US2646525A - Oscillator - Google Patents

Description

July 21, 1953 e. E. HULSTED.E

OSCILLATOR Filed Aug. 1, 1945 Q3 3 MN VN mum IN VEN TOR.

GEORGE E. HULS TE DE ATTORNEY Patented July 21 1953 OSCILLATOR George E. Hulstede, Cambridge, Mass., assignor to the United States of'America as represented by the Secretary of War Application August 1, 1945, Serial No. 608,320

- This invention relates generally to electrical apparatus and more particularly to an electronic device for generating oscillations at'high frequencies.

Amplitude modulated electronic vacuum tubes have long been used as generators of radio frequency oscillations. frequencies so high that the transit time of the electrons from cathode to plate becomes appreciable with respect to the duration of a cycle, efficiency drops considerably and difliculty is encountered in maintaining oscillations. This is due in main to the shift in phase betweenthe plate current and the grid voltage anddecrease in eifective reactance between grid andvcathode.

A partial solution to the problem of operating at such high frequencies has been found in velocity-modulated electronic tubes, the operation of which depends upon modulating the speed of the electrons passing through it and thus bunching electrons.

Great difficulties have been encountered in the use of this type of tube generally known as and hereinafter referred to as klystrons because they are very critical to adjust since the tuning and spacing of the cavities are interdependent.

An improvement of the first developed type of klystron is the reflex klystron in which the same set of grids is used for both bunching and catching the electrons and a negative repeller plate is provided to force the electrons to retrace their paths.

There still remain in these reflex kl-ystrons many undesirable characteristics, such as frequency drift and instability, dead-spots in the tuning range, and undesired and troublesome operation on spurious modes. In the reflex kl-ystron frequency drift and instability arise from the numerous contacts required between prefabricated parts and from the reflection of reactances into the principal cavity from auxiliary cavities and resonating loops or probes. Despite most careful manufacturing processes and well However, for operation at 2 Claims. (Cl. 315-5) thought out design, it has been found impossible in the conventionally used waveguide type of cavity to eliminate all of these objectional features.

Operation on spurious modes is due to the fact that at very high frequencies the principal cavity itself is usually so large with respect to the corparticular voltage and current distribution within that cavity resulting in magnetic fields containing the oscillatory energy. The configuration of these fields within a cavity can take many forms;

that is, a cavity can usually permit oscillations at several fundamental frequencies as well a at harmonics of the fundamentals, depending on the manner in which the cavity'is energized and the shape and dimensions of the cavity. Thus the electromagnetic fields within a cavity can assume many patterns; each of these patterns is called a mode. e V

The frequency range over which reflex klystrons including a waveguide type of cavity'may be tuned hasbeen limited because with the cavities used tuning has been non-linear and asymptotic in characteristic. Further, since oscillations at two modes at the same frequency are'possible for a given position of the tuning means, the tuning means may upon further adjustment follow the wrong mode.

It is an object of the present invention to provide a velocity-modulatedelectronic tube in which these disadvantages are eliminated by reducing the number of critical contacts, by decreasing the dimensions of the resonant cavity, and by improving the configuration of the cavity. There results a tube which may be tuned over a range of high frequencies with stability at any chosen operating frequency and without any frequency within its frequency range, at which there is a failure of oscillation.

It is a further object of the present invention to provide a velocity-modulated tube which contains tuning means having a substantially linear characteristic thus extending the tunable range over the prior art.

An advantage of the present invention is that a variety of types of plunger, such as the choke type, may be used.

These objects are accomplished by means of a mode of operation arising therefrom would be at frequencies much higher than those of the operating range. The cavity is readily adaptable to the use of tuning means having substantial linear change in frequency with respect to its movement. Frequency tab l t at the chosen fr qu n y f operation is achieved by the simplicity of con- 1 struction of devices made by applying the principles of the present invention. In such devices there are not necessaryiany auxiliary elements is a cross-sectional view of a device representing 7 a preferred embodiment of the invention.

A coaxial cavity resonator H3 is formed by two concentric hollow cylinders consisting of outer cylinder II and inner cylinder i2. Between these two cylinders is a plunger I3 which is in the form of an annular ring and which is movabl to change the length of cavity resonator I thus changing the operating frequency. At the end of cavity resonator I0 opposite plunger I3 there are two grids I4 and I5, each formed by a circular washer having small concentric orifices Ma and 15a. The washers forming grids I4 and I5 preferably have flanged portions Mb and I5?) surrounding orifices I ia and a for purposes of mechanical strength.

Grid I 4 is at its outer edge attached to and makes electrical contact with the end of outer coaxial conductor II. Similarly grid I5 is attached to and makes electrical contact with the endof inner coaxial conductor I2.

Repeller electrode I6 which is made in the shape of a hollow cylinder with an aperture ifia to give maximum concentration of returning electrons is mounted within inner. conductor i2 in close proximity to grid I5. A central conductor I'l attached to repeller I6 and terminating in a terminal cap It provides means for applying the desired direct current potential to repeller i5 Central conductor II for a portion of its length is surrounded by an insulating material I9 of low dielectric loss.

The cathode assembly consists of a heater element 20, cathode 2|, and beam forming cylinder 22. pins 23, extending through terminal base 2d of conventional design.

, The space between grids Id and I 5 should have a length less than one-half the wavelength corresponding to the highest frequency in the range of operation for which the device is designed.

The evacuated portion of the tube is enclosed by glass envelope 25. Envelope is sealed to terminal base 24 and extends into inner conductor l2 a sufiicient distance to enclose repeller I5. Envelope 25 is sealed to conductor II where that conductor passes through it. Seals also are made in envelope 25 where grids I i and i5 pass through it.

An inductive coupling loop 26, or any well known means such as a capacity probe, is attached at one end to outer coaxial conductor ii and at its other end through hole 2i provided for that purpose in outer coaxial conductor I I to any conventional connecting means such as 28.

The physical arrangement of the components of this invention favor the prefabrication of the device in two major units. The part of the device inclosed in and integral with glass envelope 25 is made as one unit. This includes besides those parts entirely within the envelope 25, grids I4 and I5, inner conductor I2, conductor II, insulating material I9, and terminals I8 and 24. The second'unit consists of outer coaxial con- Connections to these electrodes are made to 4 ductor II, tuning plunger I3, and coupling loop 26. Outer conductor II is maintained at ground potential and means are provided for maintaining cathode 21 and repeller I6 at direct current potentials negative to ground with repeller I6 more negative than cathode 2|. V

In operation, electrons are emitted at cathode 2| and are kept in a narrow beam'by beam forming cylinder 22. These are attracted through orifices Ida and I50. by the positive potentials, with respect to cathode 2|, on grids I4 and I5.

Electrons pass through these orifices and into the cavity formed in repeller I6 whence they return to cathode 2 I. On each trip some of the electrons will be absorbed by grids I4, I5 and their energy dissipated as heat but sufiloient pass through orifices Ma and I5a, properly bunched and in right phase relationship to set up an oscillating magnetic field in the drift space between grids I4 and I5. The presence of grids I i and I5 in this oscillating field causes cavity resonator II! to oscillate at its resonant frequency, The resulting radio frequency field between grids I4 and I5 due to oscillation of cavity resonator I9, velocity-modulates the electrons in their initial flight through the drift space. Upon the return trip of electrons toward cathode 2|, caused by the negative charged repeller electrode I6, which,

since it is located within inner coaxial conductor I2, is protected from the intensity of that field, the velocity-modulated electron beam will, if in the proper phase relationship, transfer additional energy to cavity resonator i0, thereby reinforcing existing oscillations.

Oscillations in cavity resonator III may be set up in accordance with any mode, that is, at any frequency at which the dimensions and configuration of cavity resonator I8 will permit. It is important that in the cavity formed by the arrangement hereinabove described there is no dimension, other than the length determined by the position of plunger I3, within the frequency operating range at which this cavity can oscillate. The reason for this is that all other dimensions of cavity resonator iii are appreciably less than that of its frequency determining dimension. It is a feature of this invention that the commonly used type of cavity resonator is replaced by a coaxial cavity resonator which permits not only choice of relative dimensions as herein taught,-

' that could occur would be at frequencies outside those in the operating range of the oscillator and would not cause failure of oscillationdead-. spotswithin the range. For the above dimensional relationship to be insured over a wide band of high operating frequencies, the use of a coaxial conductor is necessary.

Energy at the desired frequency may be extracted from cavity resonator ID by use of an inductive coupling loop 26 which should be of such physical size as to be nonresonant within the,

range of operating frequencies. In order to maintain oscillations, loop 26 should be placed in an electromagnetic field of a desired density, or angularly disposed with respect to the magnetic lines of the field sothatthe proper impedance match and loading is reflected back into the cavity resonator.

While there has been here described what is at present considered to be the preferred embodiment of the invention, it will be obvious to those skilledin the art that various changes and modifications may be made therein without departing from the spirit or scope of the invention.

What is claimed is:

. 1. A reflex velocity-modulated electron oscillator having first and second component units adapted to be engaged together, said first unit comprising: a cylindrical evacuated envelope containing, in spaced axial succession, an electron gun for generating a beam of electrons, a first apertured disc, a second apertured disc, the distance between said first and second discs being less than one-half the wavelength corresponding to the highest frequency within the range of operation of said reflex electron oscillator, said first and second discs being circular in shape and radially extending without said envelope, the diameter of said first disc being larger than that of said second disc, said secand disc comprising a cylindrical element of a given diameter axially aligned with said electron gun, said cylindrical element having an outwardly extending flange at one end and'a base at the opposite end with an opening whose diameter is less than said given diameter, an electron repeller, said repeller being disposed within said cylindrical element and having the shape of a hollow cylinder with a central aperture therein. the diameter of said repeller being less than said given diameter, said repeller providing a maximum concentration of returning electrons, and an axial central conductor having one end attached to said repeller within said envelope and having its other end extending axially without said envelope; a dielectric member surrounding said other end of said central conductor; a terminal cap connected to said other end of said conductor and to said dielectric member for providing a connection to which a potential that is negative with respect to said discs may be applied to said repeller; and a hollow cylinder coaxial with said central conductor and forming an in-" with a diameter substantially equal to that of said first disc, and being in contact with said first disc, said cylinders forming a single cavity resonator therebetween such that the efiective axial length of said resonator with respect to the remaining resonator dimensions is largeand is the main determinant of the frequency of osciltract energy from said cavity resonator; and an annular plunger slidably engaging both said cylinders within said resonator, said plunger serving to linearly vary the effective axial length of said cavity resonator and thus similarly vary the frequency of said reflex oscillator. v

2. A reflex velocity-modulated electron device comprising, as a single unit: a cylindrical evacuated envelope containing, in spaced axial succession, an electron gun for generating a beam of electrons, a first apertured disc, a second apertured disc, the distance between said first and second disc being less than one-half wavelength corresponding to the highest frequency within the range of operation of said reflex electron device, said first and second discs being circular in shape and radially extending without said envelope, the diameter of said first disc being larger than that of said second disc, said second disc comprising a cylindrical element of a given diameter axially aligned with said electron gun, said cylindrical element having an outwardly extending flange at one end and a base at the opposite end with an opening whose diameter is less than said given diameter, an electron repeller, said repeller being disposed within'said cylindrical element and having the shape of a hollow cylinder with a central aperture therein, the diameter of said repeller being less than said given diameter, said repeller providing a maximum concentration of returning electrons, and an axial central conductor having one end attached to said repeller within said envelope and having its other end extending axially without said envelope; a dielectric member surrounding said other end of said central conductor; a terminal capconnected to said other end of said conductor and to said dielectric member; and a hollow cylinder coaxial with said central conductor and attached to the flange of said second disc, said cylinder having a uniform cross-section with a diameter substantially equal to that of said second disc.

' GEORGE E. HULSTEDE.

References Cited in the file of this patent UNITED STATES PATENTS 7 McArthur Oct. 18, 1949

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