US2460540A

US2460540A - Control circuit

Info

Publication number: US2460540A
Application number: US650372A
Authority: US
Inventors: Shaw Hubert Rice
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1946-02-26
Filing date: 1946-02-26
Publication date: 1949-02-01
Anticipated expiration: 1966-02-01
Also published as: GB643150A

Description

Feb. 1, 1949. H. R. SHAW CONTROL CIRCUIT Filed Fb. 26, 1946 HORIZONML HOR/Z'OA/HL o1 DAMPER DEFLECT/ON powm TUBE] 00/15 14 was w. E l

f 50 34 32 cwPu/va Sc/3E5 5+ TRANSFORMER a [V6 IL H El 2 47 h f IV [V 2 4 d JL v INVENTOR HUBERT R. SHAW ATTORNEY Patented Feb. 1, 1949 CONTRQL CIR'GUIT Hubert Rice Shaw, Drexel Hill, Pa, assignor to Radio Corporation of America, a corporation of Delaware Application February 26, 1946, Serial No. 650,372

9 Claims.

The present invention relates to television systems employing electromagnetic deflection of a cathode ray beam, and more particularly relates to means for overcoming one form of scanning non-linearity which results from the inductive nature of the beam deflecting circuit.

In television systems in which the horizontal deflection circuit consists of one or more power output tubes connected to a pair of beam deflecting coils through a coupling transformer, it is customary to employ some form of damping means connected across either the primary or secondary winding of the coupling transformer. Such damping means may, for example, comprise a series combination of resistance and capacitance, but more frequently consists of an electron discharge tube, the dual purpose of which is to substantially eliminate the high-frequency oscillations which normally result from the inductive nature of the deflecting circuit, and also to assist in the deflection of the cathode ray beam during the trace portion of each scanning cycle. The latter function of the damper tube reduces the amount of input power which must be delivered to the secondary circuit of the coupling transformer.

One form of damper tube which is conventionally used to obtain the above results is a diode. However, it has been found that when a diode is so utilized, it is often difficult to obtain linearity of deflection of the cathode ray beam, and, as a result, distortion of the reproduced image may occur.

It has been further discovered that a linear summation characteristic of power tube current and damper tube current is obtainable by moditying the conduction characteristics, or internal resistance, of the damper tube. In order to accomplish this, the diode is replaced by a triode, or multi-grid electron discharge tube, so that voltage variations of a particular waveform may be applied to the control electrode of the damper tube to alter its conduction characteristics. When the internal resistance of the damper tube is thus varied during each deflection cycle, approximate linearity of deflection may be obtained.

As above stated, a control-led damper tube, or, in other words, one having an impedance which may :be altered by application of a voltage of particular waveform to the control electrode thereof, is effective in substantially eliminating the high-frequency oscillations which occur following the retrace, or snapback, portion of each deflection cycle. However, a secondary action takes place in such circuits which is detrimental to the linearity of beam deflection during the initial part of the trace period. This secondary action consists of an oscillation, or ringing, of the coupling transformer in combination with various shunt capacitances, especially those associated with its primary winding, and occurs when its secondary winding is shunted by the relatively low impedance of the damper tube immediately toll-owing retrace. The frequency of this oscillation, or ringing, depends on the leakage in:- ductance of the transformer and the value of the shunt capacitances.

An oscillatory transformer action of the above nature causes the voltage across the deflection coils to vary correspondingly, so that the Waveform of the current through the beam deflection coils is not linear during the entire trace period. As a consequence, the image raster traversed by the cathode ray scanning beam contains alternate light and dark vertical bands, or bars, near the edge corresponding to the beginning of trace, with considerable distortion of the reproduced image in this region.

According to a feature of the present invention, means are provided for eliminating the nonlinearity of scan which is the cause of these vertical bars within the image raster, as well as the image distortion associated therewith. In one embodiment, such means include a separate, or auxiliary, coil wound on the horizontal output transformer. An oscillatory voltage is developed across this coil which is substantially in phase with the voltage induced into the secondary winding of the transformer, the relative amplitudes of these voltages being determined by the turns ratio. The oscillatory voltage present across the output terminal of the auxiliary coil, however, is relatively much greater than the oscillatory voltage present across the ends of the secondary Winding. The latter is not the same as the voltage induced into the secondary winding due to the action of the damper tube.

The output .of the auxiliary coil is then applied to the control electrode of the damper tube to vary its conduction characteristics to a greater degree than could be obtained by the application of the secondary voltage alone, and this variation may be such as to efiect substantially complete cancellation of the transformer oscillation, or ringing, insofar as its efiect on the linearity of current flow through the deflection coils is concerned.

One object of the present invention, therefore,

is to provide, in a cathode ray beam deflection circuit having means for coupling a power output tube to a pair of beam deflection coils through a transformer, means whereby the inductive nature of the coupling circuit is prevented from affecting the linearity of deflection of the cathode ray beam.

Another object of the present invention is to provide, in a circuit for electromagnetically deflecting a cathode ray beam, means whereby the effect on scanning linearity of high-frequency oscillations in the circuit may be substantially completely overcome.

A further object of the invention is to provide, in a cathode ray beam deflecting circuit in which a power output tube is coupled to a pair of beam deflection coils through a transformer, and in which a grid-controlled damper tube is connected across one winding of the transformer, means associated with the transformer for developing a voltage variation, and means for applying the voltage variation so developed to the grid of the damper tube.

Other objects and advantages will be apparent from the following description of a preferred form of the invention and from the drawing, in which;

Fig. 1 is a circuit diagram of a preferred embodiment of the present invention; and

Fig. 2 is a set of waveforms useful in explaining the operation of the circuit of Fig. 1.

Referring now to Fig. 1, there is shown a horizontal (also frequently termed line) deflection circuit including a power output tube It. Power tube H) is adapted to supply, when voltage variations of substantially sawtooth waveform such as indicated by the reference character iii are applied to the control electrode thereof, cyclically varying current, a portion of each cycle of which varies in a substantially linear manner with respect to time, to a pair of horizontal deflection coils Hi through a coupling transformer IS. The anode of power tube Ill is connected to a source of positive potential through the primary winding l3 of transformer l6, so that this winding forms part of the anode-cathode circuit of tube ill.

Coupling transformer I6 is provided with a secondary winding 20, usually of the step-down variety, the ends of this winding 26 being respectively joined to the ends of the series-connected horizontal deflection coils l4, as shown in Fig. 1.

Across the secondary winding 20 of transformer it there is connected the series combination of a damper tube 22 and a parallel resistance-condenser network comprising resistor 24 and condenser 26. Damper tube 22 is provided with a control electrode 28 which acts to vary the current flow through tube 22 upon a variation in the potential applied thereto.

The output of power tube it], as a result of the voltage variation [2 applied to the control electrode thereof, is a current which increases in value during the scanning portion of each deflection cycle. The voltage variation l2, however, acts to cut off plate current in tube it at the beginning of retrace time. However, the

current in the deflecting coils It does not disappear instantaneously because of the inherent distributed capacity of the circuit. This distributed capacity is, at the instant of the beginning of retrace, charged to a relatively low voltage.

The inductance of the deflection circuit, together with its distributed capacity, forms a tuned circuit in which continuing high-frequency oscillations of considerable amplitude would be produced in the absence of the damper tube 22. The reason for this action is well known in the art, and hence will merely be summarized hereinafter.

As previously stated, a current of increasing amplitude is caused to flow through the primary winding 18 of transformer it during scansion as a result of the reception of the voltage variation !2 on the control electrode of power tube [0. This flow of current through the primary winding $8 of transformer 15 produces a flow of current in the horizontal deflection coils M, as a result of the inductive coupling between windings l8 and 29, which is substantially in phase with the current flow through winding Hi. When the current through the deflection coils l4 and winding it reaches a maximum, the energy stored in the magnetic fields surrounding the deflection coils l4 and transformer i5 is also at a maximum. At this instant, the voltage variation 22 drops to a negative value, and cuts off current through power tube It. The result of the sudden termination of current flow through the primary winding i8 is to cause the magnetic fields surrounding the deflection coils l4 and transformer 56 to collapse. This action initiates a series of high-frequency oscillations in the tuned circuit consisting of the deflection coils M,

the transformer windings l8 and 26, and the distributed and stray capacitances of the circuit. These elements oscillate at their natural or free frequency.

The current through the deflection coils I4 decreases rapidly during the first quarter-cycle of such oscillation, passes through zero, and increases to a maximum in the reverse direction at the end of the second quarter-cycle of oscillation. At the end of the first half-cycle of oscillation, the voltage applied to the anode of damper tube 22 is such as to cause it to conduct, and the low impedance of the damper tube, which is in shunt with the deflection coils M, results in a damping out of subsequent oscillations.

During the first half-cycle of oscillation the energy in the deflection circuit flows out of the magnetic fields into the circuit capacitances and back into the magnetic fields with some loss due to the resistance in the circuit. Following the end of one-half cycle of oscillation, the energy stored in the magnetic fields of the deflection coils l4 and transformer I 6 causes current to flow through the deflection coils i4 and through the damper tube 22. This stored energy is substantially dissipated at the end of approximately one-half period of the scanning portion of each deflecting cycle. The power tube l0 begins conducting shortly before the middle of the scanning trace to produce further deflection of the beam until the end of that particular deflection cycle and until this time the damper tube is substantially independent of the power tube.

In order that linear deflection of the cathode ray beam may be produced, it is necessary that the rate of change of the current in the deflecting coils i l be maintained constant during the scanning portion of the deflection cycle. The voltage variations applied to the control electrode 28 0f damper tube 22, therefore, should have a waveform such that the current flow through the damper tube 22 will properly match the waveform of the current output of power tube It to result in a linear summation characteristic, or, in cases where it may be desirable, a predetermined departure from linearity of angular deflection so as to produce a linear rate of movement of the cathode ray scanning beam across the viewing surface of the cathode ray tube. The desired Waveform of the voltage variations which should be applied to the control electrode 28 of damper tube 22 is obtained by differentiating the voltage appearing across the deflection coils I l.

The means for difierentiating the voltage appearing across deflection coils 14 may be of -any suitable type, such, for example, as that disclosed in U. S. Patent No. 2,382,822, issued August 14, 1945 to O. H. Schade. In the embodiment of the invention shown in Fig. 1, it comprises a condenser one plate of which is connected to the high-potential end of the deflecting coils M by means including three

conductors

32, 3d and 36.. The conductor 34 is shown as a broken line for reasons which will later become apparent. The other plate of condenser 30 is connected to the control electrode 28 of damper tube 22 through a resistor 38.

A further resistor 40, which is adjustable as shown, has one end connected to a point between condenser 30 and resistor 38, and its other end to one terminal of a parallel resistance-condenser combination 42, the other terminal of resistancecondenser combination 42 being connected to the low-potential side of the horizontal deflection coils l4, and also to the low-potential side of the transformer secondary Winding 20.

Condenser 30 and resistor form a differentiating network for obtaining a voltage variation of particular waveform for application to the control electrode 28 of damper tube 22. The time constant of the differentiating network 30, Mi may be varied by adjustment of resistor 40, and since this acts to change the waveform of the voltage on control electrode 28, adjustable resistor 40 is also a linearity control. The parallel resistorcondenser combination 42 acts to provide a suitable D.'-C. bias for damper tube 22..

An alternative form of differentiating network which may be employed in conjunction 'with the present invention is set forth in a copending application of Otto H. Schade, Serial No. 572,712, filed January 13, 1945.

The waveforms of the voltages appearing across the primary winding l8 and the secondary winding 20, respectively, of transformer 1.6 may be somewhat as indicated by the curves a and b in Fig. 2.. The high-frequency oscillations which are produced by the ringing of transformer l6 following the retrace periods are indicated by the

reference characters

46 and 47. The amplitude of the oscillations 4! is less than the amplitude of the oscillations due to the damping action of tube 22, as mentioned above. These voltage variations 41 across the deflecting coils 14 cause the current (curve 0) through the coils M to fluctuate during the initial portion of the trace period, as shown by the reference character 48, and this results in distortion of the reproduced image in the manner set forth above.

It has been found that these voltage variations 46 and 41 are due to a secondary action of the deflection circuit which may be described as follows: Immediately following the retrace period, the transformer secondary winding 20 is shunted by the relatively low impedance of the damper tube 22, since the reactance of condenser 26 is chosen to be negligible at the frequency of horizontal deflection. When the transformer I6 is shunted in this manner, it oscillates, or rings, at

a frequency-determined by the leakageinductancl oi the transformer and by the shunt .capacitances of the deflection circuit, especially those asso-' part oft-he scanning portion of each deflect-ion cycle.

In accordance with the present invention, means are provided for substantially counteracting the effect of a ringing of transformer l6, nd the non-linearity of scan caused thereby. This means comprises an auxiliary, or linearity, coil wound on the coupling transformer 16. One end of coil 50 is joined to conductor 32, and the other end of coil 50 is joined to conductor 35. Conductor 34, shown by the broken line, is omitted from the circuit. By such an arrange.- ment, coil 50 is effectively connected in series. with condenser 30 between the control electrode 28 of tube 22 and the high-potential end of the deflection coils l4.

It will be apparent that any change of flux in the core of transformer it will induce a voltage across coil 50. Hence, when the transformer l6 oscillates, or rings, following retrace, an oscillating voltage is induced across coil 50, and this voltage may be as shown by the waveform d of Fig. 2. The .amplitude of the high-frequency oscillations indicated by the reference character 52 in curve (I is, like the oscillations 4.6 in curve a, greater than the amplitude of the oscillations 41 in curve 12,. As stated above, the coil 50 is so connected that the voltage induced thereacross is applied to the differentiating

network

30,40 in series with the voltage appearing across the horizontal deflection coils Hi. When this combined voltage is diflerentiated and applied to the control electrode 28 of damper tube 22, the com duction characteristics of tube 22 are altered so that the tube 22 acts in effect as a variable iin pedance connected across the secondary winding 20. With a proper number of turns on the auxiliary coil 50, the internal resistance of damper tube 22 may be varied so as substantially completely to cancel the voltage oscillations 4B, and to yield a waveform (curve e) which is practically constant during the trace portions of the deflecting cycle. As a result of such a constant voltage across the deflecting coils I4, the current therethrough will be substantially linear as shown in curve f.

It will be appreciated that the waveforms of the currents and voltages shown in Fig. 2 are given merely illustratively as an aid in understanding the operation of the circuit of Fig. 1, and do not necessarily represent the exact or actual amplitudes or durations of these currents and voltages.

Having thus described the invention, what is claimed is:

1. In a cathode ray beam deflection system of the type in which a power tube is adapted to 6 supply cyclically varying current through a coupling transformer to a cathode ray beam deflection coil, and in which a grid-controlled variable impedance damper tube is connected'across the secondary winding of said transformer, the combination of electrical difierentiating means, electrical connections for applying the voltage developed across said cathode ray beam deflection coil to said differentiating means, an auxiliary coil electromagnetically associated with said coupling transformer, said auxiliary coil being so wound that the voltage developed thereacross is substantially in phase with the voltage. developed across the secondary winding of. said transformer, means for applying the voltage developed across said auxiliary coil to said difierentiating means in additive series with the voltage developed across said cathode ray beam deflection coils, and means for applying the output of said diiferentiating means to the grid of said damper tube to control the impedance of said damper tube.

' 2. In a cathode ray beam deflection system of the type in which a cyclically varying current is generated and applied to a cathode ray beam deflection arrangement through a coupling member possessing inductive reactance, and in which controllable electrical damping means are'provided in shunt with said inductive coupling member, the combination of circuit connections for obtaining a first control voltage variation from the voltage developed across said cathode ray beam deflection arrangement, an auxiliary winding on said inductive coupling member for obtaining a second control voltage from the reactive energy developed in said inductive coupling member, means for combining said control voltages, and means for applying said combined controlled voltages to control the impedance of said damping means.

3. A cathode ray beam deflection system in accordance with claim 2, in which said controllable damping means comprises a grid-controlled electron discharge tube.

4. A cathode ray beam deflection system in accordance with claim 2, further comprising means for electrically difierentiating the waveform of the said combined control voltages prior to their application to control said damping means.

5. In a cathode ray beam deflection system'of the type in which a cyclically varying current is generated and applied to a' cathode ray beam deflection coil through a coupling member possessing inductive reactance, and in which a variablei pedance device is connected in shunt, .the combination of means for obtaining a voltage variation from the reactive energy developed in said inductive coupling member, electrical differentiating wave-shaping mean-s, means for applying said voltage variation to said electrical differentiating means in phase with the voltage developed across said cathode ray beam deflection coil, and means for applying the output of said electrical differentiating means to vary the impedance of said variable impedance device.

6. A cathode ray beam deflection system in accordanoe with claim 5, in which said electrical differentiating wave-shaping means comprises an electrical differentiating network.

7. A cathode ray beam deflection system in accordance with claim 5, in which said electrical diiferentiatin'g means comprises a differentiating network, 'in which said variable-impedance device comprises a grid-controlled electron discharge tube, and in which said-means to vary the imepdance of said variable-impedance device includes means for applying the output of said differentiating network to the grid of said electron discharge tube to thereby vary its conduction characteristics.

8. In a cathode ray beam deflection system involving a coupling transformer between a power tube and a deflection coil, a method for improving deflection linearity by reducing spurious oscillations comprising the steps of developing a first control'v-oltage having an amplitude indicative of the voltage across said deflecting coil, developing' a second control voltage by induction from said transformer, combining said first and second control voltages, and controlling the shunt impedance across said deflecting coil with said combined voltages.

9. In a cathode ray beam deflection system involving a coupling transformer between a power tube, a deflection coil and a shunt impedance across said deflection coil, 21. method for improving deflection linearity by reducing spurious oscillations comprising the steps of developing a first control voltage having an amplitude indicative of the voltage across said deflecting coil, developing a second control voltage by'induction from said transformer, adding in phase said first and second control voltages, and controlling the shunt impedance across said deflecting coil with said combined voltages.

HUBERT RICE SHAW.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,315,073 Norton Mar. 30, 1943 2,320,916 Dawson June 1, 1943 2,370,426 Schade Feb. 27, 1945 2,382,822 Schade Aug. 14, 1945 2,396,476 Schade Mar. 12, 1946