US2447461A - Resonant cavity circuits - Google Patents

Description

' Aug 1948. A. v. HAEFF RESONANT CAVITY CIRCUITS Original Filed Jan. 18, 1941 4 Sheets-Sheet l I Z1 INPUT OUTPUT ANDREW V. H AEFF Aug. 1948. A. v. HAEFF RESONANT CAVITY CIRCUITS 4' Sheets-Sheet 2 Original Filed Jan. 18, 1941 MOPUWZKU 4 Sheets-Sheet 4 E B r I $15M u IIEFIEIIIIIIIJ- A. VIHAEFF RESONANT CAVITY CIRCUITS Original'Filed Jan. 18, 1941 dO-PUMJJOU nflmu ok fix k E km 3 g a HQ 3% H g. m mm Mm V 5.. 3. mm RN NE. .U @m WM 3110mm ANDREW V HAEFF Patented Aug. 17, 1948 UNITED" Es PATENT, OFFICE REsoNANr CAVITY CIRCUITS Andrew V. Haeff, Washin ton. D. C., assignorto Radio Corporation of America, a orporation 1 Delaware Original application January 18,

1941, Serial a...

375,029. Divided and this application April 27. 1943, Serial No. 484,692

j lClaims. (Cl.178-'44) January 18; 1941, now Patent 2,399,223, issued April 30, 1946,- and assigned to the same assignee as the present application. 1

It has been demonstrated that tubes utilizing conventional grids for controlling current are well adapted for operation at ultra-high frequencies and retain their characteristic advantage ofpossessing high transconductance. However, one of thedifiiculties encountered inoperating amplifying tubes at ultra-highirequemcies is the presence of considerable'loadin in the input circuit which results in an excessive amount of power being required to drive the tube. l his decreases the 'efiective power gain of the tube when operated as an amplifier.

The fundamental causes of high input loading are: (1) ohmic and radiation resistance losses due to high circulating currents in electrodes and leads; (2) electron-loading which results fromthe interaction of the electron stream with the circuit, including degenerative or regenerative effects caused by lead impedance.

In'orderto' reduce ohmic resistance losses it u is necessary ,to use internal leads and external conductors made of high conductivity material and having large peripheries. In addition interelectrode capacitances must be reduced as much as possible in order to minimize circulating currents. To'reduce radiation losses a thoroughly shielded circuit of conventional design or'closed type cavity resonators must be used.

The principal object oi my invention is to pro-- vide an electron discharge device and associated circuithaving-means for substantially reducing or completely neutralizing electron loading when the device is used at ultra-high frequencies.

It is also an object of my invention to provide an electron discharge devicehaving means for minimizing ohmic and radiation resistance losses whenthe device is used at ultra-high frequencies.

A 'still further objectxof my invention is to provide improved forms of resonant cavity tank circuits or resonators suitable for use with ultra high frequency electron discharge devices and means for tuning the same.

The novel features which I believe to be characteristic of my invention are set forth with particularity in the appended claims, but the invention itself will .bestbe understood by reference to the following description taken in connection.

with the accompanying drawing in which Figures 1 and 2 are diagrammatic representations of electrodes and the movement of electrons between the electrodes; Figures 3 and 4 are diagrammatic representations of conventional tubes and meth ods of operating the same; Figures 5 and dare curves representing the relationship of the electron loading (conductance) and the transit time of the-electrons of the tubes'in Fi-guresii and 4; Figures 7 to 10 inclusive ,arediagrammatic representations of tubes and=circuits made accordingv to my invention forpraicticing my. invention; Figure 11 is a longitudinal section of an electron discharge device, made according to my invention; Figure 11a is a section taken along the line Ilia-Ala of; Figure 11; Figures 12. and 13 are.

longitudinal sections ofmodi'fications of an electron discharge device made according to my in vention. 3 V

In order to understand-better the effect of. electronloading, the mechanism of interaction betweenthe electron stream and the electrodes to which circuits may be connected will be reviewed. Consider a system of two electrodes ill and ll as shown in Figure 1. Assume that electrons. travel from theelectrodesllL-which may. bea cathode, to the electrode I I; which may be an anode. ing electron transit an image charge, appearson the electrodes equal in magnitude tothe total charge present at any moment within the interelectrode space. I he division of the image charge. between the two electrodes depends,-in general, upon the instantaneous distribution of charges moving within the interelectrode spaceandupon the configurationof the electrodes. The current induced in an electrode due to motion of a charge is equal tothe rate of time variation of the induced image charge on the electrode dueto the moving charge. The total instantaneous current induced in the electrode by the electron stream will be found by summing. the individual currents induced by all charges moving within the interelectrode. space. If a voltage exists between electrodesv l0 and the displacement current due to the interelectrode capacitance must be also taken into account. i ,1

' Consider now. a three-electrode system formed, for example, by a cathode ID, a control grid l2 and the plate I] of a triode. Two spaces have to be considered. The total current induced in the. intermediate electrode 12 (Figure 2) is contributed by moving charges in both spaces, Ill-I2 and l2-l Land the total current is equal to; the

vector sum of the two currents. The power generated or absorbed by the electron stream within the spaces I 0-! 2 and 12-! I depends upon the respective current, voltage and the phase angle between the current and voltage in each space. Thus the pews: generated or absorbedwitinimthe. spaces lll-i2 audit-41, will be:

' In a more general case, such as a low-u triode,

when there may exist considerable--penetratipn 4 then, under ideal conditions, passes through zero and becomes negative. In the case of circuit shown in Figure 4 the variation of electron loading with transit time will be as shown in Figure 6. Starting with its maximum value at low frequencyythe loading decreases-"with:- increasing frequency:

These curves indicate that for certain values of electron transit angle, that is for certain values of the electric fields from space "FF-f E into space V I0l2, one must also consider direct-interaction into account.

In order to reduce the electron loading .the, total power must be reduced to a minimum;

This can be accomplished bychoosingcurrents,

voltages and their respective phases in-isuch a way that the total power;

W='W1,2+War-}:Wa.-1+- V 1 isa'mininmm; I

In -aconventional negative grid tetrodeopen' ated-at low frequenciestheinputelectrode load-- ing' wili jbe negligibly small" if the. driving voltage is -appIied in a conventional manner-between'the grid and the cathode so--that* the voltage also voltage presentin this'regicn 'and" hence no negative power -is*-develope'din the G- S spacetc' balance the powenabsorbeclin the G-G space; As'the driving frequency is increased-the circuit of" 'Ffgm-e'-- 3 will exhibit electron loading which initially wilI-increase-withfrequency. This loading :is 'due -to-- the-fact that i withincreasing efectron-transit-time with-respect to tr period of thefirivingfrequency the 'amplitudes and phasesoi= currents'in the C-Ga-nd G S spaces' change in such a =manner that the amounts of *power ab sorbedand generated the-two spaces no longerbalance eacn other; For the'case ofa high i control grid when the spacings and DA): voltages aresuch that the=--G'-S" electron transit time isnegligible compared'*' to- 0-6? transit time ananalysis shows that the: electron loading c'enductancel will vary with= transit time as shown in; 'Figurefis Here theordinates of the curve represent the ratio 'G/Gmo where G conductanceof the --grid- G' due to: electron motions and G'mptransconductance of the grid *6- at very: low "orzero frequencyy that is :whemsthe transittimeof the electron is negligilzile in comparison to the time of one cycle of theifrequency 01 the applied :voltage; The ahscissae represent the -ratio: 'r/T,-" that is =the- 'ratio -;of the; transit time of theielectron tok the period ofazoscillatiorr of. the applied alternating :voltage. 'Ihei:electrorr loading: innreasesrapidly) Wiflh'; transit: time.

reaches a maximumiat the-value of; atransit-ti-me 1-. equal 1020.85: of the .csciilationtzperiod I. and

How

,of theratiopi l transit time the loadingrwiik beismall even for conventional input. circuits. However, the values of frequency and opfimtihg voltages for these optimum conditions frequentlytlieloutside the useful operating range of the tube. The tubes could be designed for-this optimum condition but, in general, this may necessitate a compromise, so that high transconductancesmay be pantiy sacrificed: The presentrinvention provides :means' --for vneutralize ing 6136110119 loading: for "at wide 2 range --offre quencies-.- and: operating voltages-r Without. any: sacrifice of the useful characteristics of the-dime,v such as highvtransconductancee I AA general: scheme is :Jbhatt aim addition to. the

driving voltagesappliedzbetweena the cathode and grideawcltage developed between the control.

grid and the screen ofn-suolr' a; magnitude and" phases, as :tm-generateepower win the-grid-screen space and :this :p'owereis, f edrbackrinto the. oath-- odeegridtcincuity sothat; ic-.wil1 balance the i power absorbedrin :ithecathodeegridrspace.

- zip-schematic adiagram of-fsuch: aicircuit is. rep-.

resented dug-Figural 7: Anoimpedancezzais in-.-

tnoducenzbetweenithe screen Sr-andrthegrid ;G of. suchomagnitude eandiphase angle; :that the cur+ rent'z'oes produce a: roltagesvr across this, inreratedsin the G=S space isrthenrfedto the --grid.-; cathode circuit Z1 by means ofamouplingz-circuit Zae The simpedances Z2. and Z2 usually take the iormnofntuned; circuits; and the coupling impede ance :Zlt ;maybe :the. inter-electrode ccapacitance onran auxiliary-couplingelement;i

- A modification Ofcthe circuit shown'im-Fi'gurer'lissrcprtesented :.schematically in; Figure 8, where: the impedance sZzda-showmi-ntroduced .between 0 thescreem-Szand;the-cathode G rather than betweenathescreen- Sr. andrthe, control gridrG. The coupling. betweennthe circuitsziiand Z2 is 'pro-: videdr' by tli'etcontrol-l 'grid to'screencapacitance 011% :can ibe'supplemented byanzam-riliary con-.2

- pling ch'icuiti zm ln FigureszsTandl 8 conven tiimalo output :cireuim: with: output impedancea (Z).- :connect.ed; betweenirthe anode and the screen-.- are shown. H'cwevera; OthEIJtYDGS-"Of output circnits :can': be -usedn sinceethe 'g-input. loading neutralization schemevhereaproposed-inrno -way -dependssupomithe eextractiontof: "energy from the outputvcircuitl 1 Fiancee-.9 'nhows'sschematically 1 the I input loadmementralizationwircuit in combination; with: an inductiveetype-zoutpntucircuitz: Here the output circuitnisrjconnectedibetween-:the :two screening electrodes'si S2,; The suppressor: and current collectmgrelectrodes, represented respectively-by S3: and-coll,- a-realsoishow-nu Figure 10. represents schematicaliy; the input; a circuit arrangement of. Figures 8: in combination-1 with the inductive-. output circuit. In theeabove circuit diagrams oniyxthe essential; 2: circuitsltare-eindicated. Blocking; munding. ;and-o,,by passing condensers which: are used -:foc;providingisolation ,ofelece trodes for D. 0., so that different D.-C. voltages can be applied to difierent electrodes, are not shown.

One practical embodiment of my invention incorporated in a so-called inductve output tube" is shown in detail in Figure 11. Inductive output tubes and their operation are described more fully in my United States Patent 2,237,878, issued April 8', 1941, and assigned to the Radio Corporation of America. Briefly this tube comprises a cathode for supplying a beam of electrons and a collector for receiving the electrons. A modu lating grid is placed adjacent the cathode for modulating the beam of electrons which passes to the collector. Surrounding the beam path is a resonant cavity circuit comprising a hollow memher having a passageway extendin therethrough through which the beam passes. The passageway is provided with a gap lying in a plane transverse to the beam path. As the modulated beam of electrons passes across this gap, energy is transferred from the beam to the resonant cavity circuit-which provides the output circuit for the tube and which can be coupled to a radiator or to an amplifier. 1

Referring to Figure 11, the tube is provided with a concave surface cathode l5 which can be made of tantalum. This cathode is heated by electron bombardment from an auxiliary cathode l6 made for example in the form of a tungsten spiral and surrounded by a focusing shield or cup I! for directing the electrons from the filament to the cathode it. The cathode spiral I6 is supplied with heating current by means of leads l8 and I9 and the main cathode I5 is supported at the end of a tubular member I5 to which the oathode lead is electrically connected. The electron beam is modulated by means of the grid 2| and passes through a pair of screen and accelerating tubular electrode members 22 and 23 separated by gap 24 and the electrons are collected by means of a collector electrode 25 which is provided with a cooling jacket 26 for cooling the collector. The accelerating and screening electrodes 22 and 23 are cylindrical and conically shaped to avoid absorbing electron current from the beam which may tend to spread. The output circuit is of the closed resonant cavity type and is formed by two conically shaped metal surfaces 21 and 2'|"joined at the periphery by a short cylindrical section 211. The gap in the resonant cavity registers with the gap between the accelerating electrode members 22 and 23 to which the conically shaped sides of the resonant cavity are secured and electrically connected.

In order to practice my invention the cathode I5 is mounted in the supporting tubular member 29 of cylindrical form, a collar 28 of insulating material serving to insulate the tubular cathode extension l5 from the tubular'member 29 but permitting capacity coupling therebetween. Thus the leads for the heater and cathode are shielded by means of the tubular member 29 which serves as the inner member of a concentric line circuit. The control grid 2| is supported at the end of the tubular member 30 of cylindrical form surrounding and coaxial with the inner tubular member 29 to form the outer portion of the concentric line circuit, the ends being closed by disc members 3| and 3|. The cathode gridcircuit is formed by the tubular members 29 and 30 which constitute the inner and outer conductors of a concentric line shorted by the closure disc 3|. This cathode-grid circuit, which may be referred to also as a resonant cavity tank circuit, correspondsto impedance z of Figure 9. r The large capacitance between the cathode support or ex-' tension l5 and cylindrical member 29 serves to by-pass radio frequency current from the cathode to the tubular member 29. I

The closure member 3| is provided with an aperture throughwhich the lead wires l8, l9 and 2!! extend and a collar or extension 32 to which the insulating cup-shaped member 33 is sealed and through which the conductors pass and are sealed. The cup-shapedmember 33 hermetically seals the interior of the circuits.

A third tubular'member 34 of cylindrical form is co-axial with and surrounds the other two tubular members- It is provided with closure members 34 and 31, a gap 22 being provided between the closure member 3'! of tubular member 34 and closure member 3| of tubular member 33. The space between the cylinders 30 and 34 forms a resonant space which provides an impedance equivalent to Z2 shown in Figure 9 between the control grid and accelerating or screen electrode 22.

To provide an insulating support between the accelerating and screen electrodes 22 and 23 to which a high positive voltage is applied in operation and the grid 2! to which a negative bias is applied, the cylinder 34 is supported on the wall of the tank circuit by the insulating glass cylinder or collar'35 sealed to the cylindrical collar members 36 and 31 supported on the cylindrical member 34 and the wall 21' of the tank circuit respectively. High capacitance between the end portion 31 of the cylindrical member 34 and the adjacent wall of the tank serves to by-pass high frequency circulating current so as to reduce the radio frequency potentials between the tubular member 34 and'the wall of the tank to a negligible value.- The collector 25 is supported in like manner from the other wall of the tank circuit to which is attached the collar extension 38, the collector cooling jacket being provided with collar extension 39, both sealed to the insulating cylindrical member or collar 40.

Independent tuning of all circuits is provided by means of plunger type condensers, for example the outer tubular member 30 is provided with a collar or extension 4| surrounding an aperture in the outer surface of the tubular member. Sealed-to this extension is a re-entrant insulating tube 42 extending through this aperture and an aperture in the inner tubularmember 29pmvided with an extension 43 surrounding the aperture. This re-entrant glass tube is preferably made of low loss glass or of quartz. The tuning plunger 44 is inserted within the re-entrant insulating tube and may be adjusted by means of the insulating rod 45 attached to the plunger. Varying the position of the plunger changes the capacitance between the adjacent circuit elements and thus affords a means for tuning of, the internal circuits. For tuning the screen circuit the same kind of arrangement is provided at 46, the tubular member'34 being provided with an aperture around which extends collar 4|, the re-entrant glass tubing extending within the cupshaped extension 43' in the tubular member 30-. A like arrangement is shown generally at 41 in the tank circuit.

The coupling between the cathode-grid and grid-screen circuits, which coupling corresponds to impedance Z0 in'Figure 9, is provided by means of closed loop 50, the position of which is adjustable by means of adjusting rod 5|. An extension 48 on the outertubular member 34 surrounds an,

members I36 and I31 connected at their peripheries by means of the ring-shaped member I38. Thus a second resonant cavity is provided surrounding the resonant cavity of the cathodecontrol grid circuit. The accelerating or screen electrode I21 is secured to the wall I31 of the screen electrode-control grid circuit. The resonant cavity output circuit comprises the side wall I31, the side wall I39 and the'outer ring member I40- connected at the peripheries. The ring members I40 and I38 could of course be extensions of each other. The accelerating electrode I28 is connected to and supported by the end wall I39. I

A coupling and tuning of the circuits is permitted in the same manner as in the other modifications of applicant's invention, that is the envelope is provided with a number of re-entrant portions extending through'apertures in the various tank circuits and providing passageways for coupling loops or tuning condensers. The driver circuit is coupled to the grid-cathode circuit by loop I4l extending within re-entrant portion MI". The screen electrode-control grid tank circuit and the control grid-cathode tank circuit are inductively coupled to permit feedback by means of the loop I42 within extension I42. This loop is mounted within the re-entrant portion I42 of the envelope extending through apertures in the two tank circuits. Tuning of the cathode-control grid tank circuit is accomplished by means of the tuning plunger I43 slidably mounted within the re-entrant tube I44 extending through apertures in the tank circuits and the extensions I33 and I34 between which is provided a gap. Tuning of the screen electrode-control grid circuit is accomplished by means of the tuning plunger I45 mounted within the re-entrant glass tube I46. The plunger I45 enters a tubular member I49, which is attached to member I34, and also projects into a tubular well I50 connected to the electrode I21. A similar arrangement is provided for tuning the output circuit, the plunger I41 being slidably supported within the re-entrant tube I48 and coupling extensions II and I52. The output is delivered by means of the loop I41 mounted within the extension I48, which extends through an aperture in the ring-shaped connecting member I43 of the output tank circuit. Heating current is supplied by means of potential source I52 and potential difference for causing bombardment of the rear surface of the cathode I20 by voltage source I54. Grid bias is furnished by means of voltage source I53. The potentials required for the tank circuit and the collector are provided respectively by potential sources I55 and I55.

in Figure 13 for example the resonator comprising the slightly dishshaped or cone-shaped walls I31-I39 closed at their peripheries by means of the collar-like element I and having the axially tubular extension I28, is that in case these resonators become heated during operation and expansion occurs, the walls I31 and I39 will move in the same direction, that is toward the right, keeping their same relative positions and the relative position of the tubular member I28 so that the gap width varies little if any. Thus, although the resonator may be subjectedto temperature changes, its resonant frequency remains substantially constant.

It will be apparent from the above discussion and description that I have provided an electron discharge device particularly suitable for use at ultra-high frequencies since both ohmic and radiation resistance losses due to high radio frequency circulating currents-in electrodes and leads have been substantially eliminated, and because electron loading, which results from-interaction of the electron stream and the circuit, including regenerative or degenerative effectscaused by lead impedance, has also been substantially neutralize. This is accomplished by means of leads and external conductors of highly. conducting material andlarge diameter. The radiation losses are reduced to a minimum by thoroughly shielded circuits comprising closed type cavity resonators. These resonators are provided with novel and effective means for tunmg.

While I have indicated the preferred .embodi ments of my invention of which I am now aware and have also indicated only one specific application for which my invention may be employed, it will be apparent that my invention is byno means limited to. the exact forms illustratedor the use indicated, but that many variations may be made in the particular structure used and the purpose for which it is employed without departing from the scope of my invention as set forth in the appended claims.

What I claim as new is:

1. A resonator including a hollow conducting member having an aperture, a conducting member supported on the wall of said resonator and surrounding said aperture, a cup-shaped conducting member supported from a portion of the inner wall of said resonator and registering with said tubular member and said aperture and a reentrant member of insulating material sealing said aperture and extending through said tubular member and into said cup-shaped member, and a plunger-like conducting member extending within said re-entrant member of insulating material and movable longitudinally of the tubular member and cup-shaped member.

2. A first resonator including a first hollow conducting member having an aperture therein, a second resonator including a second hollow conducting member surrounding said first hollow conducting member and having an aperture registering with the aperture in said first hollow conducting member, a tubular shield element extending from the wall of the first hollow conducting member through the aperture in the second hollow conducting member and surrounding said aperture, and means extending through said tubular shield element into the interior of said first resonator for coupling purposes, a first tubular member within said first resonator supported on a wall portion of said first resonator and extending toward the opposite wall and a 11 second :tubularmember: positioned on the oppo site 'wall -o't' said first resonator and registering: with said firsti tubular member, and conducting means: movable longitudinally of. said tubular members for tuning said' first resonator:

A- first resonator including afirst hollow conductingmember l'iavingtan aperture-therein; a seeond resonfior including a-second ho'llow conducting member surroundings-aid' in st hollow oondueting= member and having an aperture registering:- with the :aperture in said first hollowconducting member; a tubular shield element extending. from the wall of the first hollow conducting member through the aperture inthe second-hollow conducting-member and-surrounding said aperture, and means extending: through saidtubul'ar shield element into: the interior of said first' resonator for coupling-purposes, arfirst' tubular member within said first resonatorsupportedon a wall 'portionofvsaid first resonator &

and extending-toward the -opposite wall-and a second tubular member positioned on. the oppositewall of said-first resonator andfregistering with said first tubular member,- and'" conducting means movable longitudinally of saidi tubular s members fbrtuningsaid first resonator; and a first tubulabmeans supported withimsaidfsecond' liolloweondueting member-and between said first hollowconducting-memberand said seconcfhollow Conducting member, and-a seeond'tubul'an means supported within said second 'hollow conducting member and registering with: the first tubular means within said second hollow conducting member, and conducting means moving longitudinally-of saidtubular means for tuning said second resonant cavity tank circuit:

first resonatorincluding a: first hollow conducting-member having arr-aperture therein, a-seeond=resonator-including a second hollowccon dueting= member. surrounding said? first hollow I2 eendli'cting-- member, a:firsttubular memhenwithe in said-first hollow conducting" memberand -sup ported on: a wall and extending :t'oward 'thetopnor site walrandia second"-tubulanmemberpositioned ing member and registeringn mth: said first-tuba lar member, and conducting means1i'n'ovalcile lone gitudinally: of said'wtubular members: for tuning saidi'first; resonator; andi a first tu'lmlarmeairrs supported-J within: said second? hollow-eoml1mting.- member andebetween;saidafirstr liollow conducting: member'and'said'second hollow eonduntingrmezmber, anda: sec-0ndtuhulan m'eansmupportecb-mithx in said. second hollow conducting; member registering; with the first tubular: means within saidrsecond hollowcon-ducting member; .andfeoneducting? means moving; longitudinally;- of: :saitb tubular means forrtuning saideseoondlresonatnm andsa third aperture-within said 'finstthoilowmone ductingv memberiandoa; coupling: loopy: extending between said? hollow. conducting: members tor coupling: setichrestimators.-v

GIEIZEIL The iollowine .ref rences.-.areaoineoondin the fileof this patentg V

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