US2265669A - Modulating device - Google Patents

Description

W410 H. w. PAEHR 2,265 69 MODULATING DEVICE Filed April 10, 1939 5&7

M/vs H/EENEE F/lE/IE INVENTOR B I T I ATTORNEYS Patented Dec. 9, 1941 MODULATING DEVICE Hans Werner Paehr, Berlin-Zehlendorf, Germany, assignor to Fernseh Aktiengesellschaft, Berlin-Zehlendorf, Germany Application April 10, 1939, Serial No. 267,058 In Germany April 8, 1938 6 Claims.

My invention relates to a means and method for modulating cathode ray tubes.

At present most cathode ray tubes are modulated by means of a grid, an apertured diaphragm, or so-called Wehnelt cylinder, to which signals are applied as is well known in the art. If such tubes are to be operated at anode Voltages of the order of 50,000 volts or more and at beam currents of the order of 4 milliamperes or more, difiiculties arise. Inasmuch as there is considerable fluctuation in the amount of anode current drawn, this would tend to cause corresponding fluctuations in the anode voltage ii the power supply is not well regulated to deliver constant voltages for a wide range of current values. Such well regulated power supplies, however, perate very uneconomically at high currents and voltages. In cathode ray tubes with luminescent screen's, such as are used for instance for television purposes, a further disadvantage lies in 1.

the fact that the size of the luminescent spot varies in accordance with the modulating signal, which is detrimental to the quality of the reproduced image.

It is known in the art to control the intensity of 'a cathode ray beam by deflecting in accordance with the modulating signal a concentrated beam in one direction across an aperture in a diaphragm, thereby intercepting a portion of the beam and allowing the remainder to pass through the aperture in accordance with the modulating signal. Deflection can be effected by electrostatic or electromagnetic means, whereby a signal corresponding to a white picture element of a tele- K vision image deflects the concentrated beam in such a manner that none of it is intercepted by the diaphragm and the full beam intensity arrives at the luminescent screen, While a signal corresponding to a black picture element of a television image deflects the cathode ray beam in such a manner that it is fully intercepted by the diaphragm and no electrons arrive at the luminescent screen. In this type of modulation the anode current remains constant inasmuch as merely the ratio of the portion of the beam intercepted to the entire beam intensity is varied. This allows the use of considerably simpler power supplies. The slope of the gride voltage-anode current characteristic is also considerably steeper than that for gridor cylinder-type modulation.

It has been found, however, that deflection modulation is not usable in tubes with extremely high anode currents, such as are used in television projection tubes. The portion of the cathode ray beam intercepted by the diaphragm produces such an extreme amount of heat by electron bombardment, which is of the order of for instance 100 watts in an area of square millimeter, that the diaphragm would be destroyed within a short time. It is also not readily possible to provide effective cooling because the diaphragm material is not sufficiently heat-conductive.

My invention has for its object to overcome the aforesaid disadvantages; to provide a means and method of modulating a cathode ray beam in accordance with a signal; to provide a method and means for modulating cathode ray beams of extremely high intensity; and to provide a means and method of modulating a cathode ray beam in a television picture tube so as to produce a luminescent spot of constant size. Other objects and features of my invention will become evident from the following description.

Referring to the drawing:

Fig. 1 shows reduced to the simplest terms the modulating device as provided by my invention;

Fig. 2 shows a preferred type of modulating aperture;

Fig. 3 schematically shows a modulating circuit reduced to the simplest terms;

Fig. 4 schematically shows apparatus required in connection with television transmission of socalled positive polarity; and

Figs. 5, 6, '7 and 8 show curves explaining the function of the apparatus as set forth in Fig. 4.

Broadly considered, my invention provides for directing a cathode ray beam of constant intensity and elemental cross-section upon an aperture disposed in a diaphragm. In the undefiected condition the entire beam current is allowed to pass through the aperture. I then provide two deflecting fields perpendicular to each other in front of the aperture so that I may deflect the concentrated cathode ray beam in two directions perpendicular to each other about the aperture. The signal in accordance with which the intensity of the cathode ray beam is to be modulated is modulated upon a carrier frequency substantially higher than that of the modulating signal.

This modulated carrier-frequency signal is then directly applied to one deflection system. I then provide for shifting the phase of the modulated carrier-frequency signal through an angle of degrees and for applying this shifted carrierfrequency signal to the other deflecting system. From the foregoing, it may be evident to those skilled in the art that for a constant modulating signal the cathode ray beam will be deflected along a circular path about the aperture, in such a way that the radius of the circle is directly proportional to the intensity of the modulating signal. The portion of the cathode ray beam passing through the aperture, that is, not intercepted by the diaphragm, is then scanned across the luminescent screen of a television reproducing tube by any of the suitable means well known in the art, in that I prefer to use such an electronoptical system that an image of the aperture is produced upon the fluorescent screen so that a luminescent spot of varying intensity but of constant size is obtained.

For use in television, the radius of deflection of the cathode ray beam with respect to the aperture is preferably zero for picture White and increasingly larger for decreasing brightness of picture intensity, and finally sufliciently large to intercept all beam electrons by the diaphragm in case of picture black. In so-called negative polarity of transmission of television signals wherein decreasing brightness is represented by increasing carrier amplitude, it is possible to apply th intermediate frequency in a television superheterodyne receiver directly to one of the deflecting systems and to the other after shifting the intermediate-frequency phase through an angle of 90 degrees. It maybe seen that the correct polarity of deflection is then obtained. In the case of so-called positive transmission, however, where increasing picture brightness is represented by increasing carrier amplitude, it is necessary to reverse the polarity of transmission before applying the signal to the deflecting system. In order to do this I may prefer to produce the rectified television signal with such a polarity that increase in amplitude represents decrease in brightness, and to then modulate this signal on an auxiliary carrier of considerably higher frequency. This carrier is then directly applied to one of the deflecting systems, and after a phase shift through an angle of 90 degrees to the other deflecting system.

My invention will now be explained in detail in connection with the drawing.

Fig. 1 shows a sectional view reduced to the simplest terms through the modulating system only, omitting all other elements of the tube which are well known to those skilled in the art. Fig. 1 shows a section of what may be the neck of a cathode ray viewing tube for television provided on the left hand with a thermionic constant source of electrons and a fluorescent screen at the far right-hand end of the tube. Electrostatic or electromagnetic focusing and deflecting means for the modulating portion of the beam may be provided inside or outside of the tube on the right-hand side of the modulating device shown in the drawing. The boundary rays of a cathode ray beam of elementary cross-section produced by a constant source of electrons are indicated by lines I, as is well known in the art. Two sets of electrostatic deflecting plates 3 are mounted at right angles to each other. A metal body I is provided with an aperture having a short wide-angle conical portion facing the cathode and a slender long conical portion facing the side remote from the cathode. Body I may be attached to a metal cylinder 2 provided with cooling ribs, as shown in the drawing,

which may constitute a portion of the tube neck, whereby both ends of cylinder 2 are sealed to the glass portion of the cathode ray tube. In operation the cathode ray beam of elemental cross-section is directed upon the aperture in body I, which is at a positive potential with respect to the electron source. Signals of the aforementioned character are applied to the two sets of deflecting plates 3, whereby the cathode ray beam is deflected around the aperture in body I causing a certain portion of the cathode ray beam to be intercepted by body I, while a portion in accordance with the signal is allowed to pass through the aperture. The slender conical portion of the aperture prevents electron reflections and secondaryemissicn from body I.

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An electron-optical image of the aperture is produced upon the fluorescent screen of the cathode ray tube and is scanned across the same in a method and by means well known in the art. Metal cylinder 2 in conjunction with the cooling ribs provides for cooling of body I by radiation of the heat produced by electron bombardment.

Fig. 2 shows a different type of aperturecarrying body which may be a hollow figure of rotation made of metal which may be provided 'with tubulation I through which a cooling medium is caused to flow in the direction indicated by the arrows.

Fig. 3 schematically shows the circuit for operation of my invention. Two sets of electrostatic deflecting plates I 3, I 4 and I5, I6 are provided. .A square aperture I9 in the diaphragm is provided, through which the desired portion of the beam current is allowed to pass. Plates I3 and I4 are connected across resistor 5, while plates I5 and I6 are connected across condenser 5. Resistor 5 and condenser 6 are con-v nected in series. The carrier signal modulated by the picture signal is applied at terminals 8. The voltage produced across resistor 5 and applied to plates I3 and I4 will have the same phase as the signal applied at 3. The voltage produced across condenser 6 and applied to plates I5 and I5 will be shifted through an angle of 90 degrees with respect to the signal applied at 8. Thus, for picture black, at which the signal applied at Ii is to be a maximum, the cathode ray beam will be guided about the aperture along the circle indicated by numeral II. All electrons will be intercepted by the diaphragm and none will pass through aperture l9 so that no light is produced on the fluorescent screen. For picture white, which is to be represented by minimum or zero signal at 8, the cathode ray beam becomes either totally undeflected or deflected only to such a degree that it passes through area I8, which falls entirely within aperture I9, so that the entire emission is allowed to pass through aperture I9, arriving at the fluorescent screen and producing a light spot of maximum intensity.

As mentioned above, for so-called negative polarity of television transmission the intermediate frequency produced in a superheterodyne television receiver can be directly applied after amplification if necessary to terminals 8. This is not possible for positive polarity of transmission, and Fig. 4 shows an arrangement for conversion of the polarity of the signal. In Fig. 4 the intermediate frequency Z which may have the wave shape shown in Fig. 5, is rectified in stage 9, which may consist of a second detector, as is well known in the art. In Fig. 5 the direction of the arrow indicates increase in picture brightness. Fig. 6 shows the rectified picture signal. Thi picture signal is then applied to an inverter stage II) which may consist of any of the suitable means well known in the art. The output of this inverter stage is shown in Fig. '7, wherein the change of the direction of the arrow may be noticed, indicating the change of polarity of thesignal. This signal is modulated upon an auxiliary carrier of considerably higher frequency in modulator stage II, and a signal as shown in Fig. 8 is then produced across resistor IS. A portion of this signal is then fed to the circuit, as shown in Fig. 3, which is symbolically indicated by numeral I2 in Fig. 4.

While I have described my invention in connection with a cathode ray tube for reproduction of television images and in combination with a superheterodyne receiver, I do not wish to be limited to any such apparatus inasmuch as it is quite evident that my invention is applicable to various other types of apparatus within the scope of this invention. I also do not wish to be limited to electrostatic means of beam deflection in my modulation device inasmuch as magnetic deflection of the electron beam is deemed a full equivalent thereof.

What I claim is:

1. Method of modulating a cathode ray beam comprising the steps of generating a beam of constant intensity, deflecting said beam in one direction in accordance with a signal of varying amplitude, shifting the phase of said signal through a phase angle of substantially 90 degrees, deflecting said beam in the direction substantially perpendicular to said first-named direction in accordance with said phase-shifted signal thereby to cause said beam to describe a substantially circular path having a varying radius proportional to the amplitude of said signal, and intercepting a portion or" said deflected beam proportional to the radius of said circular path.

2. Method of modulating a cathode ray beam in accordance with a signal of varying amplitude comprising the steps of generating a beam of constant intensity, modulating said signal upon a carrier signal, deflecting said beam in one direction in accordance with said modulated carrier signal, shifting the phase of said modulated carrier signal through a. phase angle of substantially 90 degrees, deflecting said beam in the direction perpendicular to said first-named direction in accordance with said phase-shifted modulated carrier signal thereby to cause said beam to describe a substantially circular path having a varying radius proportional to the amplitude of said signal, and intercepting a portion of said deflected beam proportional to the radius of said circular path.

3. In a cathode ray tube including a source for producing a beam of electrons of elemental crosssection and of constant intensity, means for deflecting said beam in two directions mutually perpendicular, and a member suitable for intercepting electrons possessing an electron-permeable portion of defined area in alignment with said source and said deflecting means, said beam being directed upon said electron-permeable portion when undeflected, the method of modulating said electron beam in accordance with a signal of varying amplitude comprising the steps of generating a beam of constant intensity, modulating said signal upon a carrier signal, deflecting said beam in accordance with said modulated carrier signal, shifting the phase of said modulated carrier signal through a phase angle of substantially 90 degrees, deflecting said beam in the direction perpendicular to said first-named direction in accordance with said phase-shifted modulated carrier signal, thereby to cause said beam to describe a substantially circular path oscillating back and forth across the border lines of said electron-permeable portion in accordance with the amplitude of said signal and utilizing that portion of said deflected beam passing through said electron-permeable portion.

4. In a superheterodyne receiver for reception of television transmissions of negative polarity including a cathode ray viewing tube possessing a source for producing a beam of electrons of elemental cross-section and of constant intensity, means for deflecting said beam in two directions mutually perpendicular, and a member suitable for intercepting electrons possessing an electronpermeable portion of defined area in alignment with said source and said deflecting means, said beam being directed upon said electron-permeable portion when undeflected, the method of modulation comprising the steps of deflecting said beam in one direction in accordance with the intermediate-frequency picture signal produced by said receiver, shifting the phase of said intermediate-frequency signal through a phase angle of degrees, deflecting said beam in the direction perpendicular to said first-named direction in accordance with said phase-shifted intermediate-frequency signal, and utilizing that portion of said beam passing through said electron-permeable portion to scan the fluorescent screen of said cathode ray tube.

5. In a superheterodyne receiver for reception of television transmissions of positive polarity including a cathode ray viewing tube possessing a source for producing a beam of electrons of elemental cross-section and of constant intensity, means for deflecting said beam in two directions mutually perpendicular, and a member suitable for intercepting electrons possessing an electronpermeable portion of defined area in alignment with said source and said deflecting means, said beam being directed upon said electron-permeable portion when undeflected, the method of modulation comprising the steps of demodulating the received picture signal, inverting the polarity of said demodulated signal, modulating an auxiliary carrier signal with said inverted signal, deflecting said beam in one direction in accordance with said modulated auxiliary carrier signal, shifting the phase of said modulated auxiliary carrier signal through a phase angle of 90 degrees, deflecting said beam in the direction perpendicular to said first-named direction in accordance with said phase-shifted modulated auxiliary carrier signal, and utilizing that portion of said beam passing through said electron-permeable portion to scan the fluorescent screen of said cathode ray tube.

6. A cathode ray tube system comprising means for generating a beam of electrons of defined cross-section, a source of electrical signals of varying amplitude, means for shifting the phase of said signals, means for deflecting said beam in one direction in accordance with said firstnamed signals, means for deflecting said beam in another direction in accordance with the phase-shifted signals, thereby to cause said beam to describe curved paths, corresponding points of which having a varying distance from the axis of the undeflected beam proportional to the intensity of said signals, means for intercepting a portion of said deflected beam proportional to the distance of said deflected beam from the axis of the undeflected beam, said last-named means possessing an aperture having an outline similar to the general shape of said paths, and means for utilizing the non-intercepted portion of said beam passing through said aperture.

HANS WERNER PAEHR.

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