US2405175A - Ultra high frequency oscillator - Google Patents

Description

5 1946- A E. ANDERSON ETAL ULTRA HIGH FREQUENCY OSCILLATOR Filed. April '4, 1941 2 Sheets-Sheet 1 .AEA/VDERSON gj A 1. SAMUEL A from/EV EAQA 75 A.E. ANDERSON ETAL ULTRA HIGH FREQUENCY OSCILLATOR Filed April 4, 1941 2 Sheets-Sheet 2 A. EA/VDERSON gfi A L SAMUEL A 7'TOR/VEV Patented Aug. 6, 1946 ULTRA HIGH FREQUENCY OSCILLATOR Alva Eugene Anderson, New York, N. Y., and Arthur. L. Samuel, Summit, N. J., assignors to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application April 4, 1941, Serial No; 386,794

This invention relates to oscillators or generators of ultra-high frequency electromagnetic waves and currents and is particularly applicable to the generation of waves of a few centimeters or less in wave-length.

An object of the invention is to produce an oscillator having a higher degree of frequency stability and amplitude stability at the higher frequencies than have been heretofore obtainable.

Another object is to reduce the weight and bulk of an oscillator of this kind.-

A further object is to simplify the manufacture, assembly and adjustment of the device.

The oscillator is of the type employing a single resonant cavity which is maintained in oscillation by direct interaction between the electromagnetic field of the resonator and the electrons in an associated electron stream. The electron stream interacts with the field at two gaps separated by a shielded drift space. In the first gap the electron stream is velocity varied by the electric field. Passing then through the drift space, the electron stream undergoes a grouping or bunching of electrons resulting from the velocity variation. The transit time of the electrons in the drift space is So adjusted that the electron groups arrive at the second gap in proper phase to contribute energy to the field. When the system is properly adjusted oscillations start spontaneously and are maintained by the energy 7 Claims. 250-275) contributed to the field cyclically by the electron stream.

Afeature of the invention is the construction of the main body of the resonator in two half sections to facilitate manufacture and assembly. The resonant chambers are completed by a pair of disc electrodes extending through and sealed into the walls of the envelope which encloses the electron stream. Within the envelope and enclosed by the resonator is mounted a drift tube the ends of which, together with the two abovementioned electrodes, form the gaps hereinbefore referred to. are provided with aligned apertures for the passage of the electron stream therethrough.

Another feature of the invention lies in improved means for joining the half sections of. the resonator together and clamping them to the disc electrodes. 7 A further feature is connected with the use of a single battery or other suitable source to provide all the biasing potentials required for the operation of the oscillator. The heating current is not necessarily supplied from the same source as thebiasing potentials and any or all of the The drift tube and the electrodes currents and potentials may be supplied, if desired, from ordinary electric power mains. The collector current is preferably kept out of the potentiometer and returns directly to the biasing battery thereby permitting the use of a relatively light and compact potentiometer. In applications where the power output is the primary consideration and efficiency secondary, the electrons arepreferably collected at the maximum battery potential in order, as above mentioned; tokeep the collector-current out of the potentiometer and also to avoid the alternative of an additional battery opposed to the biasing-potential to reduce the effective potential at the-collector electrode.

Another feature of the invention is the use of a permanent magnet as a source of focussing flux and a pair of magnetic collars fitting over the vacuum tube for the direction of flux.

Other features include a suitable shaping of the interior walls of the drift tube to further facilitate the focussing of the electron beam and the use of an improved electron gun as the source of the electron stream.

In the drawings, I

Fig. 1 is a'perspective view of a preferred embodiment'of the oscillator, shown partially cut away;

Figs. 1a and lb show alternative details which may be substituted in the arrangementof Fig. 1;

Fig, 2 is an end view of the oscillator of Fig. 1, shown partially cut away;

Fig. 3 is an enlarged detailed view of an electron gun suitable for use in an oscillator in accordance with the invention;

Fig. 4 is an end view of 3; and

Fig. 5 is a schematic diagram showing the auxiliary connections associated with the oscillator.

Referring to Figs.-1 and 2, a cylindrical insuthe'electron gun of Fig.

lating envelope ID, for example, a glass tube, en-

closes an electron gun shown generally at H, and in greater detail in Fig. 3, together with an electron collecting electrode or collector l2. Between the electron gun I I and the collector l2 there are .sealed into and through the walls of the envelope c2 of the electron beam. The electrodes l5 and H3, in cooperation with the respective adjacent ends of the drift tube l'l, form a, pair of gaps l8 and I9 which will bereferred to respectively as the input gap and the output gap. The discs l3 and I4 and the drift tube I! are preferably of highly conductive material. or example copper. The discs l3 and ['4 are hermetically sealed into and through the walls of the tube It by any suitable process or form of seal, for example, a copperglass seal. The drift tube I! has attached to it a conductive rod 20, by which itis supported in position and by means of which electrical contact may be made from the exterior of the envelope ID. The conductor is sealed into and through the wall of the envelope in through a glass bead or other suitable hermetic seaL- Alternatively, three supporting rods H, 12 and 13 maybe employed as shown in Fig. 1a, or the drift tube'may be supported by a disc electrode 14 as shown in Fig. lb.

The discsl3 and 14 form a portion of the walls of a. resonant chamber or cavity resonator together with a casing in two half sections 2| and 22, which sections fit closely together and are provided with milled semiannular surfaces 23 and 24, respectively, which fit snugly inside the edges of the disc members 13 and 1-4. A pair of cylindrical collars 25 and 25 of magnetic material, provided with flanges 21 and. 28, respectively, are placed over the tube 1 0 and against the outer surfaces of the respective disc members l3 and i4. on one side of the casing a ring 6! and screws 29 are provided so that when the screws are tightened, the disc 13 and flange 21 are clamped securely between the ring BI and surfaces 23 and 24 on the'casing. A similarring 62 and'screws '29 are provided on the other side of the casing. A flexible lead is connected to theconductive rod 20 and during assembly the lead 30 is passed through a small hole 3| in the casing section 22. When the assembly of the resonant chamber is completed the lead 30 is secured and a good electrical contact .between the lead and the casing is obtained by tightening a set screw 32 in a larger threaded hole 33 adjacent to'the hole 3!, thus bringing the head of the screw 32 down upon the lead 30. v

A pluralityof plungers, in the form of screws '34, 35, 36 and '31, are'threaded into the walls of the casing sections 2| and 22 for altering the size and shape of the resonant cavity for tuning purposes A coaxial transmission line comprising an inner conductor '31 and an outer conductor 38 is coupled into the chamber through a hole 39, the inner conductor 3'! terminating in the form of a small loop 40, the end of the loop being connected to the outer conductor '38. The end of the outer conductor serves as a plug which slides into a jack member 42 surrounding the hole 39. The coaxial line is extended, preferably by a flexible portion 43, to any suitable load circuit or point of utilization. A set screw 44 is provided for securing the plug in the 'jack 42:? I

A permanent magnet 45 has pole pieces extending perpendicular to the main body, the polepieces being milled out with cylindrical depressions at 46 and 4-1 to form-a cradle to support the cylindrical collars 25'and 26, respectively. The

'tube assembly including the collars is held to the 1 permanent magnet by the magnetic force and, in addition, the lower half section '22 of the casing maybe secured'to'the'middle portion of the permanent-magnet 45 by "means of a screw 48. If

desired, the magnet 45 may be attached to a base 4 plate or other mounting in any suitable manner. A U-shaped magnet or one of other suitable shape may be used in place of the one illustrated.

The electron gun, as shown more clearly in Figs. 3 and 4, comprises a dished cathode 49 arranged to be heated by a heating element '50. The electrons emitted from the cathode 49 are formed into a conical beam by means of the shape of the cathode and by the presence of a shaping electrode 5|. The electrode 5i is electrically connected with the cathode. An accelerating electrode 52 having a cylindrical portion 520. bounding a frusto-conical aperture 521), serves to regulate the beam current and in conjunction with the shaping electrode 51 to focus the beam at approximately the center of the input gap 13. The accelerating anode 52 is provided with a plurality of apertures 520 to enable viewing of the gap between the juxtaposed portions of the electrodes 5i and 52 whereby these electrodes may be mounted accurately in a desired relation. One side of the heating element '50 may be connected to the cathode within the tube l0 and brought out by means of a common lead 53. In that case, the lead 53, together with the remaining heater lead 54 and a lead 55 from the accelerating electrode 52 constitute the external connections from the electron gun. The remaining electrical connections to the oscillator comprise a lead 56 connected with the Walls of the resonant chamber and to the drift tube through lead 30 and rod 23, which lead 55 may conveniently be grounded or connected with the mounting plate, and a lead 51 connected to the collector 12. The leads 53, 53, 55 and 51 are brought out through the Walls of the tube It} in suitable presses or seals.

The interconnections of the oscillator with suitable sources of biasing potential and heating current are shown schematically in Fig, 5. 58 is a power transformer or other source of suitable current for operating the heating element 50. The lead 53 is connected to the negative terminal of a biasing battery or other source of biasing potential 59, the positive terminal of the battery being connected directly to the collector l2 through the lead 51. celerating electrode 52 is connected to the variable contactor of a potentiometer 60, that is, one of the two potentiometers 60 and 10 connected in shunt across the battery 59., The lead 56 from the resonator is connected to the variable contactor of the potentiometer 15. If desired, the potentiometers B0 and'lll may-be replaced by a single potentiometer with two variable contacts.

In the operation of the system as shown in the figures, the heating element 58 is supplied with suitable heating current from the source 58 to produce a suitable beam of electrons emanating from the cathode 49. The electrons emerge in the form of a solid cone. The voltage Em between the leads 53 and 55 is adjusted by means of the potentiometer 30 to a suitable value to insure a copious supply of electrons in the beam. Under the combined influence of the dishing of cathode 43, the effect of electrode 51 and of the accelerating electrode 52, the electrons reach an appropriate point focus preferably in the center of the gap is, diverge again slightly in the interior of the drift tube i1, and by virtue of the magnetic field of the permanent magnet directed axially along the tube by means of the collars 25 and 26, are brought to another approximate point focus in the gap l9. The electrons again diverge slightly before being intercepted by the collector l2. The shape of the interior of the drift tube [1 is The lead '55 from the acpreferably curved to conform with the contourof the'beam, which has been found fromtheoretical and practical considerations to approximate a sinusoidal curve, the walls of the tube being concave toward the inside. p

\ Any non-uniformity in the densit of the electron stream will cause a transient electromagnetic wave or disturbance to originate within the resonant chamber as the electron beam traverses the output gap 1 9. The transient wave will be propaated through the interior of the resonant chamber to the input gap l8 where it will produce a velocity variation in the electron stream at that point, The electrons, thus differentiated a to velocity, will tend to arrange themselves into groups or bunches as the beam traverses the interior of the drift tube l1. Upon reaching the output gap I9, the resulting bunches of electrons, if they arrive in the proper phase, will contribute energy to the electromagnetic field of the resonant chamber by building up the transient oscillations. The energy delivered to this field by the electron stream at the output gap will, with proper adjustment, be greater than the energy required to maintain the field within the cavity. Accordingly, the oscillations in the resonant chamber will increase until limited by non-linear eifects in the generating mechanism. Oscillations will then be maintained at a level where a .condition of equilibrium exists between the energy taken from the electron beam and the energy dissipated in the cavity and its associated load circuit.

For oscillations of a given frequency to be sustained, it'is necessary that the electron transit time in the drift space have one of a certain series of critical values. It is readily determined that this transit time must be approximately threequarters of the cyclic time of the oscillations or else it must have some other one of the values determined by the following equation:

where 0 is the transit time in cycles and n is either zero or a positive integer,

To derive Equation 1, consider the instant of time when the electric field in the input gap 18 is changing from the direction opposing the passage of electrons to the direction aiding their passage. This instant will hereinafter be referred to as the instant of reference. Assuming that the gap is sufiiciently short so that the force acting upon an electron while it traverses the gap may be considered constant, the electrons passing at the time above designated will I- be neither accelerated nor dec-elerated by the field. The electrons which have passed the gap I 8 during the preceding half cycle have been slowed down and the electrons which will pass the gap in the succeeding half cycle will be speeded up. Consequently, as the electrons traverse the drift space, the electrons that have been accelerated will tend to catch up with those which have been decelerated and the center of a bunch or group of electrons will evidently reside in the neighborhood of those electrons which pass the gap at the instant of reference. In order to have this bunch of electrons contribute energy to the electric field to offset the damping of the field, the electron bunch must reach the output gap at an instant when the field across that gap is in opposition to the direction of motlon of the electrons and to be most efiective the electron bunch should arrive when the field strength is at a maximum. Evidently such an instant occurs three-quarters of a cycle after'the instant of reference. Other similar instants occur at intervals spaced a complete cycle apart and. hence the appropriatenes of equation (1) isdemonstrated.

In order to secure a suitable value of 0 in accordance with Equation 1, the transit time of the electrons may be adjusted by means of the potentiometer 70. The proper adjustment depends upon the wave-length of the oscillations desired, 1 the length of the drift space, and ED the direct current potential between the cathode and the resonant chamber. The following close- 1y approximate relationship has been found to exist between 0 and the above-mentioned variables:

500i 0 NF (2) Combining Equations 1 and 2 and solving for ED the following value is obtained:

500l 2 Y WIAW) 3) where ED is in volts, and Z and A are centimeters. In an oscillator which has been built and successfully operated in accordance with the invention, the important dimensions and adjustments are as follows: The diameterv of the resonant chamber is approximately 2% inches and the thickness of the chamber approximately inch. The oscillator was operated in the range of wave-lengths from about 9 to 10 centimeters. The biasing battery potential was about 720 volts of which in the neighborhoodofZOO to 350 volts was impressed'upon the accelerating electrode as Em. To illustrate thevalues of the other constants which were found to operate successfully, the values are given for a case in which the oscillator was adjusted by means of the tuning screws 34, 35, 36 and31 tooperate at a wavelength of 9.75 centimeters. The length l was inch and n had the value 2.. Using these values in Equation 3 it was determined that the proper value of ED was approximately 600 volts which is within a few per cent of the experimentally observed value. The power output ranged from approximately 100 to 200 milliwatts, according to the particular adjustment of Em within the above-mentioned limit of 200 to 350 volts. The corresponding values of i0 ranged from 10 to 20 milliamperes. The current in was in the range from 1 to 4 milliamperes while the current 5A1 was negligible.

The potentiometers and provide substantially independent adjustments. The setting of potentiometer l0 governs the value of transit time and is used in the frequency adjustment. The setting of potentiometer 60 governs the collector current and consequently serves to vary the power output of the oscillator.

The power adjustment had substantially no efiect upon the tuning adjustment and vice versa. The two potentiometers and one or more tuning plungers, such as 34 to 31, inclusive, shown in Fig. 2, are the only adjustable elements required in the practical operation of the oscillator.

What is claimed is:

1. An ultra-high frequency electronic device comprising a cavity resonator substantially closed against the external radiation of power, said cavity resonator comprising a. single cavity bounded by a single surface having a pair of relatively small apertures therein, said resonator being of relatively large dimensions and containing, a relatively narrow drift tube positioned within said, cavity resonator coaxial with a line passing through said apertures, said drift tube being open at both ends, means for passing an electron stream through said chamber and said drift tube along the above-defined axis, said drift tube being only sufficiently large to accommodate the electron stream, and electrode means for accelerating the electrons in said stream.

2. An ultra-high frequency electronic device comprising an evacuated cylindrical envelope of insulating material, an electron gun and an electron collecting anode mounted within said envelope and along the axis thereof, a pair of disc electrodes extending through the wall of said envelope and hermetically sealed to said Well, said electrodes being substantially perpendicular to the axis of said envelope, a pair of conductive casing members forming together with said electrodes a substantially closed resonant chamber, means clamping said casing members and said disc electrodes together in electrical contact, a conductive tube mounted within said resonant chamber on the axis of said envelope, said disc electrodes and said conductive tube having axially aligned apertures to accommodate the passage of electrons from the electron gun toward the collecting anode.

3. An arrangement in accordance with claim 2. in which the walls of the said conductive tube are concave on the inside.

4. An arrangement in accordance with claim 2 in which the interior surfaces of the said conductive tube are concave toward the inside and have ashape in. longitudinal section substantially following a sinusoidal curve symmetrical with respect to the aXiS.

5. An. electronic device comprising an axially symmetrical electrode system for producing a concentrated conical beam, said system including a cathode having a dished electron emissive surface, an accelerating anode having a cylindrical portion coaxial with and opposite said surface and bounding a frusto-conical aperture 8 tapering away from said surface, and a beam forming electrode having a right cylindrical inner surface encompassing and coaxial with said emissive surface and the cylindrical portion of said accelerating anode,

6. An ultra-high frequency electronic device comprising an evacuated cylindrical envelope of insulating material, a pair of disc electrodes extending through the wall of said envelope and hermetically sealed to said wall, said electrodes being substantially perpendicular to the axis of said envelope, a pair of conductive casing members forming together with said disc electrodes a substantially closed resonant chamber, a pair of flanged collars surrounding said cylindrical envelope, said pair of easing members presenting together a pair of plane surfaces each parallel to an adjacent surface of one of said collars, and means clamping each of said disc electrodes between the said parallel surfaces of the flange of one of said collars and said pair of casing members with electrical contact between said disc electrodes and said casing members, said clamping means also fastening said casing members together in electrical contact.

7. An ultra-high frequency electronic device comprising an evacuated cylindrical envelope of insulating material, a pair of disc electrodes extending through the wall of said envelope and hermetically sealed to said wall, said electrodes being substantially perpendicular to the axis of said envelope, a cavity resonator, the wall of which comprises two portions, one of said portions being integral with said evacuated envelope and comprising saiddisc electrodes, the other of said portions being conductively and demountably attached to said disc electrodes outside of said evacuated envelope, and a conductive tube mounted axially between said disc electrodes and spaced therefrom at both ends, said conductive tube being inside both said evacuated envelope and said cavity resonator.

ALVA EUGENE ANDERSON.

ARTHUR L. SAMUEL.

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