed small ball caps. The pattern eleatrode cap is loaoted at the center of the face of the tube and the collector cap on the side of the bulb near the neck of the tube. The entire, tube, except for a small portion near the baee, is coated externally with an insulating moisture-repellent coating, which serves to prevent eaudensation of water vapor in a conductive Tilm over the glass eurface. :i4.ratic surface sparking, which may be produced by such a film when a high volt- age gradient is present, is thus eliminated, Design L:onsiderat lntorna1lr the 21721 conalute of an electron gun, eleetrotW and a collector. .Wlate I. a pattern These are shown diaerammatically in The electron gun, which produces a oceleratos and focuses the electron beam, must be of high Quality if the best video signal is to be obtained. ifl particular, the gun structure must have been dehigned electrically and neehanically to be measurable with two objectives. The first of these would com b be that of producing the maximum output signal. since the out- put signal is a function of the secondary emission current, the potential of the pattern electrode current are important factors. talc' the magnitude of the beam The second objective is to secure a beam whose diameter does not limit the resolution of the system. It is of interest to investigate the particular choice of bean current and final accelerating voltage employed. in the monoscope tube, sines it is essentially these values that will deter- mine the resolution and strength of the video signal fed to the video amplifiers. The final accelerating voltage, which is the potentiel of the target plate with respect to the cathode, is given by the manufacturer to be 800 volts. The choice of this particular value may be justified by realizing that the accelerating voltege employed should be consistant with two primary objectives. Ihe first of these is that of obtaining the max- imum coefficient of secondary emission for the white or alumin um portion of the target, and a minimum secondary emlaion coefficient for the black or carbon portion of the picture. Here a good. seeroximation is that the secondary emission co- efficient for a given surface is a function only of the primary electron energy and thus of the accelerating potential. The actual secondary emission current will be to sane extent dependent upon space charge effects, the angle of incidence of the primary electrons, residual gas in the bombarded materiel, and possibly temperature, but these effects are negligible 0). Curves representing the desired relationship are plotted in elate I for carbon and for a typical composite oxidized layer LIP.L,NATIM F 2I6TL Fig. 1. Simplified sehemtle of the monoseope tube Pig, L. Secondary emission ohareeteristiez of the materials compri,'ing the signal plate (1Z) 7 PLATE I GUN ELECTRON PATTERN ELECTRODE LOAD COLLECTOR RES151-0Fi Fig. 1 a 7- 200 400 PRIMARY J 600 IMPACT Fig. 2 800 ENERGY, 1000 VOLTS 1200 1400 0 3ince the secondary emission coefficent for carbon is (1:5). prectioally independent of the primary electron energy, the accelerating potential should be chosen so as to correspond with that value ehich will give the ximam secondary emission coeeficient for the alumminum oxidized layer. This maximum occurs at potentials in the vicinity of 600 volts. The potential of the pattern electrode also influences the resolution of the camera in that it is instrumental mining the diameter of the electron beam. irk deter- Accordingly this po- tential must be chosen to be consistant with a resolution ex- ceeding b25 lines. This in itself is analogous to the problem of preeictine the change in beam diameter with a change in accelerating potential. The Kluwer to this seems to be provided in some earlier papers end in the eell-kaown textbooks by orykin ana eprton (14). The general opinion expressed seems to be that the spot size ahould decrease in proportion to the square root of the increase in the accelerating voltage of the electron beam, all other conditions being held constant. Thus the formula a Where: r is :ea = constant the radius of the electron beam is the accelerating potential This theoretical prediction, however, does not agree at all with the change of spot size actually observed; the exper- imentally found change of spot size with a change of acceler- ating voltage is very much smaller than required by this theory. A rather thorough investigation of this subject :as undertaken by G. Liebmann, and it is of interest to note the result of his work since it provides an answer to the foregoing question. Briefly, Liebmann proposed a new equivelent-optioal system loon slating of three lenses to represent the electron optical system. On this besis the conclusion is reaohed mathematically that the spot si:4e is an image of the oathode and that its diemeter is not a fuection of the accelerating voltage. This prediction was checked experimentally on a number of typical oathode ray tubes employing both types of deflection. The ex- perimental results indicated that for accelerating voltages above 1000 volts, the spot size remains unchanged within the error of measurement. Below a final accelerating voltage of 1000 volta, the obtainable resolution decreases, although the decreaee is not as great as that predicted by the OTOSS over theory. The 2F21 uses accelerating voltages in the vicinity of 1000 volts, i'rx1 the above arguments. it is seen that a d crease from this ficure, while resulting in a slightly in- creased secondary omission coefficient for the aluminum oxidized, layer, would also produce a eecrease in resolution. SimiliarIy, higher accelerating voltages would be to no particular advantage. The definition would not be increased, and the secondary emission coeffivient would be adversely effected. 'Liebmann, G., "Image Formation in Oatho the Relation of Spot Size and. Final Anode Voltage" 1.R.B., 315::581, Juni 1945. Tubes Proc. am:, IC The magnitude of the beam current is another important consideration and here again a compromise is made. A high beam current is desired since a percentage increase in beam ourrent results in a like percentage increase in signal output. However, the beam current is primarily a function of grid, voltage. the control This voltage ',Idle determining the magnitude of the current, simultaneously determines the size of the emitting area of the cathode. ince the spot at the pattern electrode is an image of this cathode area, results in poorer resolution. then an increased beam current The size of the beam should not be sacrificed to obtain high beam currents. As a first appromaration it might well be argued that the signal output from the mon.oscope tube could be increaed by enlarging the area of the pattern electrode. ith an increased area, beam dLumeter could be correspondingly increased without adversly effecting the b25 line resolution. beam current, :_hich is he allowable proportional to the beam diameter, is greater, and the secondary emission current and thus output voltage is also increased. two objections manifest rile the above argument is valid, themselves. The first is the well- known defocusing of the beam as the defleution angle is increased. The second is the variation of the secondary emission coefficient for any surface as the angle of incidence of the primary electrons in changed. This effect becomes appreciable at primary impact energies above 100 electron-volts as such would be evident in the monoscope tube. (1Z), and The degree of this effect is a function of the bombarded material and is II determiaed experimentally* eltnoegh an increase in the eucoedary eetission coefficient for increasing angles of incidence is characteristic of all eubstances. For carbon the coefficient of secondary eaiseion is practically independent of the angle of incidenoe, while for aluminum it can be eoneidered constant if the angle of incidence is not greater than Le degrees. It 30 degrees the coefficient of secondary emiseion has in- creased to about 16 per cent of its normal inci:lenee value, and thereafter rises rapidly. If the effective center of deflection is considered to lie at the mid-point of the coil jo7ee, then the aaximum angle or incidence in the monoecope tube is 17 degrees, so that the effect described could be negleeted. Haaever, this variation in the secondary emiseion coefficient might well be a limiting factor in increasing the signal strength by an enlargement of the pattern electrode area. he 'at tern electrode The heart of the monoseope tube is the signal plate or pattern electrode, tee it is aumetimee called. plate is made from aluminum foil and carbon. - The siaa.1 The eurfaee of the aluminum has a natural coating of aluminum oxide whieh has high secondary emission ratio while the carbon has a relatively low ratio. eyference to Plate relative ratios obtainable. I confirms this and shows the 1:xpeldmentally the manufacturers found that aluminunl foil developed for advertising and packing purposes as well as special inks developed for printing on metal foils were satisfactory materials for the signal plate. was extremely fortunate This inee the advantages and flexibility of coLlmercial printing could be utilized. The desired picture or pattern is priated on a piece of aluminum foil approximately .034 inche thick ,A.th a blaokfoil ink. The only other process necessary before sealing the signal plate in the tube is to fire it in hydrogen. This process removes the volatile uaterial from the ink and leaves it practically pure carbon (2). Subject matter for reproduction on a signal plate can be divided into two classes: black and. white, and half tones. Cartoons are a good example of the first, while snapshots, which Contain half tone group. black and white, illustrate the 2hoto-engravings are made of the :;ubjeot matter for printing the signal plate. The black and white material is treated as a line-cut, but the half tone mterial must be broken into a number of dots of various sizes depending on the half tone value. This is done when the photo-engraving is made by photographing the material through a suitable screen. A screen is used which will break the picture into more elements than are used in the television seanning system for which the tube is designed. As a result, this technique of obtaining half tones does not limit the resolution of the television system, and the half tone effect is reproduced just as in a newspaper photograph. The actual pattern in printed on the signal plate as a 13 'negative. The pattern is printed as a negative in order to add to the convenience of the tube's use in testing iconoscope equipment. polarity. The iconoscope has a signal output of negative That is, when n picture is transmitted by an loon- oseope, a highlight in the picture is represented by a negative velue of the iconeseope's signal output voltage. In the menos- cope, however, a white portion of the pEttorn is represented by a positive value of signal output voltage. This is beceuee the aluainum omide has a secondary emission ratio greater than earbon. In order to melee the eignal output of the 2721 correspond to an iconoscope, 2F1. to the pattern is printod as a negative in the The result is that when the signal from a 21721 is applied the ieput of a chain of amplifier stages betweea an ico.ioecope and a kinescope, the pattern appears on the kinescope as a positive. The negative printing therefore is a convenience in tests of iecnoecope equipment. In tests of any television equip- ment, a positive reproduction of the pattern cen be obtained by using an oed number of video amelifier etegcs between the monoscope and the kinescope. aeam Defleetion The 21e2,1 (eaploys electromagnetic deflection end the standard rectilinear system of scanning. Sawtoeth scanaing currents are eupplied'to the respective coils by horizontal and vertical deflection circuits ehich sre a. ceeposite pert of the unit. are described separately in detail. These The vertical,horizonal, field and frame frequencies as well as the geometry of the inter- 14 laced pattern, eumber of scanning lines and the aspect ratio all conform to the present R. a. A. standard. Physically the deflection yoke is situated so as to cover the neck of tube, extending fron the end of the electron gun struoture to the beginning of the bulb. a de the to support the yoke on either ft-0'1ns bolts were llo, and two slots out in tube shield so that the yoke might be positioned to cor- rectly scan the pattern electrode. The collo are formed within the yoke in the following manner. ach coil is wound rectilinear in form, and is made up of several concentric seotions each having the proper num- ber of turns so that the magnetic field will be uniform in the finished yoke. The two horizontal coils are placed on the inside, 180 degrees apart, and the vertical coils are formed over them and displaced 90 degrees from the horizontal coils. Thus the horizontal coils provide a magnetio field at right angles to that of the vertical coils, so that the resultant horizontal deflection is at right angles to the vertical doflection (e). An important design oriteria is that the de- flecting coils provide a uniform flux distribution across the neck of the tube, If this requirement is not satisfied, that portion of electron beam which passes then through the strong- er region of the field suffers a greater deflection, and a beam of circular cross section is distorted into an eclipse. en electrostatic shield is provided between the horizontal and vertical coils to minimize any cross coupling. The cells are entirely enclosed within a case of megnetio material, to Ib insure ahielding and leo to increase the inductance of vertioal In order to prevent utray electric and magnetic fields from effecting the deflection tnid beam, a galvanized iron shield 16a,3 focusting of the electron constructed to enclose the entire tube and the deflecting yoke. wall was built within A this case encircling the monoscope tube between the pattern electrode and the yoke. This shields the pattern electrode from the electrostatic field of the coils vmd minimizes pickup by the pattern electrode and video amplifiers of high frequency components of the voltage across the yoke. Care was taken in spacing this 'a 11 from the pattern electrode and its terminal, since too close spoing would result in an in- crease in the output capacitance of the tube and thus the re- duotion of output voltages at GlICEILAL Ugh DE3CRI2TION OF The fundamental arrang frequencies. CAML,74, CIRCUITS ho monosoope nt of e camera is shown in the bloat diagram 1ate Xl. The vertical deflection generator consists of four tubes and their associated circuits. The first of these tubes amplifies the driv- ing signal received from the synchronizing Generator and gen- erates a sawtooth voltage wtich is amplified in the second tube. The fourth provides negative feedback to improve laic scanning linearity. The horizontal deflection generator inclutis three tubes and their associated circuits. The first tube is the driving 16 signal input aaplifier and sawtooth genc.ator; third aaplify the output wave Aid f(ed it deflecting coils of th, to the second and the horizontal monoecope tube. The blanking amplifier is used to provide the proper level and polarity of the blanking pulses received from the synchronizing generator before these pulses are fen into the video anplifier for mixing with the video signal. Video amplification is provided by compensated stages, end the monoscope output sienea is fen to the first of these. A clip)er stage is included to provide variation of the d-c component of the camera signal. This is acoomplished by con- trolling the vias on the clipper which in turn determines the height of the blanking pedestal. One may be used to feed a monitor, Two output stages are provided. .11i10 the other is connected to the system under test. VID14 Li F High- 2requency Compensation The video amplifiers consists of a nine tube res capncity coupled amplifier, compensated so as . frequency response. to ace insure good Actunlly only the first eight stages are compensated amplifiers, no oompensation being required in the output stage, sinoe it operates into a 7b ohm coaxiF1 line. Theoretically, these amplifiers may be adjusted so as to obtain a response uniform to 8 megacycles. difficulty Aotually, however, as experienced in extendning,the response beyond 3. EXPLANATION Or ILATL II Fie* Z. Block diaeram of the monoscope camera Allowing input end output voltnee wave forms r- VELI.THAAL. /EAT CL OUTPUT H co-t VO L-TA E ,OPE MBE SUPPLY Hont-zoNTAL DR.JNG 2a HOR170 L DEFLECTION 'ENERATOR K1 NEti.coPt_ BLANKING 0 SIGNAL BLANKING AIVIPLIFIER REGULATED; POW E. Ci SUPPLY VIDEO E NCLuDEs VIDED ,<:^1C2. 19 6.4'J megacycles without peaking at the high end. However, the maximum video frequency generated by the monoscope, aseuming equal horizontal and vertical resolution, is given by the formula (6) 2 ra/a 2 = (625) f = Where: (ZO) 6.5 megacycles N is the number of scanning lines r is the fraae ropition rate a le the aspect ratio f is the maximum video frequency. The resolution of the system, therefore, will not be limited by the video amplifiers. It is important to realize that e high frequency per- formanoe of the Amplifier determines the quality of the picture so far as the resolution and detail are concerned. in enu relay characteristics are flat, duced exactly. If both the picture is repro- If the gain is constant in the video bend end the time delay varies ith frequency, all the high frequency components are reproduced exactly in their proper relative Naplitudes, but the location of the various picture elements is not correct, because of the different amounts of time takea for passage of th different frequencies. This results la in- ferior reproduction of detail. The signal plate of the .aonoscope tube is connected to the grid of the first picture amplifier tube through a compen- sating metwork composed of the transformer, 11, and the 20 resistors, liZ and Re, as shown in rlate V. A similar network is included in the plate circuit of each adeitional amplifier tube. The compensating circuit is a four terminal network that presents a constant load impedance to the tube over the required bandwidth. A mathematical analysis of the network appears to be rather complex and will not be attempted, It may be stated, however, that there exists essentially two methods to reduce the effect of the is load, capacitance which the principle cause of loss of gain at the hiO4 frequen- cies. One method involves the use of a very small load resis- tor, whose resistance is so low compared to the reactance of the load capacitance at the highest video frequency, that the reactance has no effect on the gain or phase characteristics. This arrangement would possess no practical advantages, because of the erent loss in gain per stage entailed, by the use of a small plate resistor. a circuit The second method is that of employing containing inductance and utilizing the resonance effect to extend the gain at high frequencies. This, basically, is the principle employed by the coalpensating network. In the actual construction of the video amplifiers all possible precautions were taken as to the placement of component parts and the length of leads to insure a minimum of stray capacitance. Coupling condensers and load tires were held well off the chassis. It was found experimentally that the capacitance between a component having axlel symmetry, such as paper coupling condenser or a 1.rel, and the chassis varies xi e approximately as the function Ce ; here x is the distance of separation, C is Lho capacitance at x equal to zero, and is a constant proportional to the surface area of the coapon- ent. Thus the rate at which the capacity chaneeee with respect to distance, evaluated at any point, is proportional to the capacity at that point. This relationship is useful, inasmuch as it indicates the advantage gained in increueing the distance for small initial values of x. Low-Frequency Compensation In the frequency range below 200 oycles, the gain and phase ehift of the vid.oc amellfiers arc both subject to a variation from the aid-band values. The phase shift is particularly ob- jectionable at low frequencies for this. and. it is necessary to compensate The relationship between phase shift and time delay for a perticular frequency is =2-rrtf Thus for a given phase shift the time delay is seen to increase rapidly as the frequency is lowered. et to frequencies it is neeessary to compensate for phase shift in the cathode circuit, the screen circuit end in the coupling network. Complete coepeneation may be obtained by means of a network in the load circuit for any one, but only one, of the-e. In the video stages conetructed for the menoz- cope no compensating network is employed; however, preceutions are taken to effectively eliminate any phase shift that might arise due to the above causes. The 6AG7's use 18,000 oh screen dropping resistors which are bypassed by 20 microfarad condensers. The 6eG7's use ,O00 ee oha dropping resistors ehich are bypassed by 10 microfered condensers. This may be verified by reference to illete V. The frequency response is thus not effected by the screen grid circuit for frequeecies exceedine approximately 5 or t cycles. In the coupling network o.eb mierofarad condensers are emp7oyed with grid resistors of approximately 500,000 ohms. lere again the phase shift is neeilible down to a few cycles per second. In the cathode circuit no phase shift whatsoever is intro- duced. however, is accomplished at the expense of the This stge. in of the The method emplojed to accomplish this is sieply to omit the cathode bypess cendenser. of the stage at loe; frequencies is given by 3ince the gain 0) 1. Ge i e here: G' is + the etin of the atage taking into consideratlo. the effect of the bias network Is the load resistee,e R is 0, e the cathode resistor is the cathode bypass condenser and u, rp and ga are then if L;% equals zero, the gain is real and independent of frequency. the tube parameters This equation, with C. to zero, is equivalent to Where: G is the gain of the emplifier without feedback BG is the feedback factor This, of csourse is the well-known equation of an wJlplifier stage eaploying inverse feedback. An approximation of the gains realized, in the video atage will be made. typical J value of load and cathode resistance are 100 ohaa respectively. Considering the transconductunce of a 60 to be approximately 10,000 reicrodlos, then the absolute value of the feedbae-faetor is feedback is 10. 1 and the gain of the stage neglecting Thus the gain is deoreased by a factor of two and the overall gain of the stage is 5. i.S a first ap- proximation it may be thought that this decrease in gain is not commensurable with the result achieved. ever, other advantages resistor. ance There are, h to be had by using an unbypassed cathode The first of these is a decrease in input capacit- since the gain of the stage has beca lowered. This effect is desirable since it improves the high frequency posse of the preceding stage. res... The trunster chsraoteristic curves of the tube also become MOT linear and a sharp out off is obtained. It should be mentioned that the i'unction of the very low video frequencies is to suvly the background of the pieture. If the video stages fail to pass these frequencies without distortion the result will appear as a gradual shadine of the picture from the top to bottom (6) Output St The output of the video system is fe'ti to the paralleled grids of Ve and V10, the picture output and monitor output L4 tubes. The two output stages are identical, the additional out- put stage being provided so that the picture signal may be view- ed on a monitor while the signal is being fed to the equipment No attempt is made to achieve high-frequency being checked. compensation, since both stages operate into a 75 ohm coaxial line. The necessity of feeding into a coaxial line reduces the voltage gain of the output stages to a value less than unity. Blanking The blanking signal is also mixed in the video amplifiers. The composite vertical and horizontal blanking pulses are de- rived from the synchronizing generator and initially fed to V7, the blanking amplifier. The blanking pulses must be of nega.t- ive polarity and should conform to tha R.4.A. standard. The composite pulses are aixed with the picture signal in the fourth video stage .through the use of a common plate load, resistor. It is of interest to note here that during :anoscope tube is blanked, the polarity of ti.. the time the output video signal from the 2121 corresponds to that of a white section of the picture. Therefore, when the actual blanking pulses are mixed with the video signal, the polarity of the monoscope signal corresponding to the blankinking time is opposite of the blanking pulse. rather than additive. The that two voltages are then subtractive This necessitates that the blanking pulse be of much greater amplitude than the pulse generated by the monosoope tube during the blanking time at the point where they are mixed. Lfter the mixing takes place, the polarity of the signal during picture blanking is in the true black direction. PICTURE; BRIGHTNLS3 Following five stages of video amplification the signal is fed to the clipper tube, V4. The clipper tube operates only on the blanking pulses, v4.hich have previously been mixed with the video si nal, to establish a black level, and thus control the light intensity, that will correspond to any particular amplitude of the video signal. lien the blanking pulses are mixed with the video signal the level of the signal during the blanking period is raised far above the level of the actual signal eontali:Ate picture information. The clipper tube, therefore, has a wide range over whieh to operate on the blanking pulse, and the average picture brightness can be adequately controlled. The importance of the clipper tube in controlling the black reference level cannot be overestimated, and an analysis of the camera signal at this point nay be well justified. Fundamentally, the information contained in the camera signal consists of two components.. One is the a-c component, whose relativainstanteous amplitude contains the picture element brightness corresponding being scanned. to that point on the pattern electrode The second is the d-e component which is de- fined as that value of voltage existing between a fixed reference level and the average of the camera signal Olek (4)). The d-c component, therefore, corresponds to the average brightness of the picture, since it is an average voltage which would correspond to an average light intensity. It is advantageous 2t.1 to be able to vary these two components independently of each other. The relative amplitude of the a-c component is con- trolled by the Lain of the video amplifiers. This control is incorporated by mfeking the screen grid voltege of the first video amplifier variable with the potentiometer, 117. While this control is labled "Gain" it is in a sense also a contrast control, since a variation in contrast is an inevitable result of 'vryine the amplitude of the a-c component of the video signal. It nova remains signal cell to be be varied. shown how the d-c component oT the This is readily accomplished once the fixed bleak reference level is defined. This bleak reference will be taken es the height of the blanking pulse. Thus by varying the difference between the picture average end the blanking level, the d-c component corresponding to the average picture brightness is controlled. The average picture bright- ness ef the test pattern in the monoscoee tube is obviously a constant value. Once the d-o component corresponding to this average brightness is set for a satisfactory picture as vieeed on a kinescope, further adjustment will not be necessary. heference to Plate IiIwill help to clarify this discussion. The brightness control, N55, is a potentiometer that controls the grid bias on the el/peer tube. -Glipping is accom- plished by setting the grid bias of V4 to such a value that the top of the blanking pulse drives the tube into cut off. even number of steges is had the An between the blanking input end elippe, so that the blanking pulses appear negative on the EXIMANATION OF 2LATZ III Fig. 4. Control of the average picture brightness by varying the average of the caera signal with respect to the black reference level. ikverage light intensity of the scene being transitted is high Fig. b. Average light intensity of the scene being transmitted is low 28 PLATE III BLACK 4 D-C C REFERENCE OMPOtlEt IT vv-"Ji-or-AvERAGE 0 LEVEL OF CAMERA 51:SNAL 1,VHI-TE TitviE Fig. 4 BLACK REFERENCE LEVEL OF CAMERA SIGNAL D-C COMPONENT AVERAGE WhITE Fig. 5 of V4. Re in varying the bias on V4, determines the clipping level and consequently the d-c component contained by the signal. The action of the clipper is improved by using a 1000 ohm unby- passed cathode resistor. This gives a sharp out off character- istic and thus a well defined blanking level. 01.414PER TUBE ANAIr5I5 Fundamental Theory clamp circuit using a 6116 third picture amplifier tube, V6. operates on the grid of the The function of this eireuit is to establish the peaks of the blanking pulse at a fixed reference level in the video signal or at some potential fixed with respect to this reference level clu.rinL the retrace tiete. Coincidental with this action, spurious low frequency signals such as microphenics, power supply surges or 60 cycle hum which may have been introduced in the precedirw low level. picture LLn)lifier stages are reduced to a negligible amplitude (7;, The activation of any clamp circuit can be controlled either by the signal to be claaped or a signal independent of the cla.ipod signal (L2.). ed The particular clamp circuit employ- in this equipment uses the latter method of clamping, de- riving the keying signal from the horizontal driving circuits. By using a keying signal it is possible to obtain greatly improved d-c restoration. Keyed circuits can be made very fast with low distortion and high immunity from noise. They operate satisfactorily with signal levels much lower than for the sim-- 30 pie restorer circuits (7). The operation of the clamp circuit may be understood :.ore easily by reference to the idealized equivalent of a keyed d-c restorer or claap circuit shoLn in Plate IV. ated, or closed, for They key ia oper- n interval of time corresponding to the width of the horizontal driving pulse. The horizontal driving pulse width is approximately 10 per cent of the horizontal cycle or 6.35 microseconds. When the key is closed the output voltage goes to ground potential. A charging or diacharging current flows through C liaited only'by R. so that before they key is opened, C is small enough it becomes coapletely charged, and the current through it has drcrdded to practically zero. C now possesses a charge representing the diffrence between the signal voltage and ground. After the key is opened the charge oannot change since no path exists for the current to flow. The signal is transaitted through C as if it were in- finite in size. when the keying interval again returns, the signal may be at an incorrect level, and the charge will be changed to agree with the new difference between the input voltage and the correct output voltage. If the level, however, needed no changing. no current could flow into or out of U. This keyed circuit thus reatores the si-c component by holding the eignal during the keying pulse at a fixed voltage which may be considered the d-c axis. The signal extends always in one direction from this axis, end has a d-c coaponent exactly as in all d-c restorers or elaapers (7). In the light of the foregoing analysis the operation of PLALIATION 01? eLATL IV Fig, 6. Idealized equivalent of a keyed d-c res or or clamp circuit Fig. 7. Simplified diagram of the doable keyed diode clamp circuit use in the monoscope ca,716:0a. Keying pulses are derived from the grid and plrx te circuit of the horizontal pulse amplif 32 PLATE IV Fig. 6 R, Fig . 7 the monoscope clamp circuit may be discussed. The key is closed at the beginning of the horizontal retrace period which corres- ponds to the leading e -a of the horizontal driving pulses. These driving pulses vhile employed as the keyin g signal are simultaneously applied to the control grid of the ing pulses. Zn1 as blaz- The grid of ie, elate V, is now at zero signal level or ground potential ana this, as previously stated, is the clamping action. blanking pulse, ing. Just prior to the end of the monoscope the key is opened and the grid of 72 is float- The remainder of the blanking pulse, therefore, falls on a predetermined point on the grid voltage-plate current characteristic of V6. This portion Of the blanking pulse there- fore falls on the same point during successive cycles of hor- izontal or line frequency. In other words, undesirable low frequenoy chances in preceding amplifier stages are removed by resetting the level at the end of each line. he desired low frequency characteristics of the picture aignal are at the same tie restored by resetting the base of the blanking pedestal at a fixed refereace level at the end of each line. I]xcept for the difficulty in closing at the proper time, and opening the key the keyed circuit fulfills for a satisfactory clamp circuit. the requirements The advantages associated with a keyed clamp are that d-c level at which the blanking is held can be easily adjusted, and that the same circuit eill handle either positive or negative polarity signals. it is of.interest to note actual circuit is of for value. that the capacitor, C84 in the This keeps the time constant LXPLUIATION OF PLATL Actual circuit V diagram of the muuoicope camera i. 35 PLATE V VI V'S 6AC7 PICNIC: ANIL. 6AC7 menace 1,49 Wt. af ve GAC.7 0/C724211 cLAMP.E R.14 /V \ MONOSCOPE y ID RCA -2FZI a R -7 11. 5 A. 0 R-47 C.41, /es A r I 't r .-,ww, 4 C4113 T.3 6-24E.- IA C.37 I or --a T__ I utx( C3 ---11ruree M 1.1 If 2....4 10000 ( ..00n , OCN/ 3 :; :,..L40 ...II'S° J F- 6.3 v ?AK D C- -4 - V ' AND DISCHARGE 4-2S V 13 65N7-(-_,T VERT. S.T, n Ir e . 'lc., t. 4 l tr, ,,.,20., APJ CI'Y 4 N1 'Z.14-A P OWL S933113 'e 6 ',.. MON 44107:?...,-E.A.:0 ,cagan HI* VERT lue_SE AHR:_ 5 5 Is Vilr IWO -*-la f 'fcr r TT, ALANKING ASAC 28 nOn.,Va. evrmar A:5711 ,9 V.13 * F.1 z: Ls AwsP. nal faxW% a h I 7--11 P t, ea 11IVRLeCIA. C2 DI .12 ell,. tow ) s 011. L tAriT , TE 0, salTnhc.a vows. 4 L It 0 s-e t 1 GO FtE Fig. 8 jr4o MT% No-, yrlTf 'T 619213 1001 f ., wai...4_11 IN :*P3 I1Anal ,ALucs car inaT stt:12_,;:taty.aa, in-, 04.101 CO% T4 zeIttim*.deo.. c DWG% IS .4.Alf idalLGarim&T TAtf 0117704.0" of the circuit to a suitable low value with respect to the ation of the keying pulse. .ur- The low frequency response is not deteriorated during the picture siEnal interval as the open cireuit resistance of the diode reaches a high value, end the new RC value 1.3 of the proper order to pass these frequencies. The Double Keyed. Diode Clamp In the aotual circuit shown in the schueetic diagram, Plate the switching action is accomplished by keying two dipdes by means of the horizontal driving pulses. These driering pulses are 180 degrees out of phase and are obtained from the input end. Output circuits of the horizontal pulse amplifier seotion of tube, V18. An analysis of the keying pulse:i, and tic .t te particular requirements eill be given later. The simplified diagram of Pla e IV is coneistene with the actual circuit. The two diodes are driven through two condensers, 01 and C2, and are connoted by two resistors RI and R2, the common point of which is grounded through some voltage E for supplying the bias to the amplifier. end R2c2, are long coapareti to the For an-analysis this circuit as shown in Plate'Vlo, The time oonstants R101 duration of the horizontal isbest redrawn in bridge form The circuit and letters deeignations are the same as in the previous figure, except that the two pulses are indicated by PI and 22 with the center point of the two pulses at ground. opposite polarity. The two pulses are equal in this case, and ef In the actual circuit RI and. R2 are 2E,000 Cl 1m3, 9,11 C are .01 mierofarads, and C is 330 mio ofarads. During the pulse time the diodes are driven into eondnotion by the positive pulse on the plate of D1 and the negative pulse A ourrent i1 flows through the diode and on the cathode of DZ. the capacitance Cl and C2. A ourront 14 will flow into or out of C until point 1 is brought to an equilibrum voltage which is the clamping level. Between pulses a current 12 flows through Cl, C2, R1 and RZ slightly reducinc the charge on G1 and C2. The clamping level or output voltage to which point 1 is brought during the pulse time is equal. to IL under the conditions of equal pulse amplitudes, equal resiatanoes, and similar diodes. The voltage to ',hich condenser C is brought during the pulse time, is charged or to which point 1 depnd upon the a-c an voltage applied to the diodes at points 3 and 4. are shown in Plate VI. ay way of illustration, d-c These voltages the pulses are shown as 20 volts peak to peak, and of 5 percent width, whioh would give they a peak value of 19 volts from the d-e values rumoured at points 3 and 4. During conduction time points 3, 1, and 4 are all approximately the same instantaneous potential, which is the clamping level. The axis or d-o voltage at 4 is then min- ua 19 volta and that at 3 is plus 19 volts from the clamping level. The clamping level is therefore midway between voltage at tv.een these and. 4. If R1 and R2 are equal, 1.0 is also midway be- voltages, and equal to the clamping level.. pulses are unequal, the d-c the elamping level will equal If the whenever 12LANATIOS OF Fig. 9, ' T1 ouble keyed diode clamp eirouit redrawn in bridge for4 to simplify aaalysis Fig. 10. Clamp oirouit designed to supply grid bias for the following amplifier stage 39 PLATE VI , Et 19 v _ 20v E,+20v 1_ _L_ Fig. 4 RA Fig. 10 Et+ !gv - - 40 R1 Clamp Level = Since = 1 Then C.L. + 4 - -f- =. 2 ÷ - 1.41 + 1 P1 1. ;-; 11 21 or Via C.L.= Then If the resistors are unbalanced C, ttnd G. should be very large, or unbalanced so that he two keying signals are constant in amplitude, may be obtained in effect from the diode oirouit itself and tAle voltage eliminated, point 2 being grounded. 3uoh an arrangement is shown in :elate VI, where the °enter portion of the R1R2 combination has been replaced by a potentiometer. There is a point 2 on this potentiometer vIlich has the same 41 potential aa the olamping level. It is the junction of the now hypothetical resistors 41 Lnd R2 which have the same ratio as 2. and. If soma other point on the potentiometer ,le ground- voltage will exist between the aria and point 2, since a In the example shown, d-c voltage exists aorose 41 and 42. negative voltage exists at point 2, since the positive portion. of R, ground. Any part of R1, R2 may be grounded, except that as the grounding point leaves point 2, an inoreasing por- tion of the pulse voltage also appears on the arm* A resistor. RA should therefore be ineerted between the arm and ground in order to avoid loading the pulse. (7). /eying The aocrate tLainz of importance. Requirements the keying pulsee is of obvious This requirement, uits external Sussed. .Pulse however, to the monoscope camera is fulfilled by cir- nd will not be dis- Incorrectly timed pulses to the keyed circuit are re- vealed by a strong dark or light horizontal streaking effect on the kinescope which varies erratically. The pulses must, of course, be synchronous with the signal. The front edge of the keying pulse shoald occur approximately at the beginning of the portion or the monoscope derives which is' the signal to be clamped. since the keying signal and the blanking pulse clamped from the same source, this requirement is automatically satisfied. The keying signal should end well be- fore the end of the blanking pulse. A tolerenoe is allowed here, such that under no conditions vill the key last beyond the signal re -ranee level. The keying pulses must elso bo large enough so that under maximUm signal swings, neither diode will conduct between keying pulses.. It is apparent that even though the pulses place a largo blocking voltage on the diodes the signal also swings the other element of each diode, and eill decrease at least on one of them the bias provided by the keying pulse. Greatly increased noise immunity is given by the keyed restorer, since noise ooeurring between the keying pulses has no effect on the circuit. Since the keying pulses are apprcet imately 10 percent of the horizontal cycle, the circuit is effectively improved by a factor of 10. TH HORIZONTAL DEFleeCTION CIA CUITe Horizontal .eulse Amplifier and Sawtooth Generator The horizontal driving signal, derived from the aynchro- nizing generator, is a square pulse of negative polarity, the duration of which is 10 per cent of the horizontal cycle or microseconds. fe.;5 Its peak amplitude is from 3.5 to 5 volts held within a tolerance of .5 volts. The driving pulse is applied to the grid of V18, a 631,7, uaed as the pulse amplifier. The time constant of the grid circuit, C30 end R95, is long (10,000 microseconds) compared to the grid, duration of the horizontal driving pulse. As such, leak bies is developed and the signal is effectively Clamped at ground potential. The clamping action of the air ouit is a result of the low etatie grid reeistasee of the vale?, which can be approximated as 1000 microseconds, and the coupling eandeneer changes quickly to a value equal to the aaplitude of the input signal above its ac reference level. The bias on the tube is then zero and the signal is said to be clamped to Ground potential. At the time of the next negative pulse the time con stant of the Grid circuit is, of course, its original value of 10,000 micro-seoonds, aad the waveshape of the negative pulse is preserved. If the amplitude of the input signal is above 4 volts the tube is probably driven into out off during the negate ive pulse since Ch 80 the of cut off) (at volts The input horizontal driving pulse is also fed directly to one section of a 61M, which is used in a keyed diode clamp cire cult in the video amplifiers. A voltage diVider in the plate circuit of the input section of V18 supplies a signal of opposite polarity' but of OV: 1 amplitude to the other diode section of the eHee The total output of the pulse amplifier sections of Vi8 is fed to the second section of the 62147 which acts as the dis- charge tube for the sawtooth generating circuit. The peak to peak amplitude of the driving pulse from the pulse amplifier is 20 volte, ehich drives the discharge tube into full conduction 44 duriag the driving pulse period. ing the interval between the The tube remains out oft dur- driving pulses. Glamping is not had in the grid oircuit, a case, due primarily to two reasons, in the previous The 2irst is that the time durieg which the signal goes positive col-resads to 6.4 micro- con de rather than 57.1 microseconds, since the waveform has been inverted by the pulse amplifier. To determine whether °Lmping action is present the must be compared to this. However, the resistance usei to cal- time content of the grid circuit culate the RC product is no longer merely the etetio grid resistance of the tube, but or the drivingsource, mist else include the internal impedances iiich in sistance of the pulse amplifiee. this case is the plate load re- This consideration could be neglected in the grid circuiteof the pulse eaplifier since the internal impedance of the driving source is only the characteristic impedance of the coaxial cable. Thus the hC product, as determined during the time which the grid is positive, leis in- . creesed greatly and the time to which the RC product must be compared has deeecaeed. the grid. circuit during The result is a long time constant in the period of the positive driving pulse end no clamping action is obtained. The savitooth generatin g. circuit actually produces a trap- ezoidal voltage waveform. This is necessary to produce the saw- tooth of current in the inductive-resistive circuit of the de- flection coils, The mathod employed to develop this voltage waveform is conventional and is based on the gradual aoeumulation of charge on a condenser, C32, following by its rapid dis- charge. The output volte is then taken across the condenser and a .peaking resistor, R126, and applied directly to the grid of the horizontal output tube, V17. The horizontal width con- trol is incorporated in this circuit by varying the potential to which C32 charges. This potential is controlled by the set- ting of the arm of the potentiometer, R92, since the potentio- meter is part of a bleeder across the to voltage power supply. R92 will also control to a certain degree the horizontal linearity. The Horizontal Damper Circuit An analysis of the horizontal damper circuit will undertaken* The actual circuit is simlified to a w be fundamental damper circuit: shown in Plate VII, Fig. II, and reference will be made to this and to the associated waveforms in Plato VII, Fig. 12. The tubes, VI and V2, correspond respectively to tubes, 717 and 118, of thc actual circuit diagram. The initial assumed condition will be that V2 is not con- ducting and that VI is eondueting but with an absence of signal on its grid. If a linearly increasing voltage is on the grid of V1, the plate current, nee with the grid signal. i, ow inpressed will rise in accord- This plate current flowing through the horizontal deflection (soils* Lh, builds up an assumed pos- itive magnetic field, the respect to time. to give magnade of which is increasing with When the magnetic field strength is such as the desired deflection, the plate current of V1 is cut off by driving its grid into cut off. This time is designated 1ATION OF PLAT Z VII Fig. 11. A fundamental damper circuit diagram employing an inverse power control tube as the resistaaee. The direetion of eleotron current flow is assumed Fig. 12. AssoeiaXed current and voltage waveforms of the fundamental damped circuit 47 PLATE VII 8 eg A Fig. 11 0 eLRET-Lci1/4 1 T 0 d = L e= Fig. 12 i$R 0 ,4 48 by the letter B in Fig. 12. iitiatc e. A period of energy reversal is now tuned plate circuit, in the retrace period. LhC, corresponding to the Thus the magnetic field surrounding Lh collap- By virtue of this induced ses, inducing a voltage in the coil. the polarity voltage the coil now appears as a voltage source, being such as to drive current in the same direction as it was originally flowing. This current is designated as necessarily go to charge the capacity in accordance with the maximum when large. and must since V1 is no longer The voltage across the condenser is now increasing conducting. is also a C i i formula equals zero. maxim at this e,= 1/C f idt, and reaches a The voltage appearing across Lh time, since the expression di/dt is The actual peak value of voltage across the deflection coils is so large as to warrant its omission from the waveforms of Fig. 12. The potential energy, now stored in the condenser, is released in the form of a condenser discharge current, -i. has increased to a value which is approximately 70 per cent of the original value of il at point B. This decrease is a result of the power loss that is inevitable in a tuned circuit with a finite Q. time is now Since the rate of change of current with respect to zero the voltage across Lh is zero and the waveform representing ce is seen to cross the zero axis. The system has now cocepleted a complete cycle of energy reverse' or one half cycle of free oscillation. Again the magnetic field collapses and a voltage is induced in the coils such as to substantiate the current -1. This ne- cessitates that the coil, now appearing as a veltage source, have a polarity such that point 1 is negative with respect to Thus the plate of the inverse power control tube,. V2 point becomes positive bhd the tube will caldnet. sistanceof V2 now shunts fie and the circuit is highly damped. a result of the shunting effect is damped cut at point 4« Lh 0 and. V2 The low plate re of the tube, free oscillation The current that now flows through as a result of this action will be called 12. This current is forced to devrease in any desired manner by controlling the plate resistance of V2 with a proper signal voltage on its grid* At point A the tube Vi is simultaneously driven into con- duetion by its control grid voltage* The current that flows through the deflection coils is therefore the summation of the two currents, iz and. i. These combined currents result in larger and linearly changing total scanning current, Thin i summation current, ij, produoes the field which causes the for ward trace of the horizontal sweep. Its characteristics deter- mine the amplitude and linearity of the horizontal deflection. The results obtained with this type of damper circuit are superior to that had 'when a conventional diode daapeT'is used. Apparently better utilization of available power is secured and certainly better control of the deflection linearity is provided by being able to control the eifective damping resistance. 2late VIII oomparison. shos the aetual horizontal damper circuit for The modifications and refinements aade to WAO fund- amental damper circuit will now be justified. . The insertion of the coupling transformer, T11 week the ZULANATION Fig, 1Z, O1' 2LATL 11111 .Atual circuit of the monoscope horizontal damper, ffC" is the lumped coil uld circuit capacities in shunt with the deflection coil. R126 &rid R102 are linearity adjustments for the horizontal sweep, and control the bias and Erid voltage ',aveforms respectively of 716 the inverse power control tube 51 VIII PLATE A.C. L OAD di* C. AP- -r .S! V16 V1T L HcIZi (11(ff-11C-1 V N _ 4PAAAAr COILS V L N GRID OF PULSES T3 MONoSC. Qp TLj si6 Fig. 13 e. 52 power tube, 117, and the damper tube, T16 allows the dumper tube This in turn .laminates the necessity of opere to be reinverted. eting the cathode of V16 at a relatively high potential above ground. Thus it is u eoessu.ry to use a special heater voltage supply trunsformer. The design requirements of the transfer= critical (3). T111., ar quite, First the transformer should have a primary and secondary winding of low induotance and low distributed capacity. This is aeoessary if the natural resonant frequency or the horizontal sweep. It is desirable that this frequenoy should be high since it insures that the half cycle of free oacillation, which results in the retrace of the beam, will be ohort. A high ratio of retrace to scanning velocity is consequently obtained. The transformer, Til, should eleo have good trelueney end phase oharacteristics. It should be designed for the frequency renge between the fundamental frequency of the deflecting current and at least the tenth harmonic of this frequency. It has been found (5) that magnetic deflection amplifiers dive satisfactory performance when the fifteenth harmonic is transmitted And may serve adequately when only the tenth harmonic is included. For 525-line scaaning with 30 frames per seoond, the transfertier should be designed for the frequency range between 15,760 and 1604000 cycles. The capacitor, C3,. and the resistors, R100 and R122, fora a differentiating circuit across the output of the transformer, Til. This provides the correct sawtooth control vol- tage on the grid or V16 neaeeeary to obtain the desired current change of 1L. The adjustment of 12, and thus the liner- ity or the deflection, is accompl.:.ohea by varying the bias on the inverse power control tube by means of resistor, 1a0L. ;,pproximately 12 percent of the hair sine wave negative pulse voltage appearing across the horizontal deflection (toil is taken from the junction of R120 and. R119. to he monoscope This is applied tube control grid for the purpose of blanking out the picture durine.the: retrace time. In this manner, the fixed reference level is established on which the clamp circuit of V9 is set at the end of each line. clai7lping If this were not done the circuit would. operate on some erratic transient voltage generated the beam swept the pattern electrode on the retrace. The resistor, R103, is the horizontal centering control. It supplies a steady d-e current of controllable magnitude to the deflection coils. trol, polarity and By the adjustment of the con- the beam can be centered horizontally on the pattern eleotrode. ;lie method employed to obtain the large current ;:e- quired fur the horizontal centering control was to use the entire reeletanee of R103 au the ground return for the 40 and b0 microfarad filter condensers in the low voltage power supply. R1OZ was then bypassed with a 600 microfarad condenser to provide adequate filtering of the .i.ictually (1.-c deflection coil current. R103 is a 20 ohm variable copper alloy resistor, so that a condenser of low voltage rating can be used. Ve:eTICAL JLPLZUTICN CIRCUITS The vertical driving pulses, derive from the synchron- izing generator axe applied to the grid of V14 through the inJ4. put jack, amplitude. pr The driving pulses are critical es to width end The duration of the pulse should correspond to 4 cent of the vertical cycle and the amplitude may vary, within a tolerance of .5 volts, from 3.5 to L volts. The vertical deflection circuits are essentially the same as those employed throuehout the horizontal deflection system. Therefore, only a brief description is eziven in order to avoid repetition. In general, the circuit constants em- ployed will differ because of the difference in time between a horizental and vertical oyole. cy of the system also preeludes The lower repetition frequenthe use of a damper circuit across the output tube, since the transient voltage developed across the coil is greatly reduced. Amplification of the input signal is obtained from the first triode section of V14, amplifier. a, 67L7 employed as the pulse The RC product of the grid, circuit, C22 and R63, is notably large, since the repetition freeuency is but 60 cycles. The second triode ecction of this tube is utilized as a sawtooth generator. The amplitude of the sawtooth wave- form is determined, by the height control, R69. The two triode sections of the tube, V13, are connected in a cascade coupled amplifier circuit to increese the sawtooth vo)tege to the proper driving level for the output tube, 112. Both sections of this output tube, a 63N 7, are connecte1 in parallel and coupled to the vertical deflection ooil through the transformer, 110. One half section of the tube, V11, is used as a vertical feedback amplifier to improve the linearity of the sawtooth waveform. The grid and cathode section of V11 is coznected through the capacitor, C27, across the resistance, N79. This resistance is in series with the vertical deflection coils. The plate of this tube is connected in parallel with the plate of the input section of V1Z. The variable resistor, R81, is part of the grid leak, and determines the operating bias of the tubes This is the verticA. linearity control and has effect over the ''hole raster. Tlii; I LOW V0ITAGE The low voltage power supply was constructed as a composite part of the monosoope camera. Alile this feature may be considered undesirable from the standpoint of the stray magnetic fields introduced by the power transformer it was felt advantageous to have a and, choke, self-contained unit. 1,11 precautions were takea to insure the ezeclusion of the fields produced by the power supply from the menoscoos tube and from the deflection circuite. The specifioations of the supply are given as follows: or input, 110-120 volts a-s 60 cycles; output voltage, 280 volts regulatS4 (1-.0; maximum load current, 250 milliamperes; power consumption, 75 watts. No particular difficulty 13 eacountered in constructing 56 a power supply to meet these requiremente. However, a dis- cussion of the theoretical and pran.ical design considerations is of interest. he necessity or voltage regulation for the d-c source should be apparent when the ultimate purpose of the monoscopc camera is considered. Obviously every refinement should be taken in the design of this equipment to insure the perfection of the test pattern gettera.ed. koor regulation would. react on the deflection circuits resulting in linear displacement or a change in picture size and on the video amplifiers oausing a variation in signal strength. In addition to supplying a staple voltage under change in load, the inherent property of a voltage regulated supply to practically eliminate ripple is also of marked importanee. the theoretical aspects of the voltage regulating section will first be eonsidered. 14, Reference is made to 21Lte L, hioh shows a generalized circuit of a degenerative or cathode follower type of regulator. 2.ssuming ideal tube characteristic's it is found that the output voltage can be expressed by the following equation (1) Ei(R2+ [ Introdueini,' the (RE + rp2) rp::,,, ) e- (WI +rpi) ulu2RrEa +Rin(1+sa! approximations that J.% az >.) 1 114141121c " Ail(a2 r2e) 11 Fig. EXPLANATICE OF 2L4TE IX Fig. 14. Basic circuit diagram of a degenerative type voltage regulator Where: El is the rectified and filteed direct voltage to be regulated. Zo is the regulated output voltage be is the negative bias voltage required for TT2 R1 is the parallel resistance of load circuit including bleeders, control potentiometers, and actual load Re is the adjustable control for E 11, .t-te is the load is the voltaGe from grid tap to ground; of Fig. 15. .4. representing a percentage f.ctual circuit of the regulator section of the monoscope power supply 58 PLATE IX Fig. V 6AS7 10 2 14 70 L vv 10 10 riAA' 470 Ei OFD '27C 470 I< V6 K I V: 1i`- 651_7 M FD VR 150 r-^AAN)14-65.7 270C V VR 150 750C 473 K LI MED Fig. 15 L,19 el+ rpi) itilye1iu4 then the equetion will. reeuee to the form =E This relationship is an important design equation Ir- respective of the fact that it predices perfect reguletioni From an inspection of the equation several features of the regulator become evident. The first is that smooth (lent of the output voltage can be obtained by manually varyine the value of E0 or A. Reference to the actual circuit diagram, Plate IX, rig. 15 shows that this control is incorporated through the use of v 470,000 ohm potentiometer in the grid of the second amplifier tube. And, This is a front panel eontrol through it the output voltage can be varied from ZZO to ZOO volts without the loss of regulation. The range through which the output voltage may he varied is limited. Obviously, the output voltage is the difference between the input voltage end the drop across the pass tube, which in turn is current. a, functien of it control grid bias end pieta One limit is therefore reached ehen the amplifier tube is biased to out off resulting in zero bias on the pass tube. The output voltage will then be at its maximum value and will be determined by the load ourrent, From the tube characteristics of the peas tube, the voltage drop across it can be found by finding the plate voltage required to sueeort the deslred load current, whieh will be its plate current, under zero bias conditions. It may be noted that when the amplifier tubs. VT2, is cut off its its resistance is ia- finite. Thus the equetion expressing the output voltage trans- ends from that of perfect regulation to = RIP]. rp1 and the output voltage is now a function of the input to the regulator. The other lie:lit is reached when en attempt is made to adjust the output voltage to the voltage across the very low value. a pass tube is high. is required on the pass tube when only to that of the bleeder. suvly or VTL. Thus a negative bias the output current amounts However, the output voltage must this bias, which is developed across the plate potential In this cz:se 112 in addition to Hence it becomes iereossible to reduce the output voltage below a certain value determinerl by E, bleeder load, VT1, and VT e characteristics. The choice of the pass tube, 7T1, is an important eonsider- ation since the characteristics of the regulator are essentially determined by it. The recently introduced tiA37, a twin triode, was chosen, since it was expressly designed for this service. Their extremely low plate resistance allows the use of a lower voltage drop across them, end thus a higher output for a riven input voltage. The heater cathode rating of 300 volts is ed- vantageous in eliminating the necessity of a separate filament transformer. However, a separate filament transformer wee ee- played in the power supely constructed, since the 6e37 is En expensive tube. Among its other advantages are its extremely high current handling capacity. Previously four 2e3ts vere 61 connected in parallel az pass tubes to supply a load curaent of 2.5.0 milliamps. One &AS? with its triode sectious in parallel will auffice. Standard practice dictates the use aad plate circuits when current opacity. Li3 ssors in grid a Libes are paralleled :or greater Their elimination results in highly unatable or erratic, operation. It is preferable that these suppressors be mounted with one end directly supported by the tube socket lug. to which it connects. Resistance values of 10 to 50 ohms for the plate and 100 be 300 ohms far the grid prove satisfactory. The circuit diagram, 2late II, high reu lifier. twizi 2i. 15, shows a tL7, a' triode, uaed in a two stage dire et-coupled amp- The grid of the first amplifier tube should receive as large a perceatage aa possible of the total fluctuation present on the output, Variations in the output voltage are developed across the 7500 oh resistor in the cathode of V. Th e 1 mi- . orofarad coupling aondenzer represents a low inpedanee path for high and medium frequent:31es. Attempts to teed back a greater portion of the output voltage by increasing the value of Li condenser will result in erratic operation or osoillat- ions TEE 1G 1.70 AG L WER StWLY The high voltages necessary for the operation of the mono- aeope tube are obtained from a self-contained high voltage power supply. The high-voltage power supply is conventional in that it consists of an iron-core stop up transformer, energized from the power line, end a rectifier circuit with a :eaothing half-eave rectification as obtained using a '.;e and the filter circuit consists of a 1500 henry choke, T9, and two 1 mierofarad condensers, C19 and the circuit, The filter is in the negaeive aide of k;e0. the positive side of the high voltage being ground- ed so that the signal plate of the monoseope tube may be at ground potential. This placed the heater and cathode of the 2.X.2 at a high potential and neceseitated the use of a high-voltaee socket and a heater transformer with a high voltage rating. flowever, rasons in that this system is desirable for safety the high potentials are made less accessible. It is also ad- vantageous in that the pattern electrode can be eenneeted di- reetly to the grid, of the following video amplifier without a high-voltage blocking condenser intervening. Fairly elaborate filtering of the high-voltage power supply is necessary since the ripple voltage must be kept considerably smaller than the video signal generated. of 100 millivolts is about the AnIce a video signal maximum obtainable under usual operating conditions, the ripple voltaes should be no more than Millivolts and preferably be no more than I millivo The required degree of filtering is echelveu 07 the use of one half of a 61,7, V11, as a voltage reguletine ating type circuit is used, with with the output voltage. till tube. A degener- merely inserted in series The regulating aceion of it is res- tricted. in that the voltage fed baok to its control grid is through an 'kW network. If the d-c level of the output voltage changes it will not, therefore, be corrected by the regulutor 63 tube. This, however, is not perticulerly important since the load on the power supply, ehieh essentially the beam current of the nonoscope tube, is eenstant. Any ripple voltage appear- ing in the output, however, eill be fed through 021, a 2 mi- crofarad coupling condenser, to the grid of V11 and thus reduced in magnitude through the regulating action. The enee, V11, is also bypessed by a 100,000 ohm resistor for safety reasonue The output voltage 1500 volts and. it is ()" the power supply is approximately capable of eupplying 5 milliamperes. All voltages for the monoscope tube are obtained from a tapped bleeder resistance across the output of the power supply. cause the beam current for the 21.'21 Be- is small, large bleeder current is net required. A 991 glow tube is shunted control potentiometer. the control :rid of aerosa 1t60, The potentiometer arm is eonnected to the monoscope and the positive side of the petentiomeeer is connected to the cathode. against excessive be the beam current This is a preoeution current should the output voltage of the power sup)l7 change, since the 991 providee a constant drop of 50 volts across R50. The hieh-voltage power supply is ale() used to provide the negative hies ter the clipper tube, 'LIST 114, in the video amplifier. 2eTTIRN ANALYSIS The correct ieteepretation of the test pattera is funda- mental to the intellieent use of the monoecpee oamera in testing television equipment. The particular test pattera coeteiaed 64 in the 2F2.1- and refs is shown in Plate in the following diecussion. e i a me de to this The pattern is simple, yet so de- signed that a comprehensive analysis of the equipment under teat say be made. The ratio of the pattern's width to height is four to three in accordance with the standard aspect ratio. The outside d' ameter of the largest circle is, therefore, three fourths of the width of the paetern, so that when the deflection is edjusted to this circle, the standard aspect give an undistorted form to ratio will be established. in the center of the pattern there are six concentric cir- cles, the center of which is. lebled 30. The radial spacing be- tween the circles is the same spacing that would exist between 300 horizontal lines equelly spaced in the vertical dimension of the pattern. If the equipment undertest can reproduce the ircle pattern with these central separate and distinct, then the equipment is capable of resolving 390-line detail. The four resolution we es radiating from the central cir- cles are calibrated in a similar manner. The resolution lines are of equal eldth in both the horizontal and vertical direetions. Thus the total number of lines whieh can be contained in the ;idth of the picture is greater than that etioh can be contained in the height of the picture by a factor equal to the aspect ratio or 1.33. so that The center line of each -wedge is dashed the resolution corresponding to any radial distance is sore readily determined. The break in the center line occurs at the same distance from the een!,er as the arc of the resol- LX2LANATION OF PLALL Fig. 16. Test pattern which i transmitted 14 the aonoscope camera using a 2721 act zj 0) 67 ution numbers, and the distance between the breaks in the center line corresponds to a change in resolution of 50 lines. The calibration numbers should be aultiplied by 10 to obtain the number of lines resolution. The upper vertical and right hand horizontal wedges vary in spacing from 300 lines at the outer ends of the wedge to 500 lines at the inner end. The lower vertical and left-hand horizontal wedges vary from 150 to 350 lines. In all four wedges, varies linearly along a the number of equivalent lines radius. The point in the wedges where distinction between individual lines just disappears, indicates the resolution of the system under test In measuring resolution, distinction must be made between the ability of the equipment under test to resolve detail along a horizontal line is necessarily measured by the vertical wedges and resolution along 4 vertical line is indicates by the horizontal wedges. If the situation should arise where the vert- ical resolution of the system under test is comparable to the nurser of scanning lines, then a spurious diamond-shaped- pattern appears in the reproduction of the horizontal wedges. This spurious pattern is made up of the inteesections of wedge lines with scanning lines and the resultant pattern should not be regarded as a defect in the reproducing epuipment. Also enclosed within the inner circle are two wedges set on a 45 degree angle to the horizontal. of four distinct sections, Jach wedge is composed the difference between each section arising in the degree of shading. The variation ranges from 100 per cent bleak to 25 per cent black in equal divisions. These w provide a. test for amelitude diatortion video signal. To the right end to the left of the central wedges are two vertical rowe of smell rectangles. The figure 50 above the right hand column indicates that the top rectangle has a width equal to 1/50 times the height of the pattern, and is thus in- dicative of 500-line detail.. The rectanglee in the right hand column progreseively decrease in width in steps equivalent to 25 lines up to 300-line detail, and similarily on the left hand column from 325 lines to 6"7elineee These two rows of recteme gles are useful in testing for undesired transients, since traile ing is eometlues shown up more clearly by the rectangles than by the wedges. elow the central wedges is a set of 11 horizontal lines whose length varies legarithmicelly. is 71 per cent of the The length of each line lehgth of the line above it. The short- est line has a length 1/50 times the heieth of the pattern; the longest line is 1/1.5 ties the height. These lines are useful in obscrvin, defective low frequency responses in the video range, and trailing in the reproduetion of these lines is iadiaeUve of the improper adjuetment of the low-frequency cumpenseting cireutts. eeeve the central wedges is an Indian head which provides a test of the general quality o' the reproducing system. This is especially true eith respect to contrast and average bright- ness which are moat easily judged on a Pictorial subject. The largest circle and the circle surrounding the central. 69 wedges provide a test of linearity of scanning in the horizontal and vertical direction. outline, If the circles have an egg-shaped then the rate of scanning is nonlinear, in the verti- cal direction nhen the axis of symmetry is vertical and in the horizontal direction when the axis is horizontal. Linearity of scanning is also tested by the 4b degree lines and by the grid squares into which the pattern is divided. The fine lines which form these squares have a width of 1/600 times the height of the pattern and will immediately reveal any orthogonal dis- tortion in any part of the image. In each of the four corners of the pattern there are re- solution we es. If the resolution in the corners of the pat- tern is poorer than that given by the control wedges, clusion is that spot defocusing is present. defocusing is inherent to all kinescopes, the con- since some spot the resolution in the corners as compared to the resolution of the system is a meas- urement of the relative defocusing present. USES OF THE UDNOSCOPE The raonoscope camera is a coaprehensive piece of television test equipment and may be used for a variety of purposes. commercial af?lications, the In television transaitting station may aaploy the camera to transmit a test pattern during t,arm-up and stand-by periods. Station identification can be simultaneously provided if the station call letters are printed on the pattern electrode. Another interesting commercial application is the use of 70 the monoeeope in obtaining a fixed background for studio works The final signal is combination or the video signal from an loonoscope which might represent aotion, and that from the monoseupe for backgroand. The aonoscope camera is also used in many laboratories and factories to obtain a television signal whioh can be to test televiaion receivers. titled In conjunction with a synchron- izing generator and a distribution amplifier it produees a sAindnrd R.IA.A. video signal for use in testing video aseplifiers and picture tubes. .eith the additiorrof an I-F sweep generator and an R-F generator it produces a complete televis ion picture signal simulating that received off the air and receivers under conditions thus provides a means eciailfaXent to actual use. Plate i.i indicates the arrange- ment of the necessary equipment. Primarily however, the main application of the mnoseope camera remains in'testing television transmitting systems.. it furnishes the transmitting station with an always cvailable source of video, signal of known quality which can be substituted for the studio cameras whenever it is desired to check or adjust the operation of the following units in the system. In this respect the monoscope has aided materially in th advancement of the television art. ...1A4,14:,TION Fig, 17. OF 2LATi; ti Lquipment arrangement for testing a television receiver fed from a monoseope camera MoNOSCO PE DISTR1BOT ION CArAERA A mPLIFIER R-F 1-F SIGNAL SIGNAL GENERATOR 7 SYNCHRONIZING GENERATOR GENERATOR - V IsT 2!4° ^RP 2Ho sT DET. IF IF DET. \Ape.° 2 tii) VIDEO 73 The wrib1, e:,,teads his sincere J. _Jdaond :solfe for his thi;$ thesis, areciation to rotcs8or aid throughout the preparation of °wean for his help- r1 to Instructor Kenneth ful sugi;estions during the cantruoiion o.. the eamera. 74 (1) Abate, A. Basic theory and design of electronically regulated eveer supplies. 2roo. IRE 33: 478-480. July, 1945. (2) Burnett, C. E. The monoscope. (3) (4) (5) RCA Rev. 2(4):414. Engstrom, E. W., and R. S. Holmes. Television deflection circuits. 12(1):19-21. January, 1936. Everest, F. Alton. Wide band television amplifiers. 11(b):241. May, 1938. April, 1938. Electronics. Electronics, Fink, Donald G. Principles of television engineering. McGraw-Hill, 1940. (6) (7) New York: Goldsmith, Alfred N., and others, editor. Television volume III. Prifleeen, E. Review, 1946. ac, Goldsmith, Alfred N., and others, editor. Television volume IV. Princeton, N. J.: Review, 1947. RCA (8) Liebmann, G. Image formation in cathode ray tubes and the relation of flourescent spot size and final anodevoltage. Proc. IRE 33:381-389. June, 1945. (9) Mather, N. W. Clipping and clamping circuits. 20(7): 11. July, 1947. Electronics. (10) Cwen, Robert P. Linear sweep circuits. December, 1946. (11) Reintjes, Francis J., Electronics. 19(12) :136. editor. Principles of radar. New York: McGraw-Rill, 1946. (12 Ridenour, Lolls J., editor. Radar system engineering. 1947 New York: ACCraw-Hill, 75 (1Z) (14) 3 Karl R. Vacuum tubes. New York: axi 05enbere, Zworykin, V. K., and G. A. Television. New York: A'eGraw-Hill, 1948 ;.lorton. Viiley, 1940.