US2510027A - Cathode-ray tube deflection system - Google Patents

Description

May 30, 1959 c. E. ToRscH 2,510,027

cA'rHoDE-RAY TUBE DFLECTION SYSTEM Filed May 2a, 1947 n HomzoNTAL 7 PAnAaoLA FoRMlNG sPHERlcAL sunFAcE 'IDFA or RADlus=BAM THRow g FROM APPARENT CENTER 51'24 or MAGNETIC DEFLECTION SAWTOOTH FORMING "FIGAI /o A 5 B lo G lsnzogzspaogssww; 4sJsoKss so es mo'r) o /TRACE ns1-RACE SECTOR INVENTOR GW m ENSDCNJJ CHARLES ETORSCH ATTORN EY 55 mlcroseconds BY Patented May 30, 1950 UNITED STATES @ENCE CATHODE-RAY TUBE DEFLECTION SYSTEM Charles E. Torsch, Towson, Md., assignor to Radio Corporation of America, a corporation of Dela- Application May 28, 1947, Serial No. 750,934

Y 11 Claims. 1

This invention relates to television, and more particularly to improved cathode ray beam deflection signal generators.

Image reproduction by electronic viewing devices has been generally accepted as the most desirable and has met with outstanding success. Consequently, the cathode ray viewing tube is almost universally used today in television receivers.

The reproduction of images is accomplished by analyzing the scene into its picture elements by a predetermined sequence of scanning and transmitting energy representative of each picture element for reconstruction at the receiving point by the same scanning sequence.

In order to produce scanning, the beam must be moved across the screen independently in two directions and at right angles toeach other. It is preferable to deflect the beam through an angle at a position close to the end of the gun. This deection of the electron beam can be accomplished by either an electrostatic field produced by a pair of deecting plates or a magnetic field set up at right anglesto vthe direction oi deflection and produced by an inductance coil. Each of these methods has been successfulV in practical application as a means for producingrelectron beam deflection. V

Magnetic deflection for both vertical and horizontal displacement of the electron beam has become almost universal in application for employment on large cathode ray tubes.

Magnetic deflection is accomplished by estab- If the tube is provided with suitable shaped coils and cores, a perfect scanning pattern can be obtained only if they are supplied with current 01 voltage having a very specific time variation. The wave form of current through the inductance coils producing the vertical deflection of the electron beam must be sawtoothed andrhave a recurring frequency equal to the neld frequency of the pattern. The current through the horizontal coils must also be sawtoothed, but its repetition frequency is much higher and corresponds to the desired line frequency. The current in each case builds up substantially linearly with time for most of the cycle, and then returns rapidly to its initial value.

The deflection signal generator which supplies the power to these deflecting elements generally consists of an oscillator, a sawtooth generator and a deflection amplifier. The oscillator is normally set off by a series of synchronizing pulses which serve to coordinate the timing of the scanning rasters at the transmitter and receiver.

Although the early forms of cathode ray deflection systems generally employed a simple gaseous discharge sawtooth Wave generator, the exact requirements of television in its improved forms have placed stringent requirements for both speed and velocity on the deflection system to make it necessary to employ thermionic devices together with wave shaping circuits to furnish the necessary deiiection energy.

There are three distinct portions of the line deecting cycle, which may be outlined as (a) the major portion of the go sweep, during which time the output tube of the deflection generator supplies current to the deflection yoke, (b) the interval of free oscillation (retrace time) during which time the output tube plate current is cut off and the wave form is determined by the circuit constants of inductance, capacity, and resistance, and (c) the portion following the return sweep, during which the free oscillation must be damped and energy which has been stored in the yoke inductance is permitted to be discharged, preferably in a controlled manner.

The driver tube portion of the go cycle was of major interest in early designs of cathode ray tube systems. Particular emphasis was placed upon obtaining a yoke current, which was a linear function of time.

However, the cathode ray tubes in general use today for televisonV systems and the like are of the fiat-faced wide-angle kinescope types.VV It therefore becomes necessary to modify the yoke current to allow for the deflection geometry, ofY such tubes. For a properly designed deflection yoke, the deflection angle is approximately a linear function of the yoke current. Therefore, for small deflection angles, particularly when the tube face radius approximates the deflection radius, the spot, displacement is a linear function of the yoke current. However, when the tube face radius and the deflection radius differ radically, and particularly for Wide deflection angles, the yoke current must be distinctly non-linear in order to obtain a linear spot displacement.

According to this invention, there is provided an improved deflection system wherein the deflection current is conveniently adjustable to provide the necessary non-linearity in change of cur- 3 rent to obtain the necessary linear scanning spot displacement.

The interval of free oscillation or retrace time during which the output tube plate currentV is cut off has heretofore been taken care of by utilizing a high degree of damping. Part of this damping was in some cases inadvertently slighted by the output tube which, although supposedly at cut-ofi insofar as grid signal Was concerned, frequently was not cut oil. Additional damping was introduced by losses in the coupling transformer and yoke, and in some cases sufficient additional resistance Was added in shunt with the yoke in order to make the circuit aperiodic. this method of damping had the disadvantage of causing losses during the go sweep, as Well as during the return sweep, and hence the deection efficiency of the system was low.

It has been proposed to improve the circuit characteristics in efciency by using a series resistancecapacity combination for damping. This tended to decouple the damping resistor during the slower go time and yet provide close coupling to the damping load during the rapid return time. Generally satisfactory results were obtained for the relatively slow 441-line standards, however, with the introduction of the 525- line standard, the return time of an aperiodic system is rather lo-ng, so that new damping procedures were proposed. Y

Circuits employing triode and diode damping in the secondary of the deection transformer have met with considerable success, however, a perfect coupling in a deection transformer is difcult to obtain and would be costly to produce, due primarily to the high voltage insulation which is required.Y A certain amount of decoupling with the damping element across the secondary of the de-Y flection transformer results in a carry-over voltage in the primary of the deection transformer which causes the output tube to produce spurious oscillation, such as the Well-known Barkhausen oscillations.

The cause of the Barkhausen eiects can be attributed to the negative excursion of the anode potential of the output tube due to the'transients obtained upon a rapid change in current through an inductive element such as the deection transformer.

According to this invention, there is provided a' other and incidental objects of the invention' will be apparent to those skilled in the art from a reading of the following specification vand an inspection of the accompanying drawing i which:

Obviously, Y

Y circuit comprising the capacitive element 3 and Figure 1 illustrates schematically one formfof Y a resistance element 5.

The generation of sawtooth Wave forms in accordance with a predetermined synchronizing pulse is wellnown in the art and needs no detailed explanation here, except perhaps to refer to Chapter IV, beginning on page 118 of Principles of Television Engineering by Donald G. Fink, wherein the formation, deflection and synchronization of scanning beams is very Well described in detail. Deilecting circuits are also well known in the patented art, such as, for example, the U. S; patent to W. A. Tolson et al. No. 2,101,520, dated ,December 7, 1937; Vand the U. S. patents to W. A. Toison Nos. 2,108,152, dated February 15,

1938, and 2,280,733, dated AprilV 21, 1942.

The horizontal or line deilecting yoke 1 of the cathode ray tube sis coupled to the output of tube I by transformer I I.

Image producing tubes of the kinescope type are also well known -to the art, and a description may be found'in an article byDr. V. Zworykin" entitled Description of an experimental teievision system and kinescope, in the Proceedings of the Institute of Radio Engineersfvol. 21, No. 12, for December 1933, or the information begin ning on page 329 of the book Television by V. K.

Zworykin and G. A. Morton, published in 1940-by vention is, of course, applicable to` any scanning.

device.

A typical deflection yoke is shown and'described in detail in the U. S. patent to W. A. Tolson, No. 2,167,379, dated July 25, 1939.

It is well established that the' magnetic iield set up by the deection yoke 1 is proportional to the product of the number of turns in the coil and the current flowing in' them. Therefore, the amount of current required in the coils dependsv directly on the number of turns in the coils. It

-is important, particularly in circuits involving `rapid deflection, that not too large a value of inductance be employed. VIt is therefore necessary to supply a relatively large value of current to the deiiecting yoke '1. It follows that lhigh currents will necessitate the consideration rin the deilectionv system of extremely high peaked voltages which are `generated in the inductance coils when the-V current through them changes suddenly.

The scanning yoke has associated with. it a distributedcanacitance which, acting with the in-v ductance of the yoke, forms a resonant circuit. The sudden shck'caused by the sawtooth wave form of the driving'current tends tov set the reso-` nant circuit into oscillation. The undesired oscillations in the voltage wave form producev corresponding irregularities in the current wave K form which are reproduced in the scanning motion.

It has been Athe general practice to employ al rectifier, such as the diode damping tube i3 or any other form of runit suchv .as one ofthe semiconductor crystals, to reduce the undesired oscillation. The' diode I3 is connected across the circuit involving the secondary of the transformer II and the deilecting yoke 1 and is connected in series with a capacity I5 and resistance element I1.. Resistance element I1 may be made variable as shown.

The operation of the damping tube I3 is well knownin the art, and additional information thereon may be obtained by reference to a copending U. S. application of Otto H. Schade, Serial No. 593,161, filed May 11, 1945, wherein there is shown and described a damping tube, together with power feed-back for utilization of the rectified currents in conjunction with the output tube anode potential.

If the transformer II were so constructed that it would have perfect coupling, the effect of the diode damper, tube I3 would be projected back to the primary of transformer II in such a manner that there would be no negative excursion of voltage on the plate of tube I due to the extremely high peak voltages employed. However, in the practical consideration and design of transformer II, it is necessary to arrive at a compromise on coupling such that the Veffect of the inductance on the transformer is to cause a negative excursion of Voltage on the tube I.

The negative excursion referred to may best be understood by reference to Figure 3, which for the most part is believed to be self-explanatory, except perhaps to state that the wave shape shows the plate voltage of a typical power output tube such as a 6BG6--G, which oscillates to 1900 volts at the beginning of the forward trace for go sweep. This negative excursion will cause the output tube to produce Barkhausen oscillations, which may readily affect the radio frequency section of the television receiver at high frequencies. The oscillations will be carried through the circuit to modulate the kinescope grid and will appear as vertical bars at the left edge of the image. The effect of the Barkhausen oscillations will not be observed whenthe scanning circuit alone is tested. The `effect is generally found .only when the complete receiver is in operation and tuned to the higher frequency channels and when high intermediate frequency gain is employed.

Returning now to Figure l, there is included a tube I9 having its cathode 2| connected to an auxiliary control electrode 23 of tube I. The anode of tube I9 is connected to a source of positive potential. The control electrode 26 of tube i9 is connected to the anode of diode I3 through a wave forming circuit comprising, for example, a pair of potentiometers 25 and 21. Potentiometer 25 is adapted to supply a variable current to a parabolic wave forming network including resistors 29 and condensers 3l. Potentiometer 21 is adapted to supply a sawtooth wave forming circuit including resistors 33 and condenser 35. 'Ihe output signal from the parabolic wave forming circuit and the sawtooth wave forming circuit is fed to the control electrode 24 through capacity 31. A D.C. blocking condenser 38 is included in the circuit between diode I3 and potentiometers 25 and 21.

The function of tube I9 is to drop the potential of auxiliary control electrode 23`of tube I 'during the negative excursion of plate voltage of tube I. A suitable delay of the negative surge on the secondary of element II through adjustment of 33 and 20, and amplitude adjustment with 25 and 21 during this interval will cause control electrode 24 togo negative. This in effect opens the circuit at tube I9 so that the potential of control electrode 23 of tube I will go immediately towards or below its associated cathode potential. By dropping the potential of control electrode 23 to a sufficiently low value, there will be prevented in tube I any tendency toward Barkhausen oscillations.

It may also be added that the useless negative excursion referred to unnecessarily abuses the output tube I insofar as interelectrode insulation stresses are concerned and furthermore, the power obtainable from the output tube I is normally limited by the screen dissipation. The employment of this invention, therefore, reduces the abuses of the output tube, but permits a greater eiciency of operation. It is also possible to employ lower plate voltages in the output tube by obtaining greater eiliciency in its operation.

In addition to preventing Barkhausen oscillations, the tube I9 may be utilized to control the wave shape of the deflection voltages through the parabola forming and sawtooth forming networks including elements 20, 3|, 33 and 35. By properly selecting the values of these elements, many desired wave shapes may be obtained. Other wave shapes may be obtained by returning 25 and 21 to an extension of the secondari7 of II below the tap herein connected to +B, instead of grounding 25 and 21. This extension would generate positive voltage surges during retrace and permit the generation of opposite polarities of parabola and sawtooth waves.

The adjustable wave shapes of the deflection current may be used to advantage, for exampie, in providing the non-linear yoke current required to obtain a linear spot displacement in the modern kinescope tubes having a substantially flat tube face. This will be more readily understood after a brief reference to Figures 2 and 4.

In Figure 2, there is shown a graphic determination of required yoke current wave shape on the basis of face-plate curvature. The curva ture as illustrated in Figure 2 is typical of commercially produced kinescopes and is taken by way of example from the '1DP4 and 10BP4 types.

In Figure 4 there is shown a yoke current curve. The dashed line is straight and would be desirable for a tube having a spherical screen surface whose radius is equal to the beam throw from the apparent center of deflection. When, however, the surface is flattened out, as illustrated in Figure 2, there is less deflection rate required at the beginning and end of the course, hence it is necessary to provide a yoke current as shown in the solid line of the curve shown in Figure 4. Such a yoke current curve, as well as other forms, may be provided by the use of this invention.

Linearity measurements usually refer to meas urements made at the face of the kinescope. The unit length commonly used as the basis of production linearity testing is 10% of the useful scan. If the number of bars per unit length is within i10% of the number scanned at the center of the screen, then the linearity is said to be 10%. Strictly speaking, the spot velocity is what is actually being measured if the intervals of measurement are sufficiently small. Methods forfmeasurement of the'angular velocity of the deection beam may be coordinated with the velocity data of Figure 2 to provide an approximate method of measurement. In this event, the free-running speed of the blocking oscillator should be adjusted to .the nominal horizontal line frequency.

Having thus described .the invention, what .is claimed is:

1. In a beam deflection system, ibeam deflection means, a power tube .having fan output cir cuit and an `auxiliary control electrode, ,a coupling circuit between said output circuit and .said beam deflection means, va control voltage circuit connection between said beam deflection :means and said auxiliary control electrode, said control voltage circuit connection Y.including parabola Wave forming network.

2. In a television deflection system, a-beam ideflection coil, a power tube having an voutput circuit and an auxiliary control electrode, a coupling .circuit between said output circuit andsaid beam deilection coil, and means for reducing spurious oscillations in said beam deection coil comprising a control voltage circuit connection between said beam deilection coil Vand v`said auxiliary control electrode, saidcontrol voltage circuit connection including a tube having a cathode connected to said auxiliary control Velectrode and a control electrode connected to said beam deflection coil through a wave form changing network.

3. In a television deilection system, beam deflection means, a power tube having -an output circuit and an auxiliary vcontrolelectrode, -a coupling circuit between said output circuit and said beam deflection means, and a control voltage ci-rcuit connection containing a cathode follower type tube connected between said beam deflection means and said auxiliary control electrode to control the potential of said auxiliary control electrode during predetermined time intervals.

4. In a television deflection system, beam deflection means, a power tube having an output circuit and an auxiliary control electrode, a coupling circuit between said output circuit and .said beam deflection means, and 4-a cathode follower connected amplilier tube connected between Vsaid. beam deflection means and said auxiliar-y control electrode to controlthe potential of said auxiliary control electrode during voltage surges .across said beam deflection means.

5. In a television deflection system, beam deilection means, a power tube having an output circuit and an auxiliary control electrode, va coupling circuit between said output circuit and said beam deflection means, and a wave shaping vcircuit connection between .said beam .deflection means and said auxiliary .control electrode, said wave shaping circuit comprising a cathode -coupled amplifier tube connected serially vwith `a parallel connected sawtooth wave and parabola wave forming electrical networks.

y6. In a television deflection system, beam deflection means, a power tube having an input circuit adapted to receive a sawtooth wave, an output circuit and a current control electrode, a coupling circuit between said output circuit land said beam deflection means, and a parabola wave forming network connected between said beam deflection means and said current control electrode.

7. In a television deflection system, beam deflection means, a power tubehavingan input circuit adapted to receive a sawtooth wave, an output circuit and a current control electrode, a Vcou-- pllng circuitrbetweensaid loutput circuit andsaid beam 'deflection means, yand alsawtooth wave and parabola. wave. forming network connected between .said beam deflection means fand-,said current control electrode.

8. In ya television deflection system, beam de# flection means, a power tube having an input circuit adaptedto receive Ia sawtooth wave, van output circuit and a current control electrode, a `coupling. circuit between said outputcircuitand said beam deflection means, an auxiliary control tube having a cathode, a control .electrode and an anode, said cathode Vconnected to said current control electrode, .said .anode connected to .a source of positive potential, and .a control voltage connection between said beam deflection means and said control electrode.

9. Ina television deflection system, beam deflection means, a power tube having an input eircuit adapted to receive a vsawtooth wave, an output circuit anda current controlelectrode, acoupling circuit between ,said output circuit and said beam deflection means, an vauxiliary control tube have a cathode, a control electrode and an anode, said cathode connected 'to said .current control electrode, said anode connected to a source of positive potential, and a circuit connection between said beam deflection means .and .said control `electrode including serially 'connected resistances and parallel connected capacity,

110. In a television deflection system, beam deilection means, a-'powertube .having an input circuit adapted Yto receive a sawtooth wave, an :output circuit and a current control electrode, a coupling circuit `betweensaid output circuit and said beam deflectionv means, an `auxiliary control tube having a cathode, a .control electrode and an anode, said cathode connected to said current control electrode, said anode connected to a source of positive potential, and a circuit connection between said beam deflection means and said control electrode having serially lconnected resistance and parallel .connected capacity.

1'1. In a television deflection system, beam deflection means, -a power tube'having an input circuit adapted'to receive asawtooth wave, Ian output circuit and a'current control electrode, .a coupling Vcircuit between said output circuit and said beam deflection means, an auxiliary control tube having a cathode, a control electrode Vand an anode, said cathode connected to `said current control electrode, said anode connected to a source of positivepotential, and a sawtooth wave and parabola wave forming networks connected between said beam deflection means and `said control electrode.

' CHARLES'E. TORSCIL REFERENCES CITED The following references Vare of record in the le of this patent?" l UNITED z's'rA'rfEs PATENTS

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