US2794151A - Electronic tuning means - Google Patents

Description

May 28, 1957 B. D. KUMPF ER ELECTRONIC TUNING MEANS Original Filed May 5, 1950 Il l//l/ /l/ FUU UUUU' UUU I //I//I II l5 i I FIG. 2

United States Patent ELECTRONIC TUNING MEANS Beverly D. Kumpfer, Spring Lake Heights, N. J., assigner to the United States of America as represented by the Secretary of the Army 1950, Serial No. 159,800,

Original application May 3 dated June 14, 195s. Di-

now Patent No. 2,710,919,

vided and this application September 23, 1952, Serial The invention described herein may Abe manufactured Iand used by or for the Government for governmental purposes, without the payment of any royalty thereon.

This application is a division of application Serial No. 159,800 iiled May 3, 1950, now Patent No. 2,710,919.

The inventio-n relates to a device for electronically tuning resonant structures and more particularly to microwave generators employing wave guides `and cavity resonators whose frequency characteristics can be varied by electronic means. While the invention is subject to a wide range of applications, it is especially suited for use with, microwave generators -such as magnetrons and klystron tubes whose output wavelength depends upon the dimensions of an external tuned circuit.

In conventional microwave generator circuits, the output -frequency is controlled primarily =by the resonant frequency of the cavity resonator. When klystrons or magnetrons are employed, the operating frequency is primarily a function of the cavity resonator dimensions which may be changed mechanically or electronically. Mechanical tuning is generally accomplished Iby employing plungers to change the physical dimensions of the cavity resonator while electronic tuning is yachieved by injecting electrons into the resonant system in order to change the electrical characteristics thereof. Heretofore, electronic tuning for both klystrons and magnetrons has been accomplished by injecting an electron beam of variable intensity into a region of high R.F. electric fields within the resonator. For magnetron generators, the variable intensity of the eletctron beam is analogous to a variabledielectric constant in the cavity resonator, 'hence a variable resonant frequency of the oscillator.

When magnetrons are employed, the tuning range for eicient operation is limited by the fact that suiiciently high beam current necessary for wide frequency deviations are difficult to achieve, and moreover, in view of the high R.F. fields present in the magnetron cavity, no control of the high density beam within the cavity would be possible. In tuning reflex klystrons, it is well known that for efficient `operation the tuning range can be varied electronically only between narrow limits, the frequency deviations varying only slightly on either side of the fundamental resonator frequency.

Accordingly, it is an object of this invention to provide an electric discharge device which operates to electronically tune a resonant structure over a Wide range of frequencies.

It is still another object to provide a resonator whose frequency is electronically varied by adjusting the position of an electron space current within said resonator.

In accordance with my inventiona microwave generating apparatus comprises a resonant circuit and means v for producing and controlling an electron space current, or space charge, within said resonant circuit to effectively tune the output of the microwave generator through a wide range of frequencies.

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For a better understanding of the present invention, together with other and further objects thereof, reference is made to the following description taken in connection with the accompanying drawings in which:

Fig. l illustrates schematically and in longitudinal section one embodiment constructed in accordance with the principles of the present invention;

Fig. 2 illustrates schematically and in longitudinal section a preferred embodiment of my invention;

Fig. 3 is a transverse section through the resonant structure shown in Fig. 2, the plane of section being indicated by the line 6-6 of Fig. 2;

Fig. 4 illustrates an embodiment of my invention ernpioying resonant cavities of the pill-box type; and

Fig. 5 is a perspective view of the grid and cathode structure employed in Fig. 4.

Similar characters of reference are used lin all of the above figures to indicate corresponding parts.

Referring now to Fig. 1 there is shown `a lbroad-band microwave tuning device comprising an evacuated wave guide 10, a high frequency generating source 15 enclosed therein, a cathode 16 centrally positioned within a portion of wave guide i0, a variable spaced grid 17 surrounding cathode 16 for the entire length thereof, `a potential source 23 which is applied between wave guide 10 and cathode la, and grid voltage modulating source 24. The showing of Fig. l is a schematic in that the construction of the high frequency generating source, which may comprise a suitable magnetron oscillator, the `arrangement lfor applying operating potentials thereto, and means for its support within wave guide 10 `are not shown. Such details are well known 4and do not constitute Iany part of my invention.

As shown in Fig. l, evacuated rectangular wave guide 10 is closed at both ends to form ya resonant structure. The rectangular' cross-section of wave guide 10 jhas a major dimension somewhat larger than one-half of the maximum Wavelength of the energy corresponding to the lower limit of the frequency range `through which it is desired to operate, and a minor dimension much smaller than one-half of said wavelength. Generating source 15 may be spaced from closed end 1l by dimensions correspending to odd-quarter multiples of the 4wavelength in the guide of the frequency which is mid-Way Abetween the' upper and lower frequency limits of the desired operating range. The output of generating source 15 is so polarized that a transverse electric wave, or TE mode, is generated within wave guide 10. Thus, electric lines of force extend between the two wide walls 13 and 14 of the wave guide. Ribbon-like cathode 16 is centrally positioned along the longitudinal axis of wave guide 10 be'- ween closedA end 12 and generating source 15 `and is coated with an electron emissive material only on the twoV surfaces facing said wide walls. It is supported in posi-v tion by lead 18 which extends through `an aperture in closed end l2 and through glass seal 19. lf further support is necessary a mica spacer (not shown) may be employed in the conventional manner at the generator end. The length of cathode 16 Within wave guide 10 is determined by the desired maximum operating frequency asexplained below. Grid 17 surrounds cathode 16 for the entire length thereof and the spacing between successive grid turns progressively decreases at Ia constant rate so that the grid wires surrounding cathode 16 at the generator end will be closer together than the grid wires lat the other end of said` cathode. Thus, as shown in Fig. 1 the pitch between grid wires are progressively decreased as they approach generating source 15. Wave guide 10 is maintained at a positive potential with respect to cathode 1 6 by D. C. voltage source 23 which is applied between saidV cathode and theouter surface of the wave guide. f t

In the operation of the device illustrated in Fig. l, an electron cloud, or space charge, is created in the region between the coated surfaces of cathode 16 and wide walls 13 and 14 due to the electron space current between said cathode and wave guide 1t). lt will be shown in the following, that variations in the length of the space current charge along the cathode and incidentally along the longitudinal axis of the wave guide 1t), may be used to modify the frequency of resonance of the structure, hence vary the output frequency of generator 15.

As has been previously explained, transverse electric lines of force are generated in wave guide 1G by high frequency generating source 15' and, since the space charge is parallel to the lines of electrostatic force, said space charge forms a transverse electronic short between wide walls 13 and 14. The spacing between the innermost position of the electronic short and generator' 15 determines the operating resonant frequency. This is equivaient to the complete elimination of the section of wave guide 1t) between closed end 12 and the innermost position of the transverse electronic short end will have results similar to decreasing the length of the wave guide. Reducing the length of the wave guide will decrease the resonant wavelength of the structure and thus tend to increase the frequency output from source 15, with which it is associated. Thus, with the space charge distributed along the longitudinal axis of the wave guide for the entire length of cathode 16, the resonant frequency will be at a maximum. Accordingly, the length of cathode 16 within wave guide 10 is determined by the upper limit, or maximum frequency, of the desired frequency range.

The innermost position of the transverse electronic short across the wave guide may be varied by applying to grid 17 a source 24 of variable voltage with which it is desired to modulate the output of generating source 15. Due to the graduated spacing of the grid Wires, variations in the potential supplied by source 24 will produce corresponding changes in the position of the transverse electronic short, or space charge, within wave guide 10. For relatively small negative potentials, electron space current will be cut-off from only that portion of the cathode surrounded by the closer grid spacing. Similarly, as the applied grid voltage is increased negatively, electron space current will be cut off from a greater portion of the cathode. The ultimate result will be to modify the electrical length of wave guide and tuning of the resonant structure, and hence to change the frequency of the voltage developed by generator source in accordance with the signal pattern applied to grid 17. Thus, for example, if the grid voltage is varied linearly by applying a negative sawtooth voltage thereto as shown, the electron space current, hence the space charge, is periodically moved linearly along the longitudinal axis of wave guide 10. Moving the transverse space charge toward closed end 12 will decrease the resonant frequency, the minimum frequency limit being achieved when the space current is cut orf for substantially the whole length of the cathode.

While in the particular embodiment there has been described a rectangular wave guide resonant structure, it will be understood that other resonant structures such as a coaxial line may be used. It should be further understood that the particular spacing of the grid wires which has been described in connection with Fig. 1 is not essential to the purposes of the invention. For example, the grid spacing may vary exponentially if it is so desired.

In the arrangement shown in Fig. 2, a double ridged wave guide 30 constitutes the resonant structure within which cathode 16 is longitudinally positioned between ridges 31 and 32. Cathode 16 is coated with an electron emissive material on only those surfaces facing inner ridge surfaces 31 and 32 so that the space charge emitted therefrom is concentrated within the ridged area. Thus, the electronic short is concentrated in the region of greatest density of electric lines of force. The operation of this device is identical to that explained in connection with Fig. l.

A still further application of the invention is represented in Fig. 4 which shows its use in a pill-box cavity resonator to adjust the resonant wavelength thereof. As shown in Fig. 4, high frequency generating source 15 is axially positioned within pill-box resonator 40. The construction of the high frequency generating source and means for its support within resonator 4t) and the arrangement for applying potentials thereto are not shown, since such details are well known in the art.

Annular cathode 44 is centrally positioned within the resonator and the outer periphery thereof is radially spaced from lateral wall 41. Cathode 44 is coated with an electron emissive material only on the two surfaces facing top and bottom resonator plates 42 and 43, respectively. Surrounding cathode 44 is a toroidally wound grid 45 so that the spacing between adjacent grid wires increases from the inner periphery of annular cathode 44 to the outer periphery thereof. Thus, adjacent grid wires are closer at the inner periphery of cathode 45 than at the outer periphery of said cathode. Both cathode 44 and toroidally wound grid 45 are supported in position by leads such as 46 and 47 which extend through lateral Wall 41 to glass seals 48 and 49, respectively. A D. C. voltage 23 is applied between cathode 44 and resonator 4!) to create a space charge between the coated surfaces of the cathode and the inner surfaces of top and bottom plates 42 and 43. This space charge constitutes a cylindrical electronic short between said top and bottom plates. The radial position of the cylindrical electronic short may be varied by applying to grid 45 a source 24 of variable voltage with which it is desired to modulate the output of generator source 15. Due to the varied spacing of the grid wires at the inner periphery and the outer periphery of the annular cathode, variations in the potential supplied by source 24 will produce corresponding changes in the radial position of the cylindrical electronic short within resonator 4t). For relatively small negative potentials electron space current will be cut olf Vfrom only that portion of the annular cathode surrounded by the closer grid spacing. Similarly, as the applied grid voltage is increased negatively, electron space current, hence the space charge, will be cut off from a greater portion of the cathode. As previously explained, the tuning of the resonant structure and, hence the frequency of the voltage developed by source 15 will be varied in accordance with the signal rpattern applied to grid 45.

While the invention has been described by reference to particular embodiments thereof, it will be understood that numerous modifications may be made therein by those skilled in the art without departing from the invention, and it is, therefore, aimed in the appended claims to cover all such modifications as fall within the true spirit and scope of the invention,

What is claimed is:

l. In combination, an evacuated cylindrical resonator, a source of high frequency electromagnetic waves having a transverse electric field axially positioned within said resonator, the output frequency of said source being a function of the resonant wavelength of said resonator, an annular cathode centrally positioned within said resonator and radially spaced from the lateral wall thereof and which is electron emissive only on the two surfaces facing the top and bottom plates of said resonator, means for producing an electron space charge coplanar with said transverse electric field whereby said ield is short circuited, and a control electrode surrounding said cathode, said control electrode having a cutoff characteristic which varies from the inner periphery of said cathode to the outer periphery of said cathode whereby the effective circumferential space charge may be varied to modify the resonant wavelength of said cylindrical resonator.

2. The combination in accordance with claim 1 wherein said control electrode is a grid toroidally wound around said cathode.

3. The combination in accordance with claim 2 wherein the grid winding turns are closer at the inner periphery of said cathode than at the outer periphery thereof.

References Cited in the ile of this patent UNITED STATES PATENTS 2,241,976 Blewett et a1. May 13, 1941 6 Schmidt Dec. 31, 1946 Fiske Feb. 18, 1947 Spencer Apr. 26, 1949 Spencer July 26, 1949 Sproull Mar. 21, 1950 Derby Ian. 16, 1951

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