US2112378A - Modulating means for cathode ray tubes - Google Patents

Description

March 29, 1938. F, H NICOLL 2,112,378

MODULATING MEANS FOR CATHODE RAY TUBES Filed NOV. 16, 1935 Mmm/nf frm/455 wma M Wm@ Patented Mar. 29, 1938 UNITED' STATES PATENT OFFICE MODULATING MEANS FOR CATHODE RAY TUBES Application November 16, 1935, Serial No. 50,121

In Great Britain November 19, 1934 13 Claims. (Cl. Z50-27) The present invention 'relates to improvements in cathode ray tubes of the hard type.

A cathode ray tube is known which comprises a cathode, a modulating electrode or modulator, one or more focusing electrodes, an anode and a fluorescent screen. Tubes of this kind are used for purposes such as television reception, received picture signals being applied between the cathode and modulator so as to vary the intensity of the ray reaching the screen and thus produce a corresponding Variation in intensity of the fluorescent light emanating from this screen.

It has been found, however, that the modulation characteristic curves of such tubes, that is to say, the curves obtained by plotting modulator volts (relative to the cathode) as abscissae against fluorescent screen current as ordinates, are excessively curved, changes in fluorescent screen current for large changes in modulator volts being disproportionately large compared with the changes in fluorescent screen current produced by smaller changes in modulator volts.

It has also been found that the size of the spot varies with the modulator volts, generally increasing as the potential of the modulator approaches that of the cathode, thus giving rise to loss of detail in the reconstituted picture.

Furthermore, it has been found that it is not easy to secure a high sensitivity of control of respect to the potential of said cathode, and means for maintaining said decelerating and modulating electrode at a potential not more positive than the potential of said cathode.

were accelerated by the first accelerator electrode and thereafter slowed up as they entered a deceleration region inthe neighbourhood of the decelerating and modulating electrode. The electrostatic field existing between the modulating electrode and the second accelerating electrode caused the electrons to be again accelerated towards the screen. The shape and disposition of the electrodes and the potentials applied to them were such that, an increase of potential of the modulator electrode, in the negative sense with respect to the cathode potential, so retarded an increased proportion of the electrons passing from the rst accelerating electrode to the modulator electrode that they did not reach the accelerating iield due to the second accelerating electrode and were reversed and flowed back to the i'lrst accelerating electrode.

With that arrangement however, the sensitivity of control of the ray intensity was not as high as is sometimes desired.

An object of the present invention is to provide an improved cathode ray tube which, when associated with suitable electrical circuits, will 30 the ray intensity overr the usefulworkng range. not only yield the advantages 0f the arrange- This sensitivity is measured by the negative voltment hereinbefore referred to, but will also proage which has to be applied to the modulator to Vide a sensitivity of control of the ray intensity extinguish the ray, the sensitivity being highest which is higher than with known tubes. when this voltage is least, provided that the cur- According to the present invention, in a cath- 33 rent at Zero modulator volts is unaltered, ode ray tube of the kind hereinbefore referred Copending application No. 745,838, led Septo, the decelerating and modulating electrode, tember 28th, 1934, describes an improved cathode which is disposed between the rst and second ray tube of the hard type which, when associated accelerating eletlOdSS, iS DYOVded With a dawith suitable electrical circuits, yields a characphragm having an aperture, the eiiective thick- 40 teristic curve of suitable shape and produces on ness of the material surrounding this aperture the screen of the tube a spot whose size does not substantially exceeding one-tenth of the diamvary substantially with modulation potential. eter of the aperture. The diaphragm may be of In that arrangement there are arranged within relatively thick material, or of thin material the envelope of the tube in the order stated, a shaped to form a flange surrounding the aper- 45 cathode adapted to emit electrons, a first accelture whereby its effective thickness is increased. erating electrode, a decelerating and modulating Alternatively the diaphragm may COIlSiSt f a electrode, a second accelerating electrode and a plurality of thin sheets spaced apart so as to screen, the said accelerating and decelerating form eiectively the equivalent of a thick sheet. electrodes being so arranged that a ray consti- There is less penetration of the anode field 50 tuted by said electrons can pass through them through the thick, 01 multiple, modulator diaon to said screen, means for deecting said ray over said screen, electrostatic or electromagnetic means for focusing said ray in a small spot upon `said screen, means for applying to said accelerating electrodes potentials which.` are positive with phragm than through a thin diaphragm; hence the negative voltage which has to be applied to the modulator to extinguish the ray is less than with a thin modulator diaphragm. However, with many kinds of cathode ray tubes the in- In that arrangement electrons leavingthe cathode clusion of a thick modulator diaphragm would effect little or no increase in sensitivity because it would cause the intensity of the ray at zero modulator volts to fall also. Where, however, a positive electrode is provided between the cathode and a modulator maintained at a potential not more positive than cathode potential, and the modulator is constructed in accordance with the present invention, the intensity of the ray at zero modulator potential is not greatly influenced by the effective length of the aperture in the diaphragm, and the sensitivity of the tube is therefore increased.

Embodiments of the present invention will now be described, by way of example, with reference to the accompanying diagrammatic drawing in which Fig. 1 illustrates a tube having a decelerator provided with a thick diaphragm.

Fig. 2 shows a circuit arrangement for supplying appropriate potentials to the various electrodes of a tube of the kind illustrated in Fig. 1.

Figs. 3 to 8 show alternative forms of decelerator electrode applicable to the tube shown in Fig. 1.

In the embodiment illustrated in Fig. 1, a cathode ray tube suitable for purposes of television reception, comprises an evacuated, sealed glass envelope consisting of a cylindrical portion I haring out into a frusto-conical portion 2. The diameter and length of the cylindrical portion I are about 11/2 inches and 8 inches respectively the diameter of the base 3 of the frusto-conical portion 2 is about 9 inches, whilst the overall length of the envelope is about 19 inches.

Sealed internally to the closed end of the cylindrical portion I is a glass tubular portion 4 which projects internally from and lies wholly within the cylindrical portion I. The tubular portion 4 is about 1/2 inch in internal diameter and 61/2 inches in length.

Disposed within the tubular portion 4 are a cathode heater 4A, a cathode 5, a cathode shield 6, a first accelerator l, a decelerator 8 and a second accelerator 9, these electrodes being arranged within the glass tubular portion 4 in the order mentioned. The decelerator 8 is adapted lto function as a modulator and the second accelerator 9 corresponds to what is sometimes known as the rst anode of the tube. Leads to .the electrodes disposed within the glass tube 4 may be passed through a pinch I 0 sealed into the closed end of the tubular portion I, the leads to the cathode and its heater also serving as supports for these electrodes whilst the cathode shield, the two accelerators and the decelerator yare mounted as sliding ts within the glass tubular portion 4. Leads to the three last mentioned electrodes pass from the pinch I0 through holes such as II, I2 in the side walls of the glass tube 4 and back through holes such as I3, I4 on to the electrodes. The second accelerator 9, about 3 inches in length, is mounted within the tubular portion 4 in such manner that 1 inch or more of the electrode protrudes beyond the tubular portion. If desired a lead may be taken to this protruding portion through the side wall of the cylindrical portion I of the envelope instead of from the pinch I0. All of the electrodes mentioned, except the heater, are of cylindrical shape and are mounted co-'axially within the envelope.

A second anode I5, in the form of a cylinder of 31/2 inches length, is also mounted as a sliding fit within the cylindrical portion I of the envelope in such manner that the second anode overlaps the second accelerator or rst anode by about 1A of an inch.

On the inner side of the base 3 of the frustoconical portion 2 of the envelope is deposited a fluorescent screen I6 of any known or suitable material. Either electrostatic or electromagnetic means such as two pairs of coils I'I, I8 may be utilized to deiiect the ray so as to scan the fluorescent screen I6 and these means may be situated, either inside or outside the tube between the second anode I5 and the screen I6 and close to the second anode.

The. cathode 5 may be of any suitable kind but preferably is in the form of a shallow open pillbox of 1/4 inch diameter, electrons being emitted from the flat circular base which is indirectly heated by means of the heater which lies within the box. The cathode may be coated with an electronV emitting substance such as a mixture of strontium and barium oxides.

The shield 6 is also in the form of a pillbox, of about 1/2 inch length and diameter, having a central aperture I9 in its base and being disposed around the cathode 5 so that the cathode lies close to (but does not touch) the bottom of the shield 6 and emits electrons through the aperture I9 in the base of the shield.

The first accelerator 'I is in the form of a very shallow open pill-box having a thin base; the box is about 2 millimetres in length and 1/2 inch diameter. It has a central aperture 20 in its base which is disposed about 3 millimetres away from the base of the shield 6 and the skirt of the first accelerator is turned toward the cathode 5.

The decelerator and modulator 8 is in the form of a short cylinder, about 1/2 inch length and 1/2 inch diameter, with two thin discs 5U and 5I spaced l mm. apart and having apertures 50 and 5I disposed along its length somewhat nearer the first accelerator 'I than the second accelerator 9. One end of the modulator 8 is about le of an inch away from the rst accelerator l.

The second accelerator or first anode 9 bears two thin discs, the one disposed at the end of the lelectrode lying closer to the modulator 8 having an aperture 22, and the other being disposed about l inch from the opposite end and having an aperture 23.

The second anode I5 carries no apertured discs, and a lead to this electrode may be taken through :the wall of the frusto-conical portion 2 of the envelope.

The apertures I9, 20, 50 and 22 are all 1.9 mm. `in diameter, the aperture 5I in the disc 5I of the modulator 8 is 2.5 mm. in diameter, and the aperture 23 in the second anode I5 is 3.8 mm. in

diameter.

The various electrodes are maintained at their yappropriate potentials by means of the circuit arrangement shown in Fig. 2.

An alternating potential diiTerence, of any convenient voltage, is stepped up by means of a transformer 34 to 5,000 volts and rectified in the device indicated generally at 35. This steady potential is maintained across a potentiometer resistance 36, 31.

A tapping 38 on the potentiometer is connected both to the cathode 5 and to the cathode shield 6, these electrodes being in this manner maintained at about 30 volts positive with respect to earth. Tappings 39, 40 and 4I on the potentiometer are connected to the rst accelerator 1, the' second accelerator 9 and the anode I5, respectively, in such manner that these electrodes are respectively maintained at about 200 volts, 1,000

Volts and 3,600 volts positive with respect to the earth potential.

Where the arrangement is to be used as a television receiver, the received signals are detected and amplified in a device shown generally at 42. 'I'he output from the device 42 is applied across a resistance. 43 one end of which is earthed and the other end of which is connected to the modulator 8.

The modulator 8 derives a negative bias from the high tension supply to the tube, and the ow of picture signal current through resistance 43 is arranged to make the modulator 8 become more positive. The resultant bias is such that the picture signals operate to vary the potential of the modulator 8, relative to the cathode, from zero down to about -12 volts, at which voltage the ray is extinguished.

It is believed that the operation of the. tube is somewhat as follows:-

Referring more particularly to Fig. l, electrons emanate from the cathode 5 with random velocities and directions. The electrostatic accelerating iield existing between the positive first accelerator 1 on the one hand and the earthed cathode 5 and shield 6 on the other hand has the effect of extracting a large number of electrons from the cathode and concentrating these electrons so as to pass through the aperture 2| in the modulator 8.

The modulator is at a negative potential with respect to the first accelerator 'l so that between these two electrodes the electrons are decelerated and a deceleration region is formed close to the fr' apertures 50" and 5|' in the decelerator 8.

The second accelerator 9 is at a high positive potential with respect to the decelerator 8 and the accelerating eld which exists between these two electrodes extends into the deceleration region of the tube. The velocities (in the direction of the axis of the tube) with which the electrons emerge from the cathode 5 are small compared to the axial velocities which they subsequently acquire owing to the electrostatic accelerating eld existing between the various electrodes, and electrons which emerge from the cathode with comparatively large transverse velocities (say not less than 0.2 volt) are to a very large extent ltered out by the modulator 8 so that the electrons drawn from the deceleration region toward the second accelerator 9 have almost the same axial and radial velocities and hence can be brought to a very sharp focus on the iluorescent screen IE. The focusing is effected in the following manner.

Between the deceleration region and the distant end of the second accelerator or iirst anode 9 the electrons tend to diverge owing to the shape ,ofthe electrostatic elds produced between the electrodes. On reaching the comparatively strong electrostatic eld maintained between the adjacent ends of the rst and second anodes, however, the diverging electrons are concentrated or focused back on to the axis of the tube and at the same time accelerated towards and through the second anode I5. The eld between these two electrodes acts upon the diverging cone of electrons in much the same way as a lens acts upon a diverging beam of light, and for this reason the arrangement of first and second anodes may be called an "electron lens. In the present case the cone of electrons which is diverging on entering the electron lens emerges as a converging cone and is brought to a focus upon the iiuorescent screen I6.

It has been found that with this arrangement a spot of very small size can be obtained upon the screen and that changes in size of the spot, with changes in modulator volts, are less than the corresponding changes which have been observed in tubes of known kind.

It is found that, if the potential on the modulator 8 is increased negatively with respect to the cathode, the current iiowing to the iirst accelerator increases, and on plotting first accelerator current as ordinates and modulator volts asabscissae a roughly straight line or" negative slope is obtained over the working range (0 to 12) of modulator volts.

The tube shown in Fig. 1 is similar to that shown in Fig. l of the co-pending application Serial No. '745.838, above referred to, with the exception that in the example shown in the earlier application the decelerating and modulating electrode 8 has a single thin disc having an aperture 1.9 mm. in diameter. With that tube, the negative voltage required to extinguish the ray is 30 volts, whereas, when the modulator is provided with a second apertured disc, as shown in Fig. 1 of the present specification, the voltagerequired to extinguish the ray is reduced to 12. The thickness of the material in which the aperture is formed is in this way effectively increased to l mm. although theactual material of the diaphragm may be thin.

With the particular arrangement of diaphragms described, the shape of the modulator characteristic is not appreciably changed from that with the single diaphragm although its steepness is increased. The focusing is also substantially unaiected. By modifying the sizes of the apertures and their separation, however', certain changes in the shape of the characteristic can be eected.

The apertures in the diaphragms may if desired be made of equal diameter, or the larger diameter aperture in the arrangement first described may be arranged, for some purposes, iurther from the cathode than the aperture of smaller diameter.

A similar effect to that above described can be obtained with a single diaphragm of thin material by increasing the thickness of the material or" the diaphragm immediately surrounding the aperture. Fig. 3 shows an example of a modulator electrode 8a, applicable to the tube shown in Fig. l, in which the diaphragm 52 is provided with a flange or lip 53 around the aperture 52. The lipped portion may be of cylindrical shape, as shown, giving an effect nearly equivalent to that of two apertures of equal diameter, or it may be conical, as shown at 54r in the modulator 3b shown in Fig. 4, or of other flared shape, giving the effect of apertures of different diameter. The behaviour of lipped apertures resembles closely that of apertures in equivalently spaced diaphragrns provided that the distance between such diaphragme (and therefore the axial length of the lipped portion in the diaphragm with a lipped aperture) is not unduly great. The distance between the diaphragms is usually less than the diameter of either of the apertures. The lips may project either towards or away from the cathode and they may flare in either direction.

A further arrangement, which is nearly equivalent to that using multiple diaphragme, involves the use of relatively thick material for the diaphragm, as 55 (Fig. 5).

In Fig. 6, a single diaphragm 56, 1.0 mm. thick,

having an aperture 56 tapering from 1.9 mm. at one face to 2.5 mm. at the other face, is approximately equivalent in its behaviour to the two thin diaphragme shown in Fig. 1. The aperture may also be made of other shapes.

Three or more diaphragms may be used and the apertures therein may be equal to or diierent from one another according to circumstances. In the example shown in Fig. '7, three diaphragms 51, 58 and 5S are used and the aperture 58 in the middle diaphragm 58 is made smaller than the apertures 51 and 59 in the two outer diaphragms. The equivalent aperture can also be made in a single diaphragm of thicker material, the aperture having three portions of cylindrical shape (o1c different diameters corresponding to the apertures in the three separate diaphragms) united by iiaring portions. Alternatively the boundaries of the aperture in the thicker material may be curved as shown in Fig. 8, for example the cross section of the boundaries in planes containing the axis may be part-circular.

When the modulator diaphragm has the form shown in Fig. 8 or the approximately equivalent form which can be made with three diaphragms having the apertures in the outer diaphragms larger than that in the middle diaphragm, the size of the aperture at the middle is preferably selected as small as possible consistent with no decrease in the current in the tube at zero modulator voltage due to the return of electrons from the modulator, and the diameters of the aperture at the faces of the diaphragm are arranged to give maximiun sensitivity and current through the tube.

I claim:

1. Cathode ray tube apparatus comprising a cathode ray tube of the hard type having an envelope, and arranged therein, in the order stated, a cathode adapted to emit electrons, a first accelerating electrode, a decelerating and modulating electrode, a second accelerating electrode, and a screen, the said accelerating and modulating electrodes being so arranged that a ray constituted by said electrons can pass through them on to said screen, means for deiiecting said ray over said screen, force eld means for focusing said ray in a small spot upon said screen and means for applying to said accelerating electrodes potentials which are positive with respect to the potential of said cathode, and means for maintaining said decelerating and modulating electrode at a potential not more positive than the potential of said cathode, characterized in that said decelerating and modulating electrode is provided with a diaphragm having an aperture, the effective thickness of the material surrounding said aperture substantially exceeding onetenth of the diameter of said aperture.

2. Cathode ray tube apparatus as claimed in claim 1, wherein said diaphragm in the decelerating and modulating electrode has a thickness substantially exceeding one-tenth of the diameter of said aperture.

3. Cathode ray tube apparatus as claimed in claim l, wherein said diaphragm in the decelerating and modulating electrode is provided with a ange surrounding the aperture therein.

4. Cathode ray tube apparatus as claimed in claim 1, wherein the cross section of the aperture in the diaphragm of the decelerating and modulating electrode is different at diierent points along the axis of the tube.

5. Cathode ray tube apparatus as claimed in claim 1, wherein the cross section of the aperture in the diaphragm of the decelerating and modulating electrode is larger at the end of said aperture nearer said cathode than at the end of said aperture nearer said screen.

6. Cathode ray tube apparatus as claimed in claim 1, wherein the cross section of the aperture in the diaphragm of the decelerating and modulating electrode is larger at the end of said aperture nearer said cathode than at a point midway between the ends thereof.

'7. Cathode ray tube apparatus as claimed in claim 1, wherein the diaphragm of said decelerating and modulating electrode comprises a plurality of apertured plates located one behind another along the electron path.

8. Cathode ray tube apparatus as claimed in claim 1, wherein the diaphragm of said decelerating and modulating electrode comprises a plurality of thin apertured plates located one behind another along the electron path, the distance between the rst and last of said plates being substantially more than one-tenth of, but not more than, the diameter of the smallest of the apertures in said plates.

9. Cathode ray tube apparatus as claimed in claim 1, wherein the diaphragm of said decelerating and modulating electrode comprises a plurality of apertured plates located one behind another along the electron path, the apertures in at least two of said plates being of different diameters.

10. Cathode ray tube apparatus as claimed n claim l, wherein the diaphragm of said decelerating and modulating electrode comprises at least three apertured plates, the apertures in the outer two of said three plates being'of larger diameter than the aperture in the inner of said three plates.

l1. Electronic apparatus including a cathode ray tube of the hard type comprising an envelope, and disposed within said envelope, in the order named, a cathode, an accelerating electrode having an apertured diaphragm, a modulating electrode having an apertured diaphragm the effective thickness of the material surrounding the aperture therein substantially exceeding onetenth of the diameter of said aperture, but not exceeding the diameter of said aperture, a second accelerating electrode having at least one apertured diaphragm, an anode co-operating with said second accelerator to form an electron lens, a screen and means to apply potentials to the electrodes.

l2. Cathode ray tube apparatus comprising a cathode ray tube of the hard type having a cathode, a i'irst accelerating electrode, a modulator, and a second accelerating electrode, said modulator being in the form of a cylinder of substantially the same diameter as said accelerators, disposed between said accelerators and provided with a diaphragm placed intermediate its ends and having an aperture, the effective thickness of the material surrounding said aperture substantially exceeding one-tenth of the minimum diameter of said aperture, means for maintaining said modulating electrode at a potential not exceeding the potential of said cathode, means for maintaining said rst accelerating electrode at a positive potential relative to said cathode, and means for maintaining said second accelerating electrode at a potential substantially more positive than that of said first accelerating electrode.

13. Cathode ray tube apparatus comprising a cathode ray tube of the hard type having an envelope, and arranged therein in the order stated a cathode adapted to emit electrons, a iirst accelerator adapted to project a beam of said electrons away from said cathode, a modulator having an apertured diaphragm and serving to retard said beam so that a proportion of said electrons, varying with the potential of said modulator, is reversed and caused to flow to said rst accelerator, a second accelerator serving to accelerate away from said cathode the remainder of said electrons, means for focusing the beam constituted by said remainder, and a screen adapted to receive the focused beam, means for maintaining said rst accelerator positive with respect to said cathode, means for maintaining said second accelerator positive with respect to said rst accelerator, and means for maintaining said modulator at a potential not more positive than the potential of said cathode and or applying modulating potential diierences between said modulator and said cathode, the effective length of the aperture in said diaphragm being more than onefth of its minimum diameter but not more than its minimum diameter, whereby the penetration through said aperture of the accelerating iield due to said second accelerator is brought to a minimum consistent with the maintenance of a suicient focused electron beam when the modulator potential equals the cathode potential.

FREDERICK HERMES NICOLLv

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