US2207389A - Cathode ray tube deflecting circuit - Google Patents

Description

y 9, 1940- K. R. WENDT 2,207,389

CATHODE RAY TUBE DEFLECTING CIRCUIT Filed June 30, 1936 3nventor Patented July 9, 1940 UNITED STATES PATENT orrlca Karl R. Wendt, Audubon, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application June 30, i936, Serial No. 88,143

11 Claims.

My invention relates to cathode ray tube deflecting circuits, and particularly to deflecting circuits for cathode ray tubes of the type having a screen structure inclined with respect to the electron gun.

An object of my invention is to provide improved means for correcting the keystone scanning pattern appearing on the screen structure in a cathode ray tube of the above-mentioned type. More specifically, an object of my invention is to provide a simplified, inexpensive, and easily adjusted deflecting circuit which will produce a substantially rectangular scanning pattern in a cathode ray tube of the above-mentioned type.

A still further object of my invention is to provide an improved generator of saw-tooth waves.

A still further object of my invention is to provide a simplified circuit for producing sawtooth waves in the output circuit of a push-pull amplifier or modulator.

In practicing my invention, I employ a pushpull or balanced modulator which is so designed that it functions both as a linear amplifier and u as a linear modulator. The modulator comprises vacuum tubes having high plate impedance, these tubes preferably being of the screen-grid type. Transformer coupling is employed both in the input circuit and in the output circuit of this a modulator, the primary winding of the input transformer being utilized as a portion of the saw-tooth generator. In the output circuit, at least one winding of the transformer is shunted by a resistor having such low impedance that the vacuum tubes feed into a substantially purely resistive load. Also, this load impedance is made so low compared with the plate impedance of the screen grid tubes that they function as a substantially linear modulator.

Other objects, features, and advantages of my invention will appear from the following description takenin connection with the accompanying drawing, in which Figure l is a circuit diagram of a preferred v embodiment of my invention, and

Figures 2 and 3 are diagrams which are referred to in explaining the invention.

Referring to Figure 1, my invention is shown applied to the deflecting circuits for a cathode ray transmitter tube in a television transmitter. The transmitter tube is of a well known type comprising an evacuated envelope I having therein an electron gun consisting of an indirectly heated cathode 2, a control grid 3, and

a first anode 4. The tube also includes a second anode 8 and a mosaic I of light sensitive elements upon which the picture to be transmitted is projected.

Preferably, the cathode ray tube is provided with horizontal deflecting coils 8 and vertical 6 deflecting coils 9, although deflecting plates may be substituted if desired. It will be understood that if deflecting plates are employed the output circuit of the deflecting circuit will be suitably modified. It will also be understood that the 10 cathode ray tube may be a receiver tube in which the screen structure is a fluorescent screen instead of a light sensitive mosaic.

Whether the cathode ray tube is a transmitter tube of the type illustrated or a receiver tube M of the type in which the picture is viewed from the side of the fluorescent screen being scanned, the screen structure is inclined with respect to the electron gun as shown in the drawing, whereby the pattern scanned on the screen structure by the cathode ray normally has a keystone shape as indicated at H in Fig. 2. The desired scanning pattern, of course, is a rectangular one, as indicated at l2 in Fig. 3.

The desired rectangular scanning pattern may 25 be obtained by gradually increasing the amplitude of the comparatively high frequency, horizontal saw-tooth waves as the cathode ray is deflected from the top edge of the screen I to the bottom edge thereof by the comparatively 30 low frequency, vertical saw-tooth waves. Of, ii the top edge is nearer the electron gun than the bottom edge, this amplitude will be gradually decreased as the cathode ray is deflected downwardly. The manner in which this correction 35 is accomplished by my improved circuit will be understood from the description which follows.

Referring first to the horizontal deflecting cir- I cuit which is designed, in a preferred embodiment, to produce saw-tooth waves occurring at o the frequency of 10,290 per second, it includes a blocking oscillator comprising a three-element vacuum tube l3 and a feed-back transformer H which preferably has an iron core. The secondary l6 of the transformer I4 is connected in 5 the grid circuit of the tube l3 in series with a grid condenser l1, while the primary [8 of the transformer is connected in the plate circuit of the tube, the direction of coupling being such that when the plate current increases the grid 50 of the tube I3 is made more positive. The blocking oscillator itself is completed by a variable grid resistor l9 connected between the grid and the cathode of the tube l3.

The relative values of the grid condenser ll 5 and the grid resistor l9 are made such that the operation is as follows: Assuming the conditionwhen voltage is.flrst applied to the plate of the of plate current is prevented until such time as a portion of the charge leaks off the'grid condenser I'I through the grid resistor l9. After enough of the charge has leaked off the condenser,

plate current again flows and the operation is repeated.

It will be noted that in the grid circuit of the tube I3 a resistor 2| is included in series with the transformer secondary l6. This resistor is included in the circuit to provide a simple way of impressing a horizontal synchronizing impulse upon the blocking oscillator. This impulse is supplied through a condenser 22 and'a resistor 23 from a suitable synchronizing generator in the case of a transmitter, or from a portion of the receiver in the case of a television receiver.

As will be more fully described later, my deflecting circuit includes a pair of screen grid tubes 24 and 26 connected in push-pull or balanced relationship, these tubes having a transformer 21 in their input circuit. The Y transformer has a primary 28 and a secondary 29, the primary being connected in the plate circuit of the blocking oscillator tube l3 and in series with the primary l8 of the feed-back transformer.

The description of the operation of the blocking oscillator given above was made with the assumptionthat the voltage was applied to the plate of the blocking oscillator tube l3 directly through the primary l8 of the feed-back transformer rather than through the two primaries l8 and 28 in series. The result of such operation would be to produce a sharp positive impulse periodically, which impulse, in the more usual saw-tooth generators, is employed to drive the grid of a discharge tube positive periodically to produce saw-tooth waves.

In accordance with one feature of my invention, I cause a saw-tooth voltage wave to appear across the primary winding 28 of the transformer 21. I accomplish this by connecting a condenser 3| acres the primary winding 28 whereby they are resonant at a frequency which is much lower than the frequency at which it is desired to produce the saw-tooth waves. It has been found that if this resonant period is about V of the frequency at which the saw-tooth waves are produced, the results will be satisfactory.

It may be mentioned that the condenser 3| may be connected either directly across the primary winding 28 or between the high voltage terminal of the primary winding 28 and ground, as shown, whereby it is actually connected across both the primary winding and the plate battery or other plate voltage supply (not shown). The results obtained are the same in both cases, the impedance of a plate battery or other voltage supply being very low at the frequencies involved.

The operation of the circuit in producing a saw-tooth voltage wave across the transformer primary 28 may be explained as follows: The condenser 3| is charged at a comparatively slow rate through the transformer primary 28 by means of the plate voltage'supply (not shown). The condenser 3| is charged slowly because of the substantial impedance of the primary winding. At the end of a certain period, the grid of the blocking oscillator tube I3 is driven positive, as previously explained, whereby the plate impedance of the tube becomes low enough to permit the condenser 3| to discharge suddenly through the tube. It may be noted that the impedance of the primary winding l8 of the feed-back transformer is so low that the condenser is not prevented from discharging rapidly.v If the condenser 3| isconnected directly across the primary 28, the condenser is charged suddenly through the tube l3 and discharged gradually through the primary 28,

The broad feature of employing in the plate circuit of a vacuum tube a tuned circuit resonant at a frequency which is low compared with the f frequency of a desired saw-=tocth wave is de= scribed and claimed in Patent No. 2,085,402, issued June 29, 1937, to Arthur W. Vance and assigned to the Radio Corporation of America.

Referring now more specifically to the balanced modulator, the tube 24 includes an indirectly heated cathode 32, a control grid 33, a screen grid 34,-a suppressor grid 36, and a plate 31, while the tube 26 includes an indirectly heated cathode 38, a control grid 39, a screen grid 4|, a suppressor grid 42, and a plate 43. The secondary'winding29 of the transformer 21 has one terminal connected to the control grid 33 of the tube 24 and the other terminal connected to the control grid 39 of the tube 28. The mid point of the secondary 29 is connected through a conductor 44 and a variable tap 46 to a point on a resistor 41, through a portion of the resistor 4! to ground, and through ground and a resistor 48 to the cathodes 32 and 38 which are connected together. The upper and lower halves of the transformersecondary 29 are shunted by resistors 49 and 5|, respectively, for the purpose of preventing undesired resonant effects caused by distributed capacity in the transformer.

The coupling between the output circuit of the balanced modulator and the output tube 52 of the horizontal deflecting circuit is made by means of a transformer 53 having a primary winding 54 and a secondary winding 55. The primary winding 54 has one terminal connected to the plate 31 of the tube 24 and the other terminal connected to the plate 43 of the tube 23. The mid-point of the primary winding 54 is connected to a suitable source of plate potential (not shown). The screen grids 34 and 4| are connected to a point on a potentiometer including resistors 58 and 51 connected between the cathodes 32 and 38 and the positive terminal of the plate supply (not shown) whereby they are maintained at a suitable positive potential.

The upper and lower halves of the primary winding 54 have resistors 58 and 59, respectively, connected thereacross. These resistors, together with a resistor 6| connected across the secondary winding 55, are for the purpose of making the load impedance of the balanced modulator substantially purely resistive, whereby the modulator will provide linear amplification, and for the purpose of providing a load impedance which is so low compared with the plate impedance of the modulator tubes that the modulator will provide linear modulation. It will be understood that the resistance across the secondary 55 is reflected into the modulator plate circuit to determine the load impedance to a certain extent. The various considerations which determine the use of resistors across the transformer windings 54 and 55, and which determine their values, will be discussed in more detail later in the specification.

The output tube 52 is of the screen grid type, having a high plate impedance whereby a sawtooth voltage wave applied to its input circuit will cause a flow of saw-tooth current through the deflecting coils 8. The tube 52 is properly biased to function as the usual Class A amplifier, the biasing being provided by means of a selfbias resistor 62 and condenser 83, in the particular circuit illustrated. The saw-tooth voltage wave which appears across the resistor BI is impressed upon the input circuit of the tube 52 through a variable tap 64, whereby the amplitude of the horizontal deflecting current may be adjusted. The output circuit of the tube 52 is coupled through an output transformer 66 to the horizontal deflecting coils 8.

It will be understood that a low impedance output tube may be employed in place of the high plate impedance tube 52, but if such a substitution is made it is necessary to supply to the input circuit of the balanced modulator a deflecting wave which includes an impulse component. It is obvious that, in this case, both the saw-tooth component and the impulse component will be modulated by the low frequency vertical sawtooth wave to provide the desired keystone correction.

Referring now to the vertical deflecting circuit which supplies a saw-tooth current to the vertical deflecting coils 9 and a saw-tooth voltage to the balanced modulator, it comprises a blocking oscillator 61 of the same type employed in the horizontal deflecting circuit. This oscillator is adjusted to produce impulses occurring at a comparatively low frequency, sixty per second in the particular circuit being described. By means of a condenser 68 connected across the primary 69 of a transformer l I, these impulses are converted into saw-tooth voltage waves which appear across both the primary 69 and the secondary 12 of the transformer H. In this circuit the condenser 68 is charged through the oscillator tube instead of being discharged through it.

The blocking oscillator 61 differs from the one previously described in that the grid condenser 13 and secondary winding 14 of the feed-back transformer are connected in series with the plate resistor 16 of the preceding amplifier tube 11. The grid condenser charging circuit may be traced from the grid of the oscillator tube, through the secondary 14, the grid condenser I3, the plate resistor 16, and a by-pass condenser shunting a portion of a voltage divider 18 to ground, and through ground to the cathode of the oscillator tube. Thus the vertical synchronizing impulses are impressed upon the blocking oscillator 61 through the plate resistor 16.

A resistor 19 and condenser 8| are connected in series across the secondary" for the purpose of straightening the saw-tooth wave, this correction of the wave shape being provided because of the diificulty of making the inductance of the primary winding 69 great enough to produce an undistorted saw-tooth at the low frequency involved.

The vertical saw-tooth wave is supplied from the secondary 12 to the resistor 41, whereby it is impressed upon the control grids 33 and 39 of the modulator tubes 24 and 26 to provide the desired modulation. As is well understood, this modulating voltage will not appear in the output of the balanced modulator, since it is balanced out by the circuit connections. Therefore, the only signal appearing in the-output circuit of the balanced modulator will be'the modulated horizontal saw-tooth waves, which are so modulated that as the cathode ray is deflected downwardly their amplitude will gradually be increased to provide on the screen I the rectangular scanning pattern shown in Fig. 3. If any of the vertical saw-tooth waves appeared in the modulator output, the sides of the scanning pattern would either be slanted and parallel to each other, or otherwise distorted depending upon whether all or only a portion of the vertical saw-tooth frequency components are passed by the horizontal deflecting circuit.

The saw-tooth wave appearing across the transformer primary 69 is impressed across a resistor 82 and supplied through a variable tap 83 to the input electrodes of an output tube 84. The output circuit of the tube is coupled to the vertical deflecting coils 9 through a suitable transformer 86.

It may be pointed out that the requirements for a balanced modulator in my keystone correcting circuit are rather severe. First, the modulation should be practically linear for at least thirty percent modulation; second, the horizontal wave shape must not be distorted; and third, the circuit must not introduce transients following the return line period.

The required linear modulation is obtained by selecting a type of screen grid tube, such as a variable mu pentode, which has a straight and steep portion on its mutual conductance, controlgrid voltage curve. By making the load impedance of such tubes very low compared with the plate impedance of the tubes, the gain of the tubes will vary directly as the mutual conductance, and consequently, the tubes will provide linear modulation, the modulation being linear up to about fifty percent modulation in the abovedescribed circuit.

It may be noted here that the gain of a screen grid tube such as the 6K7 is so great that the loss in amplification, resulting from having the load impedance low compared with the tube impedance, is immaterial, there being much more gain available than is required. In ordinary modulator practice, it is, of course, customary to make the load impedance much higher than the plate impedance, whereby the most efiicient operation is obtained even though there is a resulting distortion in the modulation. In a radio transmitter, for example, the load is a tuned circuit, and harmonic frequencies introduced by distortion can be filtered or tuned out so that the modulation distortion is not detrimental. In applicants circuit such filtering out of undesired frequencies, introduced by modulation distortion. is impossible, since these frequencies fall within the range of the frequencies being transmitted.

The requirement that the horizontal wave shape shall not be distorted is complied with by shunting the transformer with resistance of such low value compared with the transformer inductance that the modulator feeds into a substantially purely resistive load. Therefore, the load impedance remains substantially constant over a wide frequency band, and linear or undistorted amplification is obtained over this frequency range. It may be noted that the load should so closely approximate a purely resistive load that secondary. This is prevented by shunting the re-' sistor 6| across the secondary. This resistance then, of course, effectively introduces resistance in the primary circuit to provide the desired resistive load for complying with the first two requirements. Resonance between the primary distributed capacity and the secondary leakage inductance is prevented by providing the resistors 58 and 59 across the primary winding.

Attention is called to the fact that no by-pass condensers are used around the self-bias resistor 48 or around the screen grid resistors 56 and 51 in the balanced modulator circuit. This improves the linearity of the modulation and tends to cancel out any unbalance resulting from adifference in the two tubes. It also degenerates the vertical saw-tooth, making necessary more modulating voltage, but'the required voltage is still less than the voltage available.

It will be understood that other types of sawtooth wave generators than the specific generator described may be employed in combination with mybalanced keystone correction circuit. In the event of such substitution, the primary of the input transformer, of course, will not function as a portion of the saw-tooth generator.

If the output tube in a deflecting circuit has a plate impedance which is low enough, only a voltage impulse should be impressed upon the input tube of the balanced modulator, a saw-tooth component being unnecessary. In that case, the low frequency voltage wave modulates the impulses occurring at the horizontal frequency, instead of modulating the saw-tooth waves occurring at this frequency.

On the drawing the values of certain resistors and condensers have been indicated in ohms, microfarads and micromicrofarads.

From the foregoing description, it will be apparent 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 claims.

I claim as my invention:

1. A deflecting circuit for a cathode ray tube having a cathode ray deflecting device associated therewith comprising means for producing voltage waves occurring at a comparatively high frequency, a balanced modulator comprising a pair of electric discharge tubes, said modulator including an input transformer and an output transformer, means for supplying said voltage waves to said input transformer, means for coupling said output transformer to said deflecting device, means for so resistively loading said output transformer that said tubes work into a substantially purely resistive load having an impedance which is low compared with the plate impedance of said tubes, whereby said modulator will provide linear amplification and linear modulation, means for producing saw-tooth waves at a comparatively low frequency, and means for so impressing said saw-tooth waves upon said modulator that they modulate said. high frequency voltage waves whereby only said modulated wavesv appear in the modulator output.

2.A deflecting circuit for a cathode ray tube of the type having a screen structure inclined with respect to the electron gun and having a vcathode ray deflecting device associated therewith, said circuit comprising means for producing saw-tooth waves at a comparatively high fre- I quency, a pair of electric discharge tubes cennmted in balanced relation with an input transformer and an output transformer, means for so resistively loading said output transformer that said tubes work into a load impedance which is low compared with their plate impedance whereby said modulator will providelin'ear amplification and linear modulation, means for impressing said saw-tooth waves upon said input transformer, means for producing saw-tooth waves occurring at a comparatively low frequency, means for varying the gain of said tubes in accordance with said low frequency saw-tooth waves and thereby modulating said high frequency saw-tooth waves, and means for supplying to said deflecting device the modulated high frequency saw-tooth waves appearing in said output transformer.

3. In combination, a cathode ray tube having a screen inclined at an acute angle with respect to the axis of the electron gun, horizontal deflecting means for deflecting the cathode ray at right angles to the plane of said acute angle, vertical deflecting means for deflecting the oathode ray in the plane of said acute angle, a horizontal deflecting circuit including a pair of electric discharge tubes each having high plate im pedance, means including an input transformer and an output transformer for connecting said tubes in push-pull relation to form a balanced modulator, means for so resistively loading said output transformer that said tubes work into a substantially purely resistive load having an impedance which is low compared with the plate impedance of said tubes whereby said modulator' will provide linear amplification and linear modu-' lation, means for supplying saw-tooth waves occurring at a comparatively high frequency to said input transformer, a vertical deflecting circuit modulated by said low frequency waves, and means for supplying said modulated waves to said horizontal deflecting means.

4. In combination, a pair of electric discharge tubes of the screen grid type each having a control grid and a. plate, means including an input transformer and an output transformer for connecting said tubes in push-pull relation to form a. balanced modulator, means for so resistively loading said output transformer that said tubes work into a substantially purely resistive load having an impedance which is low compared with the plate impedance of said tubes whereby said modulator will provide linear amplification and linear modulation, means for producing sawtooth waves occurring at a comparatively high frequency and for impressing them upon said input transformer, means for producing sawtooth waves occurring at a comparatively low frequency, and means for impressing said low frequency waves upon a portion of said input circuit which is common to both tubes whereby said low frequency waves are impressed upon said control grids to modulate said high frequency waves by said low frequency waves and whereby only the modulated waves appear in the secondary of said output transformer.

5. In combination, a pair of electric discharge tubes of the variable mu pentode type each having a control grid and a plate, means including an input transformer and an output transformer for connecting said tubes in push-pull relation to form a balanced modulator, means for so resistively loading said output transformer that said tubes work into a substantially purely resistive load having an impedance which is low compared with the plate impedance of said tubes whereby said modulator will provide linear amplification and linear modulation, means for producing voltage waves occurring at a comparatively high frequency and for impressing them upon said input transformer, means for producing sawtooth waves occurring at a comparatively low frequency, and means for varying the gain of said tubes in phase in accordance with said low frequency waves whereby said low frequency waves modulate said high frequency waves and whereby only the modulated waves appear in the secondary of said output transformer.

6. In combination, a pair of similar electric discharge tubes each having a control grid and a plate, each of said tubes being of the type which has a straight and steep portion on its mutual conductance, control-grid voltage curve, means including an input transformer and an output transformer for connecting said tubes in pushpull relation to form a balanced modulator, means for so resistively loading said output transformer that said tubes work into a substantially purely resistive load having an impedance which is low compared with the plate impedance of said tubes whereby said modulator will provide linear amplification and linear modulation, means for producing voltage waves occurring at a comparatively high frequency and for impressing them upon said input transformer, means for producing saw-tooth waves occurring at a comparatively low frequency, and means for varying the gain of said tubes in phase in accordance with said low frequency waves whereby said low frequency waves modulate said high frequency waves and whereby only the modulated waves appear in the secondary of said output transformer.

7. In combination, a pair of electric discharge tubes each having input electrodes, a transformer having a primary and a secondary, said tubes being connected in balanced relation at said input electrodes by means of said secondary, an oscillator having a plate circuit, said primary being connected in said plate circuit, and a condenser connected in parallel with said primary with respect to said plate circuit whereby said condenser and said primary are resonant at a certain frequency, the capacity and inductance of said condenser and primary, respectively, being such'that said certain frequency is substantially lower than the frequency at which said oscillator operates. I

8. In combination, a translating device having an input circuit, a transformer having a primary and a secondary, said secondary being connected in said input circuit, an oscillator having a plate circuit, said oscillator being of the type which produces electrical impulses periodically, said primary being connected in said plate circuit, and a condenser connected in parallel with said primary with respect to said plate circuit whereby said condenser and said primary form a circuit which is resonant at a certain frequency, the capacity and inductance of said condenser and primary, respectively, being such that said cer tain frequency is substantially lower than the frequency at which said impulses occur.

9. In combination, a pair of electric discharge tubes each having input electrodes, a transformer having a primary and a secondary; said tubes being connected in balanced relation at said input electrodes by means of said secondary, a blocking oscillator having a plate circuit, said oscillator being of the type which produces electrical impulses periodically, said primary being connected in said plate circuit, and a condenser connected in parallel with said primary with respect to said plate circuit whereby said condenser and said primary form a circuit which is resonant at a certain frequency, the capacity and inductance of said condenser and primary, respectively, being such that said certain frequency is substantially lower than the frequency at which said impulses occur.

10. In combination, a pair of electric discharge tubes connected in balanced relation to form a balanced translating device having an input circuit, an input transformer having a primary and a secondary, said secondary being connected in said input circuit, a blocking oscillator comprising a tube having inductively coupled plate and grid circuits and comprising a grid condenser and a grid resistor having such values that impulses are produced periodically, said coupling being provided by a feed-back transformer having a primary and a secondary, the primary of said input transformer being connected in said plate circuit in series with the primary of said feed-back transformer, and a condenser connected in parallel with the primary of said input transformer to form a parallel resonant circuit whereby said condenser either charges or discharges through said oscillator tube upon the occurrence of one of said impulses, the capacity and inductance of said condenser and the primary of said input transformer, respectively, being such that said parallel resonant circuit is resonant at a frequency which is substantially lower than the frequency at which said oscillator operates.

11. The invention according to claim 1 characterized in that said first means comprises an oscillator of the type which produces electrical impulses periodically, said oscillator having a plate circuit, said input transformer having a primary connected in said plate circuit, and further characterized in that said condenser is connected in parallel with said primary to form therewith a circuit which is resonant. at a frequency substantially lower than the frequency at which said impulses occur.

KARL R. WENDT.

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