US2251951A - Vacuum tube for high frequencies - Google Patents

Description

AugQ 12, 1941. G. E. PRAY ET AL 2,251,951

I VACUUM TUBE FOR HIGH FREQUENCIES I Filed sept. 224, 1958 4 sheets-sheet 1 ATTORNEY Aug. l2, 1941. V G. E. PRAY ETAI. I 2,251,951

` VACUUMTUBE FOR HIGH FREQUENGIES Filed Sept. 22,- 1938 4 Sheets-Sheet 2 INVENTORS G. EMERSON PRAY BY $.RlCCOBONO ATTORNEY Aug. 12, 1941.

G. E. PRAY ET AL VACUUM TUBE FOR HIGH FREQUENCIES Filed sept. 22, 1938 l 4 Sheets-Sheet 5 INVENToRs G. EMERSON PRAY BY s.mccoeoNo ATTORNEY Aug.12,1941. @.E. PRAY ETALd 2,251,951

VACUUM TUBE FOR HIGH FRQUENCIES i Filed Sept. 22. 1938 4 Sheets-Sheet 4 INVENToRs G, EMERSON PRAY BY s RlccoBoNo ATTORN Patented Aug. 12, 1941 A VACUUlJI TUBE FR HIGH FREQUENCIES George Emerson Pray and Sebastian Riccobono, Washington, D. C.

Application September 22, 1938, Serial No. 231,215

(Granted under the act of March 3, 1883, as amended April 39, 1928; 370 O. G. 757) 10 Claims.

This invention relates to novel means for effecting amplification or detection of radio waves of very high frequencies and means for generating oscillations of like frequencies..

Among the several objects of this invention are:

To provide vacuum tubes in which cathodegrid transit time losses are reduced to a minimum;

To provide vacuum tubes having nearly uniform path lengths for electrons in the cathodegrid space;

To eliminate eddy currents in the grid structure;

To provide vacuum tubes with high shunt input impedances;

To provide vacuum tubes with low inter-electrode capacities;

To provide, in vacuum tubes, one or more beams of electrons, controllable by potentials applied to the tube electrodes. l f

In the drawings:

Figs. 1 and 3 are respectively a transverse section and a longitudinal section of a commercial tube of acorn type, on the lines I l, Fig. 3 and 3 3; Fig. 1, respectively;

Fig. 2 is a diagrammatic representation of the cathode and grid arrangement of the tube shown in Fig. 1;

' Figs. 4 and 5 are a transverse section and a longitudinal section, on the lines 4 4, Fig. 5, and 5 5, Fig. 4, respectively, lshowing aduc-diode according to the present invention;

Fig. 6 depicts schematically the tube of Fig. 4 used in a detector circuit;

Fig. 7 illustrates the tube of Fig. 4 used as a mixer in connection with a local oscillator;

Figs. 8 and 10 are a transverse section and a longitudinal section on the lines 8 8, Fig. 10, and IIJ-IIJ, Fig. 8, respectively, of an embodiment of the present invention wherein two anodes are utilized and the electron streams are restricted by focusing plates;

Figs. 9 and 11 illustrate the electron flow in the tube of Fig. 8 under changed conditions of control electrode potential;

Fig. 12 is in general similar to Fig. 8 but illustrates the use of a double wire control electrode;

Fig. 13 is an elevational View of the double control electrode in Fig. 12;

Figs. 14 and 15 are similar to Figs. 12 and 13 except 4that the control electrode is a grid structure similar to that shown in elevation in Fig. 15;

Fig. 16 illustrates the use of tubes such as those shown in Figs. 8 to 15 used. as a grid-leak detector, while Fig, 17 shows the same tube as a radio frequency amplifier;

Fig. 18 illustrates a form of our invention wherein multiple control electrodes are used and Fig. 19 is a diagram showing a circuit employing the tube of Fig. 18;

Fig. 20 shows a form of our invention using multiple control grids and a focusing plate to direct all the electrons to a single anode;

Fig. 21 utilizes the same type of focusing plate as does Fig. 20 but has two separate anodes each with its individual control electrode, for use in push-pull operation; Fig. 22 discloses a system utilizing a tube of the type shown in Figs. 8 to 15 wherein modulating potentials are impressed between the cathode and the delector electrodes.

For amplication, detection, or oscillation at frequencies of the order of 1GO megacycles per second and upward, the vacuum tube inputl impedance and the cathode to grid electron transit time are the most important factors limiting the eiciency of the tube or circuit. If the input elements of the tube present a low capacitive reactance, then the resonant input circuit must have a low value of inductance, with consequent sacrifice in the resonant gain of the circuit. If the input elements of the tube also presenta low shunt resistance, the resonant impedance of the circuit is greatly reduced, with further loss in sensitivity and loss of selectivity.

This low shunt resistance is due to transit time effect as is well known in the art. In explanation, we will consider a vacuum tube triode, with an electron emitting cathode or filament, an anode whose potential is positive with respect to the cathode, and a control electrode or grid Whose potential is varied sinuscidally from positive to negative with respect to the cathode. The cathode will emit a supply of electrons with nearly zero initial velocity, building up a space charge layer around the cathode. As electrons are drawn from the space charge layer, they are replenished by further cathode emission. Since the anode is maintained positive, the potential gradient in the tube places the space charge layer so close to the cathode that it may be considered as being at the surface of the cathode. Then an electron which receives velocity due to positive grid or anode potentials must traverse the distance from the cathode to the grid before the grid current is affected, and from cathode to 'anode before the anode current is affected. The grid potential is always low compared to the anode potential, so the velocity imparted to an electron in the cathode-grid space is very small compared to the velocity in the grid-anode space. Thus, the cathode-grid transit time is many times greater than the grid-anode transit time for a normal spacing of the electrodes, and is the important factor to consider.

In the vacuum tubes commercially available for use at frequencies above 100 megacycles, with rated negative grid and positive anode (or screen if using a tetrode or pentode) potentials, the time required for an electron to go from the cathode or iilament to the grid is of the order of -9 seconds. At low frequencies this time is negligible compared to the period of one oscillation, but as the frequency is increased this transit time becomes a larger portionA of an oscillation. Thus when the grid is driven positive an increase of electron flow is induced, but no grid current flows until these electrons reach the grid. Then as the grid is driven negative,

some of the electrons accelerated during the posi-l z' ytive portion of the cycle will ystill produce current in the grid until the grid bias becomes suf- `ficiently negative to overcome the velocity of these electrons, delecting them toward the anode or turning them back toward the cathode. Thus the grid and anode currents are shifted in phase fromtheir normal relations to the voltage, this phase shift being called the transit angle. 'I'he component of current in phase with the negative grid voltage'has theV effect of greatly reducing the input impedance of the tube and consuming powerfrom the input circuit.

`The`-distinctive features of the present invention will be made clear by considering a well known commercial tube, the acorn type triode, shown in Figs'. l, 2 and 3. The glass envelope 22 encloses the electrode elements comprising the indirectly heated cathode V23, the grid 24 and anodeZ 5. The leads 2'6 are for the cathode heaterand 521 is to theremissiveY coating 28 on the cathode. It will be observed that the cathode 23 is cylindrical while the grid 24 is oval, which results inV great differences in the pathsy of electrons from the Icathode 2,3 to grid 24 as is indicated by the dotted lines to the points A, Band f Cin Fig; 2.

Emission of electrons from the coating 28 takes place in all directions perpendicular to the cath,- ode, the density being slightly greater over the shorter paths dueto the higher voltage gradient. However, the shorter paths occupy a very small angle 4in comparisonwith the total 360 degrees around the cathode, so the greater percentage of electrons will travel over paths longer than to points `A or B. Thus the transit time for most of the .electrons is great compared to that for Y the points A and B. Consequently, there exists aphasendiiference in the current to the various points of the grid structure, resulting in eddy currents in the grid Wires and consuming energy which must besupplied from an external source. This effect reduces theshunt input impedance of the Vtube to a value lower than it would other-Y wise have. Hence, it isnecessary that the radio frequency input circuit supply the power consumed by the normal transit time losses, by additional large` transit time losses to grid structureV relatively far from the cathode, andY by grid eddycurrents dueto thephase difference between the abovelosses.

YThe electrode materials Vused in our novel tubes maybeany of those well known in the art as suitable for the various purposes, such as tungstenror nickel alloys for grids or control elec-` trodes; molybednum or nickel alloys for anodes,

Suppressors, or focusing electrodes; nickel alloy sleeves coated with oxides of barium, strontium carbonate or thorium for cathodes, and tungsten for heaters. All the electrodes are normally supported and their spacing kept constant by means of mica discs at the ends of the electrodes. 'As hereinafter used, the term strip electrode is toY be understood as `designating an electrode whereof the length is notably greater than the Width and, if transversely curved, does not subtend more than a few degrees of angle.

The embodiment of our invention shown in Figs. 4 and 5 is a duo-diode, intended for operation as a detector or rectifier at Very high radio frequencies. Within the glass envelope 29 is disposed the cathode support 30 having electron emissive bodies 3| and 32 on diametrically opposite sides thereof and facing the emissive bodies are the electrodes 33 and 34. The electrodes 33 and 34 are placed very close to the respective emissive bodies, the spacing between an electrode and its coacting emissive body being but a few thousandths of an inch. The areas of the surfaces of electrodes 33 and 34 are substantially equal to the areas of the respective coacting emissive bodies 3l and 32 and the surfaces of the anodes are substantially uniformly spaced from the surfaces of the emissive bodies, whereby electron paths of uniform length are' provided between the emissive bodies and the electrodes. The electrodes 33 and 34v may have widths of il@ of an inch and a length of 1A; of an inch which gives a low input capacitance on the order of 0.35 micro-microfarad which gives satisfactory operation atA frequencies above 500 conductive material having low resistance connectecl at their end remote from the tube by 4a bar 31, with a conductive tuning slider 38 contacting both bars. The bar 31 is provided to reduce dead end losses and the slider 38 maybe moved-to resonate the frame, as is well known in this art. Alvariable capacitance 39 may be connected across the frame to provideadditional tuning if desired. The input leads to slider 38 are designated by 4t and 4| and by means thereof the alternating potentials are applied to electrodes 33 and 34. The cathode vbodies 3l and 32 are connected through a suitable resistor 42 to the slider 38 and the output leads 43 are connected to the opposite terminals of resistor 42 which, may be shunted by a capacitance 44. This the radio frequency signals between ' electrodes 33 and 34 being substantially 180 degrees out of 'phase with each other, and the demcdulated output appears across the output impedance 42, 44. The resonant frame is found to be more advantageous than the conventional inductance for` frequencies above 200 megacycles per second.

y Fig. '1 depicts our duo-diode used as the first detector or mixer in a superheterodyne circuit. 'Ihe circuit elements are the 'same as in Fig. 6 except that the output impedance is hereY a resonant circuit comprising aninductance 45 and a capacitance 46, the output energy being taken off throughvinductance 41 coupled to inductance 45. The output impedance is resonatedto a desired intermediate frequency derived from beating the output of. local oscillator 48 with the input radio frequency.

Figs. 8 and 10 illustrate our invention embodied in a novel type of electron beam tube. Mounted Within the glass envelope 29 are the cathode support 49 bearing activated coating 58, two anodes and 52 on diametrically opposite sides of the cathode, single straight wire control electrodes 53, 54 respectively disposed between the anodes and the cathode, and the focusing platesl 55. The emissive coating on the cathode may completely encircle the cathode support or may be of the form shown in Fig. 4, and the anodes 5I and 52 may be either flat strips or transversely curved to be concentric with the cathode. The control electrodes 53 and 54 are parallel to the cathode and. disposed in the electron stream between the cathode and the anodes. Focusing plates 55 may be flat rectangular plates and are disposed parallel to the common axial plane of the cathode and the other electrodes. As in the embodiment above described in connection with Fig. 4', the electrodes are placed very close together. The anodes 5l and 52 are maintained at a relatively high positive potential while the focusing plates 55 are maintained at a negative potential of 0.5 to 5.0 volts, depending upon the spacing from the other elements, thus repelling the electrons that leave the cathode and restricting the electron stream to a. narrow beam not substantially wider than the Width of the anodes. The control electrodes 53 and 54 have impressed upon them radio frequency potentials from an external source, such as a resonant circuit, to change the potentials of the control electrodes to be alternatively positive and negative with respect to the cathode and thus the electron stream is directed rst to one anode and then to the other as depicted in Figs. 9 and 11.

The tube shown in Fig. 12 is similar to that in Figs. 8 and 10 except that the control electrodes 53' and 54 are made up of two parallel straight wires, as shown in elevation in Fig. 13.

Figs. 14 and 15 show a further modification wherein the control electrodes, designated 56 and 51 consist of two parallel wires connected together by transverse conductive members, thus forming a fine grid structure, shown in elevation in Fig. 15.

In the operation of the'tubes depicted in Figs. 8 to 15, the anodes 5| and 52 will always attract the electrons while the focusing plates 55 will always repel them, thus forming the electron stream into a definite beam. When the control electrode 54 is positive and 53 is negative there will be a large flow of electrons to anode 52 and very little electron flow to anode 5l and when the radio frequency -potentials on the control grids 53 and 54 are reversed, as shown in Fig. l1, the electron stream will be diverted to electrode 5| and there will be extremely little current passing to anode 52. As the control electrode potentials are varied the currents in the tube will vary and thus it is seen that a push-pull triode action is obtained with short and relatively uniform electron paths, and with stray fields and large transit time losses eliminated. The form of control grid shown in Fig. 13 exerts more eilicient control over the electron flow than does the F single wire shown in Figs. 8 and 9 while the grid structure depicted in Fig. 15 is still more efcient and results in higher mutual conductance and greater voltage gain within the tube.

Fig. 16 illustrates a tube of the type shown in Figs. 8 :to 15 used as a grid-leak detector. Grid resistor 60 on the order of 26 megohms has been satisfactorily used in this circuit, while the plate load resistor 6-I may be of the order of one megohm. The input radio frequency is applied to control grids 53 and 54 by means of a resonant frame 62 similar to that shown in Figs. 6 and 7 while the anodes 5I and 52 have a common output circuit that includes resistor 6I, the output energy being taken off between the terminals 63. The potentiometer 64 applies suitable negative potential to the focusing plates 55. Due to the des-ign of these tubes, the anode impedance is inherently high, values of 100,000 ohms to 1.5 megohms having been obtained. With equal energy inputs, the tube and circuit of Fig. 16 shows a voltage sensitivity 10 to 20 times that of the circuit of Fig. 6 when operating at a frequency of 500 megacycles per second.

Fig. 17 shows a tube of the type disclosed in Figs. 8, 12 or 14 used as a radio frequency amplier. The control grids 53 and 54 are biased slightly negative with respect to the cathode 50 due to cathode current through resistor 65 the use of such bias tends to increase the cathodegrid transit time, which is undesirable when working near the upper frequency limit of the tube. possible to operate without grid -bias with no damage to the tube, due to the high anode impedance of these tubes. In such a case resistor 65 and the shunting capacitor 66 would be replaced by a conducting wire. It has also been foundpractical under most conditions of operation to maintain the focusing plates 55 at cathode potential, in which case the potentiometer circuit 64 would also be replaced by a conducting wire. The output may be coupled either capacitively or inductively to the load. The antenna indicated at 61 may of course be replaced by the output of a preceding stage.

The circuit of Fig. 17 may be used as an oscillator by applying sufficient anode potential and coupling the resonant frames 62 and 68 by placing them close to Ieach other physically. In some cases it may be desirable to change the circuit from cathode bias to grid-leak bias in the conventional manner.

Fig. 18 shows a method for further reducing the cathode-grid transit time by means of auxiliary accelerating grids 69 and 10, which are maintained at a. slight positive potential with respect to cathode 50. This acceleratesV the electrons from the cathode, some of which strike the grids 69 and 10 but most of them penetrate into the space around control grids 1| and 12, which latter are negative with respect to grids 69 and 10 and also with respect to cathode 50, and so they will retard lthe electrons as they arrive at the grids 1I and 12 and build up a space charge or virtual cathode very close to the control grids. By proper adjustment of the direct current potentials on the tube electrodes the virtual cathode may be placed very nearly in the plane of the control grid, thus reducing the cathodegrid time to negligible value. The frequency limit of this tube should be well over 1000 megacycles per second, depending mainly upon the inductance and capacitance of the input electrodes and leads. The tube of Fig. 18 may also be used as a screen grid tube, by utilizing the grids 69 and 10 as control grids and the grids 1I and 12 as accelerating or screen grids? Other grids may be inserted to function as accelerating grids or as Suppressors to suppress secondary However, under such conditions, it is Vtype of tube.

emission from the anodes. Likewise, the tubes shown in Figs. 8, 12 and 14 may be supplied with additional Ygrids for the vpurposes mentioned.

The tube illustrated in Figs. 4 to 19, both inclusive, have been' shown with a common cathode and separate pairs of control, anode and focusing electrodes. It is of course possible to connect each pair of like electrodes together to constitute a tube containing effectively one cathode, one control electrode, two focusing electrodes, one anode, etc.

In Fig. the construction is modified by having a single anode 13, a single control grid 14, a single acc-elerating Vgrid 15 and a single focusing plate 16 that extends around the cathode 50 in spaced relation theretogfrom adjacent one side of anode `13 to adjacent the other side thereof, thus having at least three quadrants around the cathode enclosed by the focusing plate with the anode 13 between the terminiof the plate 1E but outside of 'the space enclosed thereby. While the focusing electrode could be extended to enclose all the electrodes, this is undesirable due to the increase in outputrcapacitance that would result. Operation at high frequency usually makes vit desirable to utilize balanced, or push-pull input circuits, feeding into a balanced tube circuit. In the case of a single sided tube, that is, one having one control electrode and one anode, it is" necessary to use two tubes to obtain a balanced circuit. The connection between the separate cathodes entails a 'length of conductor, which, though short,"presents appreciable inductive reactance to the 'signal grid, allowing a possible large difference of radio'frequency potential between the cathodes and producing losses in the circuit. In thek case of the' tubes shown in Figs. 4 to 19, balanced or push-pull tubes vhave been constructed withva common cathode and double control and anode structures, to obtain greatest efliciency and least input and output capacitances.

The tube shown in Fig. 21 differs from all the preceding types in that the twoy strip anodes 11 and 18`are placed near each other on one Side of the'cathode and utilizeV the same electron stream.' YThe control electrodes 19 and 8E) are respectivelv disposed between and parallel to the anodes 11 and 18 and the cathode 5B. In this tube the electronrstream flows continuously toward the anodes but since the control electrodes V19y and 80 have applied to them radio frequency orpush-pull unit While the electron stream continues to'ow in the same general direction. If desired, accelerating grids may be used in this A single space charge grid or a single screen grid, or both, should work satisfactorily with the double elements of this tube.Y

Electron beam tubes such as above disclosed havejan additional:` feature ofy some value. If a varying'potential be applied between the cathode andthe focusing electrodes, itmay be used to A' control the anode current to some extent. Thus the .tube may be modulated in that manner forY use as a ymixer tube -in a superheterodyne receiver, asa modulated oscillator, or as a modulated amplifier(V l One system for practicing this type of modulation, utilizing a tube such as'those shown in Figs.

8 to 15, is schematically illustrated` ln Fig. 22.

The reference characters that are common to this gure and the preceding figures denote the same elements. A modulating `potential derived from conventional -modulator 90 is applied between cathode 49 and focusing electrodes 55, producing a narrowing and widening of the electron stream which modulates the density of the electron flow impinging upon anodes 5I and 52 Y cathode on diametrically opposite sides thereof,V

a straight single Wire control electrode disposed parallel to said cathode between said cathode and eachsaid anode, and two electron-repelling focusing plates on opposite sides of said cathode much nearer said :cathode than are said anodes and extending parallel to the v common axial plane of said cathode and 'said control electrodes to carry a potential to restrict the movement of electrons from said cathode to either said anode Y to a Zone not substantially wider than said anodes the transverse dimension of each said anode being not greater than the distance between said focusing plates.

2. An electron discharge device, comprising an electron emissive cathode, two narrow strip anodes vdisposed on opposite sides of said cathode with all parts of any longitudinal element` of the surface of each substantially equidistant from the corresponding parts of a longitudinal' element of the surface of said cathode, separate control electrode means disposed between said cathode and each said anode, and two electronrepelling focusing plates disposed on Vopposite sides of said cathodeextending parallel'to the common plane of symmetry through said cathode Y and said control electrode means to carry a potential to restrictrthe movement of electrons from said cathode to either said anode to a Zone not substantially wider than said anodes the transverse dimension of each said anode being not greater than the distance between said focusing plates.

3. An electron discharge device, comprising a substantially straight electron emissive cathode, two narrow strip anodes disposed parallel to said cathode on diametrically opposite sides thereof, a control electrode consisting of two straight parallel limbs disposed parallel to said cathode between said cathode and each said anode, said' limbs being slightly spaced to provide a narrow path for the electron stream, and two focusing plates on opposite sides of said cathode extendingl parallel to the common plane of Vsymmetry through said cathode and said :control electrodes that includes the axis of said cathode, saidV substantially straight electron emissive cathode,

two narrow strip anodes disposed parallel to said cathode on dimetrically opposite sides thereof, a control electrode consisting of two straight parallel limbs disposed parallel to said cathode between said cathode and each said anode, and a plurality of transverse conductive elements connecting said limbs, and two focusing plates on opposite sides of said cathode extending parallel to the common plane of symmetry through said cathode and said control electrodes that includes the axis of said cathode, said plates being adapted to carry a potential to restrict the movement of electrons from said cathode to either said anode to a zone not substantially wider than said anodes.

5. An electron discharge device, comprising a substantially straight electron emissive cathode, two narrow strip anodes disposed parallel to said cathode on' diametrically opposite sides thereof, a plurality of control electrodes between said cathode and each said anode, and two focusing plates on opposite sides of said cathode extending parallel to the common plane of symmetry of said cathode and said control electrodes containing the axis of said cathode to carry a potential to restrict the movement of electrons from said cathode to either said anode to a zone not substantially wider than said anodes.

6. An electron discharge device, comprising a substantially straight electron emissive cathode, two narrow strip anodes disposed parallel to said cathode on diametrically opposite sides thereof, a plurality of control electrodes successively disposed between said cathode and each said anode, and two focusing plates on opposite sides of said cathode extending parallel to the common plane of symmetry of said cathode and said control electrodes containing the axis of said cathode to carry a. potential to restrict the movement of electrons from said cathode to either said anode to a zone not substantially wider than said anodes.

7. An electron discharge device, comprising an electron emissive cathode, narrow strip anode means having a width not substantially greater than the diameter of said cathode, control electrode means disposed between said anode means and said cathode, and electrostatic means to restrict the electron stream between said cathode and said anode means to a Zone not substantially wider than the width of said anode means.

8. An electron discharge device, comprising an electron emissive cathode, narrow strip anode means, a plurality of control electrode means disposed at different distances and in line with each other -between said anode means and said cathode, and electrostatic means -to restrict the electron stream between said cathode and said anode means to a Zone not substantially wider than the width of said anode means.

9. An electron discharge device, comprising an electron emissive cathode, narrow strip anode means, control electrode means between said anode means and said cathode, accelerating electrode means between said cathode and said control electrode means, and electrostatic means having a transverse dimension at least several times the width of the anode to restrict the electron stream between said cathode and said anode means to a zone not substantially wider than said anode means.

10. An electron discharge device, comprising an electron emissive cathode, narrow strip anode means, control electrode means between said anode means and said cathode, accelerating electrode means between said anode means and said control electrode means, and electrostatic means to restrict the electron stream between said cathode and said anode means to a zone not substantially wider than said anode means.

GEORGE EMERSON PRAY. SEBASTlAN RICCOBONO.

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