US2396476A

US2396476A - Cathode-ray beam deflecting circuits

Info

Publication number: US2396476A
Application number: US445187A
Authority: US
Inventors: Otto H Schade
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1942-05-30
Filing date: 1942-05-30
Publication date: 1946-03-12
Anticipated expiration: 1963-03-12
Also published as: GB610081A

Description

March 12, 1946. o SCI-[ADE CATHODE RAY BEAM DEFLECTING CIRCUIT Filed May 30, 1942 I I V EP/NG ATTORNEY v1 I vvv ENTERING 0FLEcrnv6 Coma m pvsa F A MIN 4 Patented Mar. 12, 1946 2,396,476 CATHODE-BAY BEAM DEFLECTING CIRCUITS Delaware Application May 30, 1942, Serial No. 445,187

' 3 Claims. (cl. 175-335) This invention relates to an improvement in circuits for producing voltage variations for deflecting a cathode ray beam in a cathode ray tube.

More specifically the invention relates to the production oi voltage variations such that the deflection of the cathode ray beam will be at a uniform rate. In deflecting a cathode ray beam by electromagnetic deflecting coils, it is diflicult to maintain strict linearity of deflection in view of the inductance of the electromagnetic deflecting coils, and in view of certain circuit parameters associated with the coils. Inasmuch as the deflecting coils inherently have a certain amount of inductance, a certain amount of stored energy is present in the deflecting coils at the end of each deflection cycle. By using appropriate discharge tubes a portion of this energy may be used for producing a part of the voltage variations for deflecting the cathode ray beam during the initial portion of the subsequent deflection cycle. The deflecting coils also have certain inherent distributed capacities which, together with the inductance of the coils, produce tuned circuits having predetermined resonant frequencies. At the end of the deflection cycle, when the deflection voltage is removed from the deflecting coils, the energy contained in the deflecting coils will normally cause relatively high frequency oscillations to be set up in the coils by reason of the inherent distributed capacity associated with the coils. In actual operation of present day electromagnetic deflection circuits this stored energy is permitted to produce one-half cycle of the main free period of oscillation and at the end of the first half cycle of free oscillation an inverted tube (generally an inverted diode) is used to control or absorb the energy and to substantially completely suppress the remaining higher frequency oscillations. The energy contained in the deflection coils may be utilized for assisting in deflecting the cathode ray beam during the subsequent deflection cycle.

An example of the use of an inverted diode for suppressing the free oscillations is shown and described in Schade Patent No. 2,309,672 issued on February 2, 1943. This application also shows the use of a feed back arrangement for improving linearity of deflection. Another example of an electron discharge tube for controlling the free oscillations is shown and described in Schade application Serial No. 449,076, filed June 30, 1942. In this particular application a controlled electron discharge path is provided to assist in eliminating the free oscillations, and to provide subdill stantially linear deflection oi the cathode ray beam.

In spite of the circuit arrangements shown and described in the above referred to patent applications, certain relatively high frequency arasitic oscillations of short duration will persist and cause erratic operation and deflection of the cathode ray beam, particularly during the initial portion of the deflection cycle. The distributed capacity of the deflection coils and the inherently loose coupling between turns or sections of the deflection yoke windings causes the yoke circuit or the, deflection coils 'to have more than one natural frequency of oscillation. The lowest natural frequency is determined by the lumped values of capacity and inductance and this irequency determines the return time of the cathode ray beam. The return deflection oi the cathode ray beam occurs during the first half cycle of the lowest of the natural frequencies of the deflecting coils, and the next deflection cycle begins immediately following the first half cycle of oscillation of the lowest natural frequency.

The conventional inverted tube referred to above is connected across the entire deflection coil (or coils), or is connected across the primary winding of a transformer used for coupling the deflecting coil with the deflection tube. This inverted tube or its equivalent reflected into a transformer primary damps out oscillations at this lowest natural frequency during the deflection time. However, higher frequency parasitic oscillations are not damped and cause poor focus, and in most cases wavy scanning lines during the very initial part of the deflection cycle. This is because the coil sections involved are separated by the inductance of other coil sections and the loose coupling between the sections.

It is, therefore, one purpose of the present invention to provide a circuit arrangement for producing voltage variations for electromagnetically deflecting a cathode ray beam in which all high frequency parasitic oscillations are symmetrically suppressed to thereby eliminate defocusing and non-linear deflection of the cathode ray beam during the initial portion of the deflection cycle.

Another purpose of the present invention resides in the provision of means whereby the eflect of the lack of uniformity in the distributed capac ity across the electromagnetic deflecting coils may be eliminated to thereby cause balanced high frequency parasitic oscillations in each half of the winding of the yoke which would otherwise set up undesired induced currents in the other windings of the yoke.

'Still; another purpose of the present invention resides in the provision of a circuit arrangement whereby the distributed capacity across the deflecting coils may be equalized regardless of the fact that the two halves of the deflecting coil are loosely coupled and would normally have a nonlinear distribution of distributedcapacity.

A still further purpose of the present invention resides in the provision of means whereby the distributed capacity appearing in an electromagnetic deflection yoke may be made symmetrical for each half of the yoke to thereby cause the particular deflection winding of the yoke to have no coupling with another deflection winding normal to the particular winding.

Various other purposes and advantages of the present invention will become more apparent to those skilled in the art from the following detailed description, particularly when considered in connection with the drawing-wherein:

Figure 1 shows one form of the present invention.

Figure 2 shows a modified form of the present invention.

- Figure 3 shows still a further modification oi the present invention.

Figures 4 and 5 show curves representing volt minals it are provided .in order that drivingvoltage variations may be applied between the control electrode and the cathode of tube Ill. The anode of tube l 0 is connected to positive terminal 20 through a choke coil 22. The deflecting coils 2t and 25 (which form separate sections of the same deflecting coil) are connected in parallel with the inductance 22 and include in circuit therewith a portion of the resistor element of potentiometer 26. The resistance element of the potentiometer is connected in series with the resistance 28 and the series arrangement connected between the positive terminal and ground. The purpose of this connection will be explained later.

A certain inherent distributed capacity appears across the entire

deflecting coil yoke

24, 25 and is represented by the condenser 30. In view of the loose coupling between the halves of the deflecting coil an additional distributed capacity represented at 32 appears across section 25 of the deflecting coil. Actually, part of these distributed capacitances appear as a series of relatively small condensers between turns and between sections I of the winding.

The inductance of the

deflecting coil

24, 25, together with the distributed capacity 30, form a tuned circuit having a predetermined natural frequency of oscillation. This natural period of oscillation is low as compared to the sectional or parasitic oscillations, and determines the beam return time of the cathode ray beam. Furthermore, the amplitude of the lower frequency natural period oscillations determined by distributed capacity is greater than the amplitude of the parasitic oscillations. These lower frequency oscillations are damped by the inverted tube 34.

The inverted tube, as shown in Figure 1, includes a cathode and an anode, The cathode of the inverted tube 34 is connected directly to the anode of the deflection power tube In. The anode of the inverted tube is connected to the lower end ot-the deflecting coils by the parallel combination of an adjustable resistance 36 and a 'condenser 38. The purpose of this resistance and condenser combination is to produce the proper normal bias on the anode of the inverted tube in order that a, maximum amount of the energy order that the desired current wave form will.

be produced.

In the circuit arrangement as so far described it will be assumed that the deflection coils 24. 25 are for deflecting the cathode ray beam in a horizontal direction. The conventional deflection yoke also includes another deflection winding (not shown) for deflecting the cathode ray beam in a vertical direction. The axes of the horizontal and vertical windings are normal to each other and each produces an electromagnetic deflection fleld normal to the axis of the tube, that is, normal to the path of the beam. As stated above, the deflection coils 2d, 25 which constitute the assumed horizontal deflection winding have a certain inherent distributed capacity 30 thereacross which causes a main or principal oscillation to be present in the deflecting coils 2t, 25 at the end of each deflection cycle. The inverted tube 34 stops these oscillations after one half cycle of oscillation, and utilizes the stored energy in the deflecting coils for assisting in deflecting the cathode ray beam during the next succeeding deflection cycle. The additional capacity 32 which is associated with the portion 25 only causes parasitic oscillations to be present in that portion of the coil. These oscillations do not reach the inverted tube 34 and as a result are not suppressed by that tube. The oscillations are confined principallyto the portion 25 of the horizontal deflecting winding. Their effect. therefore, in so far as the horizontal deflection of the cathode ray beam is concerned, is to cause rapid acceleration and deceleration in the horizontal deflection rate of the cathode ray beam during the initial portion of each horizontal deflection cycle. Because of this relatively rapid increase and decrease in deflection rate vertical light and dark bars are caused to appear on the screen of a television receiving tube. These bars are naturally objectionable and should be eliminated.

The parasitic oscillations which are present in the portion 25 of the horizontal deflection winding also cause additional efiects which are objectionable by reason of the presence of the vertical deflecting windings. The high frequency parasitic oscillations will induce into the vertical deflecting winding a voltage which is effective to cause undesired vertical deflections of the cathode ray beam at a relatively rapid rate and at a somewhat low amplitude. These rapid deflections in a horizontal direction persist for a short time only, but result in producing a wavy pattern along one edge of the television picture. The relatively high frequency parasitic oscillations, therefore, produce not only vertical light and dark bars along one edge of the picture, but also produce wavy lines and distortion over that same portion of the picture.

There is still another effect that is the result of the parasitic oscillations, and that is the defocusing of the cathode ray beam. In the usual cathode ray tube the cathode ray beam is focused by electrostatic fields. However, a cathode ray beam may be focused or a properly focused condition may be disturbed by electromagnetic fields. In designing a deflection yoke it is desirable to so arrange the windings that the electromagnetic fields produced thereby pass through the neck of the cathode ray tube in a direction normal to the, axis of the tube. As long as the electromagnetic field is in this direction the fleld has only an eiTect on the cathode ray beam to cause a change in its direction or to cause deflection of the beam. If, however, there is a component of electromagnetic force parallel to the axis of the tube the focus of the cathode ray beam will be disturbed. The parasitic oscillations which are produced in the portion 25 of the horizontal deflection winding set up an electromagnetic field which is not wholly normal to'the axis of the tube. with the result thata certain component of the field lies parallel to the cathode ray beam. This component is frequently of sufiicient intensity to disturb the focused condition of the cathode ray beam so that the cathode ray beam may not be in proper focus during the time that the parasitic oscillations exist.

In order to eliminate these undesirable conditions there is also provided a second inverted tube 40 including a cathode and an anode, and this tube is associated only with the portion of the deflecting coil. The cathode of tube is connected to the junction of the

windings

24 and 25 by way of resistance 42, and the anode of the second inverted tube is connected to the lower end of the deflecting coils by the parallel combination of the resistance 44 and condenser 48. The resistance 44 and the condenser 46 function in a manner similar to the function of resistance 36 and condenser 38 associated with the principal inverted tube 34.

The potentiometer 26 is provided in order that some means may be available for centering the cathode ray beam. By moving the movable contact (to which the lower end of the deflecting coils are connected) along the resistance element of the potentiometer, it is possible to control the amount of direct current permitted to normally flow through the deflecting coils to thereby prop erly center the cathode ray beam. The circuit arrangement referred to is preferably applied to the horizontal deflection of a cathode ray beam in a television transmitting or receiving system, and in which case the centering potentiometer 26 would control the normal horizontal position of the cathode ray beam. The choke 22 may be of relatively high inductance and principally aflords only a direct current path, the inductance of the choke 22 being sufficient to cause substantially the entire alternating current component to be transmitted by the deflecting coils 24, 25.

In operation a voltage variation of substan-- tially sawtooth wave form is applied to the input terminals Hi to control the deflection power tube l0. Naturally, any proper and suitable wave form may be applied to the terminals Hi to produce the desired linear deflection. The resultant voltage variations appearing at the anode of tube Ill are then impressed on the deflecting coils 24, 25 which cause the cathode ray beam to be deflected at a predetermined rate established by the frequency of the voltage variations applied to the input terminals l8. As stated above, the principal distributed capacity 30, together with the inductance of the deflecting coils, causes the coils to have a certain natural period of oscillation which determines the beam return time. The beam return time then corresponds to one half cycle of the natural period of oscillation. The principal inverted tube 86 suppresses the natural frequency of oscillation after the first half cycle, and also utilizes the energy contained in the de fleeting coils for the initial deflection of the cathode ray beam for the next succeeding cycle. The operation of the beam deflection tube in conjunction with the principal inverted tube 3 3 is explained in Patent No. 2,309,672 referred to above. The current supplied by the deflection power tube '10, and the current passed by the principal inverted tube 34, combine to produce the effective deflection current for the deflecting coils.

Due to the loose coupling between the halves of the deflecting coils distributed capacity 32 also exists which. togetherwith the inductance of the associated portion 25 of the deflecting coils 2d, 25, causes higher frequency parasitic oscillations to appear. These parasitic oscillations occur because condenser 32 cannot be maintained in a discharged condition by inverted tube 34 because of insuflicient coupling between section halves 2d and 25. The parasitic oscillations are; therefore, suppressed by means of the second or auxiliary inverted tube ii]. For best operation, it is preferable that the principal inverted tube be of high perveance in order to obtain as nearly linear horizontal deflection or distribution as is possible, unless additional means are provided for improving the linearity. Such additional means, for example, might be in the form of feed back voltages for effecting the deflection power tube 4|], or the inverted tube 3 5 may be in the form of a controlled diode or a discharge path having a control electrode. The auxiliary inverted tube 56 for suppressing the parasitic oscillations and for thereby preventing defocusing and improper deflection of the cathode ray beam, should have connected in series therewith a resistance as shown at $2. Omission of the resistance 52 may impair the focusing of the cathode ray beam for a very short time interval at thestart of each deflection cycle. Its omission may cause a strong inverted tube current to flow through the portion 25 of the deflecting coils, which causes the deflecting field set up by the coils to become unbalanced in intensity and direction with consequent partial loss of focus until the damping cur rent has decayed to negligible values. It may be possible, however, that the impedance of the inverted tube 40 may be such as to not necessitate the use of resistance 42.

Inasmuch as no transformer coupling is shown in Figure 1, the cathodes of

tubes

34 and 40 operate at a relatively high potential and are subject to a relatively high potential swing." In view of this it is desirable to energize the heaters for

tubes

34 and 40 from a special transformer having relatively low distributed capacity to ground, in order that the potential difference between the cathode and the heaters may be reduced. If desirable, however, transformer coupling may be employed between the power tube in and the deflecting coils, in which case the cathodes of tubes 34 and ill may be operated at a lower potential inasmuch as they would then be isolated from the anode of the power deflection tube ill in'so far as the direct current component is concerned. When a coupling transformer is used the leakage reaction also produces parasitic oscillations which may be separately or simultaneously elirn mated by any of arrangements shown and alescribed herein.

By means of the circuit arrangement described above and shown in Figure 1, it is possible to electromagnetically deflect a cathode ray beam,

and to eliminate the defocusing of the cathode ray beam and the erratic deflection thereof during the very initial portion of the deflection cycle which would normally be caused by the relatively high frequency parasitic oscillations set up in the electromagnetic deflecting coils. The auxiliary inverted tube to absorbs these oscillations so that their effect on the other deflecting coils of the yoke and consequently on the cathode ray beam is eliminated and, furthermore, the presence of the auxiliary inverted tube ill does not interfere with the operation of the principal in.. verted tube 35.

A modification of the present invention is shown in Figure 2, wherein elements corresponding to the elements shown in Figure 1 are given corresponding reference numerals. This circuit differs from the circuit shown in Figure 1 in that the auxiliary inverted tube E8 is not used, but in lieu thereof a different means is provided for compensating for the effects or" capacity 32 which unbalances the distributed capacities across the deflecting coils fi l, 25. In this particular figure a choke coil or auto-transformer so is provided, and is connected across or in parallel with the deflecting coils. Each end of the choke coil c is connected to a corresponding end of the defleeting coils, and one or more intermediate connections are also made between corresponding parts of the deflecting coils and the choke. The various sections of the winding of the choke or auto-transformer to have very close coupling, with the result that all sections of the yoke winding are effectively inductivel coupled so as to prevent distributed inductance or capacity effects. The reactance ratios between the sections of the auto-transformer so should be maintained equal to the reactance ratios between corresponding sections of the deflection yoke or the deflecting coils 2Q, 25.

In the circuit shown in Figure 2 only one intermediate connection is indicated, and this connection is between the center of the deflecting coils and the center of the auto-transformer 5b. In order to improve the results additional corresponding connections may also be made. The number of turns included on the choke or autotransforrner to is not particularly material, but it is desirable that the reactance ratios between corresponding parts of the deflecting coils and 55 the auto-transformer be maintained uniform.

Through the use of a circuit arrangement capacitance effects in the deflecting coils, and 60 when these effects are eliminated defocusing of the cathode ray beam during the initial part of the deflection is eliminated, and the usual undesired high frequency oscillations induced in the other set of deflecting coils are eliminated.

A still further modification of the present invention is shown irrFigure 3 where, as in Figure 2, corresponding elements similar to those shown in Figure 1, are given corresponding reference numerals. In this circuit a still different means is provided for suppressing the ver high frequency parasitic oscillations. An adjustable condenser Bll is connected across the portion 2 30f the deflecting coils, and this condenser has a value such that the lumped value of distributed capacity across the portion 24 of the deflecting coils will be exactly equal to the distributed capacity of the portion 25 of the deflecting coils. By adjusting condenser so that it will have the proper value the deflecting coils may be causedwhich may be connected across the portion 25 of the deflecting coil in series with a coupling condenser 62. The oscilloscope will then be con- ,nected to terminals 64.

In the absence of the condenser 60, the voltage trace on the screen of the cathode ray tube might be such as represented by the curve 66 shown in Figure 4. It will be noticed that during the interval immediately following the beam return time a relatively high frequency oscillation is produced which is the parasitic oscillation caused by non-linear distributed capacity efiects. By introducing condenser 60 this condenser may then be adjusted until the parasitic oscillations disappear on the screen of the cathode ray oscilloscope and the trace will then be similar to the curve shown at B8 in Figure 5. When condenser 60 is properly adjusted the effects of the unbalanced distributed capacity condition may be eliminated, with the result that the deflecting coils have only a single efl'ective natural period of oscillation, and no high frequency parasitic oscillations will be induced in other windings. Naturally, the capacity of the condenser 60 is relatively small. In adjusting the condenser 60 it is highly desirable that the oscilloscope be coupled to the portion 25 of the deflecting coils by means of a very small capacity 62 in order to prevent capacity loading due to the cathode ray oscilloscope, since any capacity at the input of the oscilloscope would be re- 50 flected as an additional capacity in parallel with the distributed capacity 32. By using a very small coupling condenser 62, therefore, the input capacitance of the cathode ray oscilloscope in so far as its effect on the portion 25 of the deflecting coils is concerned is substantially eliminated. The method for eliminating the high frequency parasitic oscillations as shown in Figure 3 is relatively inexpensive, and is efiective for producing the desired results, although individual adjustments of the condenser 60 in each deflection circuit should preferably be made by an oscilloscope in order to obtain the most complete elimination of the parasitic oscillations.

For the purpose of illustration and explanation of the present invention, relatively simple deflection circuits have been shown. However. as stated above, in connection with Figure 1 transformer couplings may be used between the power deflection tube and the deflecting coils in the circuits shown in both Figures 2 and 3. Furthermore, various wave forms may be applied to the input terminals I8 in order to produce the desired linear deflection of the cathode ray beam.

Various alterations and modifications may be made in the present invention without departing from the spirit and scope thereof, and it is desired that any and all such modifications be considered within the purview of the present invention, except as limited by the hereinafter appended claims.

What I claim is:

1. A deflecting circuit for electromagnetically deflecting a cathode ray beam comprising a source of deflection voltages of a predetermined wave form, a pair of cathode ray beam deflecting coils, means for applying the deflecting voltage variations to the electromagnetic deflecting coils in series, said deflecting coils having a nonlinear distribution of distributed capacity thereacross, and means for balancing the distributed capacity comprising a tightly coupled winding, with means for connecting corresponding turnsratio portions of the tightly coupled winding to corresponding points of the electromagnetic deflecting coils.

2. A deflecting circuit for electromagnetically deflecting a cathode ray beam comprising a source of deflection voltages of a predetermined wave form, a pair of cathode ray beam deflecting coils, means for applying the deflecting voltage variations to the electromagnetic deflecting coils in series, said deflecting coils, because of their loose coupling, having a non-linear distribution of distributed capacity thereacross, means for balancing the distributed capacity comprising a tightly coupled winding, and means for connecting corresponding turns-ratio points of the tightly coupled winding to corresponding turns-ratio points of the electromagnetic deflecting coils.

3. A deflecting circuit for electromagnetically deflecting a cathode ray beam comprising a source of deflection voltages of a predetermined wave form, a pair of series connected cathode ray beam deflecting coils, means for applying the deflecting voltage variations to the electromagnetic deflecting coils in series, said deflecting coils, because of their loose mutual coupling, having a non-linear distribution of distributed capacity thereacross, a tightly coupled electromagnetic winding, and means for inter-connecting corresponding turns-ratio points on the tightly coupled winding and the electromagnetic deflecting coils to thereby efiect a substantially linear distribution of the distributed capacity across the deflecting coils.

O'I'IO H. SCHADE.