US2207499A - Cathode ray deflecting system - Google Patents

Description

July 9, 1940.

A. W. VANCE CATHODE RAY DEFLECTING SYSTEM Filed June 26, 1936 2 Shee tS-Sheet l HrIfhu/r 8 YNC. INPUT' VV: Vance July 9, 1940. A w, VANCE 2,207,499

CATHODE RAY DEFLECTING SYSTEM Filed June 26, 1936 2 Sheets-Sheet 2 SYNC. INPUT Patented July 9, 1940 UNITED STATES mais PATENT GFFICE,

Arthur W. Vance, Haddonfield, lv. J., assignor to Radio Corporation of America, a corporation of Delaware Application June 26, 1936, Serial N0. 87,489

1 Claim.

My invention relates to cathode ray tube detlecting systems, and particularly to deecting 'systems which are employed with a cathode ray tube of the type having a screen structure inclined at an acute angle with respect to the axis of the tube.

In cathode ray transmitter tubes of the type commonly employed in television transmitters, the tube includes a mosaic of light sensitive elements, which is positioned at an acute angle to the axis of the electron gun whereby the same Yside of the mosaic which has an. optical image projected thereon may be scanned by the cathode ray. In cathode ray receiver tubes of a common type, the uorescent screen is positioned at an acute angle to the electron gun, wlhereby an observer may view a picture on the same side of the screen that is being scanned by the cathode Fay.

In the preferred television systems, at both the transmitter and receiver a cathode ray is delected horizontally at a comparatively high frefluency by means of saw-tooth waves, While it is being deiiected vertically at a comparatively low frequency by means of other saw-tooth Waves.

If the screen structure of the cathode ray tube were located normal to the elctron gun axis, such :leection of the cathode ray would cause it to scan a rectangular pattern on the screen. Where the screen structure is positioned at an acute ingle to the electron gun axis, however, the scanning pattern produced by such deiiection has a ieystone shape. Various apparatus and circuits have been proposed for correcting this keystone error, but they have involved certain objectiontble features.

It is, accordingly, an object of my invention to 2nrovide an improved method of and means for correcting keystone error in cathode ray tubes of che above mentioned type.

it is a further object of my invention to provide an improved delecting system for cathode `f'ay tubes of the above mentioned type, which is :omparatively simple in design and which will u'odu'ce a substantially rectangular scannning )atterri on the tubes screen structure.

In practicing my invention, I modulate the `icrizontal saw-tooth waves with the lower freluency vertical saw-tooth waves in a balanced zircuit, whereby the modulating vertical saw- ;ooth waves are balanced out and prevented from tppearing in the output of the horizontal deiecting circuit. Therefore, the only deflecting `vaves supplied to the horizontal deilecting plates or deflecting coils are the modulated horizontal saw-tooth waves, these waves gradually increasing in amplitude as the cathode ray is deilected vertically towards the edge of the screen which is nearer the electron gun. It is very important that the Vertical saw-tooth component be prevented from appearing in the output of the horizontal deecting circuit, as such a component distorts the scanning pattern on the screen by making the vertical edges of the pattern appear at an angle With respect to the vertical to produce a slanted pattern.

Other objects, features and advantages of my invention will appear from the following description, taken in connection with the accompanying drawing, in which:

Figure 1 is a circuit diagram of a saw-tooth deecting circuit embodying my invention, this circuit being designed to supply saw-tooth voltage Waves toa pair of deilecting plates;

Fig. 2 is a diagram showing a scanning pattern containing keystone error;

Fig. 3 is a diagram showing the scanning pattern produced by a deecting circuit such as the one illustrated in Fig. 1; and

Fig. 4 is a circuit diagram of another embodiment of my invention.

Referring to Fig. 1, my invention is shown applied to a deflecting circuit for a cathode ray transmitting tube of a well known type. This tube, which is indicated at I, comprises an evacuated envelope having therein an indirectly heated cathode 2, a control grid 3, a first anode d, and a second anode 6. It also includes a screen structure 'I comprising a mosaic of light sensitive elements, this screen structure being positioned at an acute angle to the axis of the electron gun comprising the cathode, the control grid, and first anode. The cathode ray tube I is also pro-vided with a pair of deilecting plates 8 and a pair of deecting coils 9.

The horizontal deiiecting circuit includes an oscillator II for producing electrical impulses at the frequency at which it is desired to produce saw-tooth deiiecting Waves. In the preferred embodiment of the invention, this oscillator is of the blocking oscillator type, comprising a three-element tube I2 having a grid I3, a cathode I4, and a plate I6. The plate circuit is coupled to the grid circuit through a transformer I'I having a primary I8 and a secondary I9, voltage being supplied to the plate I6 through a voltage reducing resistor 2| and the transformer primary I 8`. The transformer secondary I9 is connected at one end through a resistor 22 toground, and at the other end through a grid condenser 23 to the control grid i3.

A grid resistor 24 is connected between the grid I3 and the cathode I4. The relative values of the grid condenser 23 and grid resistor 24 are such that voltage fed back through transformer I1 blocks the tube l2 for a certain period. At the end of this period another cycle of operation of the oscillatorl is'b'egun. Each'cycle of operation produces a positive voltage impulse of large magnitude, which is supplied to the grid of a discharge tube 26. v v

The discharge tube includes a control grid 21, a cathode 28, and a plate 29. The tube 2S is biased substantially to cut-off by suitable biasing means such as a self-biasing resistor 3l connected between the cathode 28 and ground, this resistor being shunted by a suitable condenser 32. The plate 29 of the discharge tube is supplied with a suitable operating potential through a potentiometer 33 and a plate resistor 34.

A condenser 36 is connected between the plate 29 and ground, whereby it is charged comparatively slowly irom the plate supply through the plate resistor 34.*w Since a positive impulse is being applied to the grid. 21 of a discharge tube periodically, the condenser 36 discharges suddenly through the discharge tube, after having been charged up to a certain value. This slow charge and sudden discharge of the condenser produces 'the desired saw-tooth wave, the sawtooth wave occurring at the frequency at which the condenser `36 is discharged by the oscillator impulses.

In accordance with my invention, the sawtooth wave appearing across the condenser 36 is supplied to a balanced modulator circuit. This modulator circuit includes a pair of screen grid tubes 31 and 38 the-tube 31 having a contro-l grid 39, a cathode 4l, a screen grid 42, a suppressor grid 43, and a plate 44. IThe tube 38 likewise includes a control grid 45, a cathode 41, a screen grid 48, a suppressor grid 49, and a plate 5l.

The input circuit of the tube 31 includes a grid resisor 52 and a self-biasing resistor 53, the self-biasing resistor being shunted by a suitable condenser 54. The saw-tooth condenser 36 is connected across the grid resistor 52 through a coupling condenser 56. The input circuit of the tube 38 includes a grid resistor 51 and the biasing resistor 53.

The plates of the tubes 31 and 38 are supplied with the proper operating voltage through resistors 58 and 59, respectively, from a common voltage source. It'will be apparent that the tubes 31 and 38 are connected in push-pull or balanced relationship both in their grid circuits and in their plate circuits.

In order to couple the single-sided discharge tube amplifier 26 to the double sided push-pull modulator Ztl-33, a connection is taken from a point on the plate resistor 58 of the tube 31 to the grid 46 of the tube 38, through a conductor 6! and a coupling condenser G2. The connection to the resistor 58 preferably is through a variable tap 63 which is adjusted to a point where the gain of the amplifier 31 is unity, that is, the

voltage appearing between the tap 6;! and the voltage supply point 64 is equal to the voltage impressed upon the control grid 39 of the tube $1. The output of the balanced modulator 31-38 is supplied to a conventional push-pull ampliiier including amplifier tubes 6B and S'ihaving the primary 68 of a step-up transformer 69 in their plate circuits. The secondary 1I of this transformer is connected across the deflecting plates 'flecting circuit preferably comprises a blocking oscillator 'I3 similar to the one in the horizontal '-defiecting circuit, this oscillator being adjusted to produce positive voltage impulses occurring at the comparatively low picture frame frequency. These impulses are impressed upon the grid of a discharge tube 14 corresponding to the tube Et in the horizontal deflecting circuit.

Voltage is supplied to the plate of the discharge tube 'M through a potentiometer 1li and a plate resistor 11 having high resistance. A condenser 18, across which the vertical sawtcoth voltage wave is produced, is connected between. the plate of tube 14 and ground. The vertical saw-tooth voltage wave is produced in the saine way that the horizontal saw-tooth wave is produced, the condenser 18 being charged slowly through the plate resistor 11, and being discharged rapidly through the discharge tube 14 each time a positive impulse is applied to the grid of". the discharge tube.

rihe vertical saw-tooth; wave through a screen grid output tube 19 to the Vertical deflecting coils or to a pair of deflecting pla-i s, if preferred.

.in accordance with my invention, the sawtoc-th waves appearing across the condenserr'l are also impressed upon the screen grids 42 and 4t or the balanced modulator 31-38111 the hori- 4zontal deflecting circuit, whereby they modulate the horizontal sawtooth waves. The modulating waves are supplied to the screen grids 42 and 48 i a linear amplifier 8| which has voltage to its plate, through aplate resistor 82. The screen grids 42 and 'i8 maybe vsupplied through a screen grid resistor B3 with Voltage from the same source (not shown) as that supplying the plates of the tubes. The low frequency saw-tooth voltage is impressed upon the resistor 33 and thus upon the screengrids 42 and it through a coupling condenser'84.

The adjustment of the circuit is such that if the cathode ray, indicated at 86, is deflected downwardly, the modulating saw-tooth voltage on the screen grids 42 and 48 increases in amplitude, whereby` the amplitude of the horizontal saw-tootn waves gradually increases during downward deflection of the beam. By adjusting a variable tap 81 on the input resistor of the linear amplier 8l, the inagnitude'of the modulating voltage may be made just sufficient tocompensate for the keystone' e'ect.

The result of the keystone correction will be seen by referring to Figs. 2 and 3.. Before the correction, the scanning pattern on Vthe mosaic 1 of the cathode ray tube is of the keystone shape indicated at 88 in Fig. 2. By properly adjusting the value. of the lcw frequency modulating voltage, the scanning pattern on the mosaicis made substantially rectangular, as indicated at 89 in Fig. 3.

in order to obtain good keystone correction, itis necessary that the balanced modulator :i1-38 have a substantiallyv linear characteristic. It be pointed out that when a saw-tooth` Wave is modulated by another saw-tooth wave, a Wide band of frequencies is produced, the highest of which is the highest harmonic of the carrier sawtooth wave plus the highest harmonic of the modulating saw-tooth wave. The lowest frequency is the fundamental of the carrier sawtooth wave minus the highest harmonic of the modulating saw-tooth wave. Thus, in order to transmit a modulated horizontal saw-tooth wave, a circuit with a lower frequency cut-off is required than would be necessary for an unmodulated saw-tooth wave. For this reason, the values of the coupling condensers and bypass condensers are made extra high in order to pass these lower side band frequencies.

In Fig. 4 there is shown an embodiment of my invention in which the horizontal deecting .circuit supplies saw-tooth circuit Yte deecting coils instead of saw-tooth voltage to deflecting plates, as in Fig. l. In the two iigures, like parts are indicated by the same reference numerals. Up to and including the plate circuit of the balanced modulator, the circuits in Figs. l. and 4 are substantially the saine. Preferably, however, a peaking resistor t! is included in series with the saw-tooth condenser Sli in order to provide the impulse component which is desired when deiiecting coils are fed from a low impedance output tube. The deflecting coils and output tube are indicated at 92 and respectively. It m ay be noted that in some cases, especially when the tube t3 has a very low plate impedance, it will be desired to impress voltage impulses only upon the balanced modulator whereby impulses occurring at the horizontal frequency are modulated by the vertical saw-tooth waves.

Since it is desirable to feed the deiiecting coils 92 from a single-sided output amplifier, instead of from a push-pull amplier, an amplifier tube 94 is included between the balanced modulator 31-38 and the output tube 93 for connecting from a balanced or double-sided circuit to a sin glo-sided circuit. The control grid 96 of tube 84 is connected through a coupling condenser 9i to a point on the plate resistor 58, and the cathode 98 is connected to the grounded conductor 9B. 'Ihe plate lll! of the tube 94 is connected directly to the plate l of the balanced modulator tube 38, both of these plates being supplied with voltage through the plate resistor 59.

The plate circuit of the tube Sil is connected to the input circuit of the output tube 93, which may be of the three-element type having a fairly low plate impedance. The deflecting coils 92 are coupled to the output circuit of the tube S3 through the usual choke coil |02 and coupling' condenser H13.

It will be noted that in this embodiment of my invention the horizontal saw-tooth waves are modulated by the vertical saw-tooth wave in the wayV previously described, in order to obtain the desired keystone correction.

It will be apparent that in the circuits described above, the vertical saw-tooth wave does not appear in the output circuit of the balanced modulator, since such a modulator balances out the modulating signal, as is Well known in the art. As previously mentioned, this is very important if an undistorted deflecting pattern is to be obtained on the screen of the cathode ray tube.

It may be pointed out that in most radio circuits, Where a radio frequency is modulated by an audio signal, it is possible to filter out the modulating frequencies if they appear in the modulator ouput. In the present case, however, where a horizontal saw-tooth wave is modulated by a vertical saw-tooth wave, some of the modulated frequencies are higher than some of the carrier frequencies, and obviously such ltering is impossible.

In Fig.V 1 the values of certain resistors Vand condensers have been indicated in ohms and microfarads merely by way of example. Also, the types of vacuum tubes have been indicated by way of example. It should be understood that, if preferred, substantially dierent circuit constants may be employed in practicing my invention.

Since the deecting circuits of a transmitter are usually operated in synchronism with impulses supplied from a synchronizing generator, the points at which these impulses are supplied to the blocking oscillators are indicated in Figs. 1 and 4 by the legend Sync input.

It will be apparent from the foregoing description that various modifications may be made in my invention without departing from the spirit and scope thereof, and I desire, therefore, that only such limitations shall be imposed thereon as are necessitated by the prior art and set forth in the appended claim.

I claim as my invention:

In combination, a cathode ray tube having a screen structure positioned at an acute angle with respect to the axis of its electron gun, horizontal defiecting means for deflecting the cathode ray at right angles to the plane of said acute angle, vertical deflecting means for deecting the cathode ray in the plane of said acute angle, a horisontal deflecting circuit including a pair of electric discharge tubes connected in balanced relation to form a balanced modulator, means for producing saw-tooth voltage waves at a comparatively high frequency, means for supplying said waves to said horizontal deflecting means through said modulator, means for producing saw-tooth waves occurring at a comparatively low frequency and supplying them to said vertical deflecting means, and means for impressing said low freduency waves upon said balanced modulator for modulating said high frequency waves, the consta-nts of said horizontal deecting circuit being such that it passes substantially all the frequency components of both the upper side band and the lower side band of the modulated high frequency waves.

ARTHUR W. VANCE.

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