US2296355A - High frequency oscillator tube - Google Patents

Description

' towards the grid and plate.

Patented Sept. 22, 1942 i 2.296.355 man FREQUENcY oscmm'roa 'man Nyman Levin, Finchley, London. England, assignor to Radio Corporation of America, a corporation of Delaware Application May 20, 1939, Serial No.'274,721

In Great Britain May 23, 1938 2 Claims.

This invention relates to high frequency oscillators and more particularly to electronic oscillator tubes of the type in which oscillations are obtained by causing electrons to pass through an apertured electrode so as'to cause oscillatory fluctations in the energy induced in said apertured electrode as a result of electron movement towards, through, and then away from said apertured electrode.

The object of the invention is to provide r proved electronic oscillators which shall be capasolenoids of diierent shapes for focusing the .electron beam along the axis of the electron discharge device.

IOnev of the best known forms of electronic oscillator is the so-called Barkhausen-Kurz or Gill- Morell oscillator and it will be of assistance inv understanding the present invention to` consider first the -action of this oscillator. The usual arrangement ci Barkhausen-Kurz` or Gill-Morell oscillator comprises a triode whose grid is maintained at a high positive potential with respect to the cathode and plate. In use a stream of` Velectrons is accelerated away from the cathode `Some of the electrons pass right through the grid and are then decelerated, stop, reverse in direction and again approach the grid under the iniluence ofV the grid iield. YOf these electrons a proportion will -pass through the grid in the opposite direction Y and then be decelerated, stopped,'and accelerated back towards the grid by the grid field. Thus an electronic oscillation takes place about the grid and oscillations will be obtained and variation in the energy in the positive grid circuit will occur by reason (it is believed) of fluo tuations in the energy induced in the grid by electrons accelerated towards it, forced through it, and decelerated beyond it. This form of elec.

` through the grid in one direction succeeding in tronic oscillator has, however, two important dey fects:' (a) the application of modulatingpotential to the triode to effect amplitude modulation (Cl. Z50-27.5)

of the oscillations generated produces also a substantial undesired Aeflfect upon the frequency generated, and (b) `the eillciency and available oscillatory power are low., The former defect arises by reason of the fact that the electron velocity in the tube is comparatively low and is, therefore, strongly affected by modulating potentials, and, of course, the periodicity of electron "swing about the grid (the main factor in determining the frequency) is a function of the electron velocity. The cause of the second defect will best be appreciated by imagining the triode to be constituted as represented in the accompanying Fig. 1 by a cathode in the form of a fiat sheet I, a plate in the form of a parallel flat sheet 2, and an open mesh fiat grid of parallel wires 3 mid-way between and parallel to the two sheets and equally positive with respect to both. Now neglecting space charge effects, the equi-potential surfaces in the system will, as represented by the dot and dash lines, run parallel to the three at electrodes. Consider any point P on the cathode which is directly opposite a point in the middle line of the space between two adjacent grid wires. The only electrons from this point which will pass through the grid 3 will be the small proportion following paths lying in a narrow (approximate) cone C represented in broken lines and having its apex at the saidpoint P on the cathode and no wider, at the grid, than the space between the said two adjacent grid wires.` favorably situated points on the cathode for projecting electrons through the grid are those which Aare directly opposite points in a middle line of a space between two adjacent grid wires. Moreover, as regards those electrons in the (approximate) cone which pass `through the grid and which are decelerated, stopped, reversed by the grid-anode iield, it is clear that only a small` proportion of these will pass through the grid on the return journey, for the equi-potential surfaces will cause the return electron` cone to spread out into a wide ang-le (approximate) cone indicated, in part, bythe more divergent dotted lines. Thus the action proceeds, only a small proportion of the electrons 4which pass passing through the grid in the next, oppositely directed swing It is for this reason that the `efficiency and available power are so small in known Barkhausen-Kurz and Gill-Morell oscillators despite attempts to increase'cathode emission and to use open mesh grids. Hence, the

proportion of the ltotal cathode emission a`ctu` The mostV i the cathode.

ally contributing to the oscillation remains small.

The present invention seeks to avoid the above described defects and limitations.

According to this invention an electronic oscillatorcomprises means for generating a sharply dened beam or jet of electrons, an apertured electrode or electrode system, means for causing the electrons in the beam to pass back and forth a, plurality of times through the aperture (if there is only one) or (preferably) to pass in succession through the apertures (if there is a plurality) and means for re-focussing the beam in the plane of the aperture, or each aperture (as the case may be) each time it passes through. It is not considered advisable to pass the beam back and forth through the same aperture each time as the high space charge due to the congeneration centration will tend to destroy the focus. It is' careful construction and design, it is possible to obtain a large number of transits before the focus becomes so wide as to result in no appreciable gain in efliciency.

In the embodiment illustrated in Fig. 2 there is employed an electron beam projection system consisting (in the order stated) of a narrow cylindrical cathode K, a modulating or intensity control electrode G, an accelerating anode AA and a concentrating electrode S. Eachof the last three electrodes is in the form of a slotted plate with a narrow elongated slot corresponding approximately with the length and diameter of The three plates are adjacent one another and parallel to one another and the cathode, the slots being aligned with one another and with the cathode. A slotted anode SA is mounted parallel -to the three slotted electrodes already mentioned, the distance of this anode from the concentrating electrode S being relatively great as compared to the distance of the concentrating electrode from the accelerating anode AA. The slotted anode is provided with a plurality of similar slots ASI, ASZ, AS3,

vand so on, which are parallel to one another and one `of whichthe end` one ASI in the construction now being described-'is substantially aligned with the slots in the three electrodes G, AA, S, adjacent the cathode. Each slot in the slotted anode is a little longer than and somewhat wider than the length and diameter (respectively) of the cathode. It will be appreciated that the dimension of the slotted anode in a direction at right angles to thelengths of the slots therein is a great `deal more than the corresponding dimensions of the other three slotted electrodes for the slots in the slotted anode are equally spaced wel] apart. In other words, the slotted anode projects well to one side of what may be termed the electron gun. Insulated -from the concentrating electrode S and in the same plane therewith is an additional plate-like electrode AP which extends over the length of the projecting portion of the slotted anode. On the opposite suitable positive electrode.

side of the slotted anode, i. e. on the side thereof remote from the said additional electrode AP is a further plate-like electrodePP yof about ther same sizeas the slotted anode and parallel thereto, said slotted anode SA being equi-distant from the additional electrode AP and the further electrode PP. A series of magnetic focussing elds each running parallel to the directions of length of the slots, is generated by means of magnets the pole pieces of which are indicated in dotted circular lines, so as to thread the discharge space between the further and additional electrodes,

there being as many fields as there are slots in the slotted anode, each threading the space in the neighborhood of a slot. Any two adjacent fields are in opposite directions. These ilelds may be generated by a sectionalized winding the sections beingalternately reversed and shielded from one another by soft iron dividing plates. The slotted anode SA may be either positive or negative with respect to the additional and further electrodes; in the former case, the two latter electrodes may be cathode potential, while in the latter case, they must be positively biased.

In use, vwith the above arrangement, a sheet like beam of electrons is projected from the projection system K, G, AA, S, through the firsts lot ASI in the slotted anode and then bends round more or less cycloidally as indicated by the broken line, to pass through the second slot ASZ after which it again bends back more or less cycloidally to pa'ss through the third slot AS3 and so on, passing through all the slots in succession until it is ilnally taken up on a The magnitude and electrostatic conditions are such as to give a series of beam foci one at each slot in the slotted anode, and accordingly high efficiency is obtained as compared to a normal Barkhausen- Kurz or Gill-Morell oscillator tube. This arrangement may be modied by reflecting the beam electrostatically, instead of magnetically, to cause it to travel to and fro through the successive slots in the slotted anode.

In a modiiicationv shown in Fig. 3, purely electrostatic focussing without reflection is employed. Here there is an electron beam projection system, as above described, and comprising a cathode K, a modulating eletcrode G, an accelerating anode A A, and a concentrating electrode S is arranged to face the first, SAI, of a series of slotted anodes SAI, SA2, each in the form -of a plate with a slot corresponding in size to the cathode. The slotted anodes are parallel and behind one another with their slots aligned with the slots in the electrodes of the beam projection system. These anodes are equally spaced by a distance equal to lthe distance between the first of them, SAI, and the concentrating electrode S. The discharge space between the concentrating electrode and the first slotted anode is surrounded by two tubular electrodes ITI, IT2, placed edge to edge and a similar pair of electrodes 2TI, 2T2, 3TI, 3T2, surrounds the space between each successive pair of slotted anodes. That one of each pair of tubular electrodes nearer the cathode is maintained at cathode potential and the other at slotted anode potential so that ing force on the electron beam. Thus the beam'.` passes in succession through the slotted anodes-' being brought to"' a focus at each slot. The crosssectional shape of thiswhole electrode system may take any of a variety of forms, e. g. circular or rectangular. In the Yformer' case the tubular electrodes would, of course, be cylindrical and in the latter case rectangular. If desired,l the slotted intermediate plates SAI, SA2, may be at cathode potential, the beam still being focussed at each slot.

The embodiment of Fig.-3 may be modified by replacing the electrostatic lenses-the pairs of tubular electrodes-by short magnetic coils outside the envelope housing the electrode system, or by a long uniform magnetic coil outside the envelope andeltending the whole length of the tube, as in Figures 4 and 5 respectively. In the former case the coils can be corrected for rotation of the line focus by the methodwell known per se-of reversing the current in the second half of each coil. Alternatively the slots in the slotted anodes 'may be not aligned but rotated, from anodel to anode, according to the rotation produced by each coil.' In the latter case the anodes must be placed at the nodes of the beam, the current through the coil being such as to produce the number of nodes required. In thisv arrangement no rotation is produced. In both these cases a decelerating electrode should be provided mid-way between the slottedy anodes. These decelerating electrodes may be in the form of slotted plates, the slots being wider than those in the slotted anodes, or in the form of open ended cylinders or rectangularly sectioned tubes.

I claim: 1.4 An electron vdischarge device having a cathode for supplying electrons and means adjacent the cathode for focusing the electrons into a well dened beam, a plane electrode spaced from said cathode and having a plurality of apertures therein, said electrode being4 positioned with one of said apertures in alignment with said electron beam, andan electrode positioned in spaced relationship on each` side of said apertured electrode,

and means for inducing magnetic elds through said apertures and parallel to the plane apertured electrode.

2. An electron discharge device having a cathy ode fol,I supplying electrons and means adjacent the cathode for focusing the electrons into a well defined beam, a plane electrode spacedfrom said cathode and having a plurality of apertures therein, said electrode being positioned with one of said apertures in alignment with said electron beam, and an electrode positioned in spaced relationship on each side of said apertured electrode, and electromagnetic means for inducing loppositely directed magnetic fields within rsaid apertures, said oppositely directed elds alternating along the length of said apertured electrode. l

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