US3296519A - Ultra high frequency generating apparatus - Google Patents

Description

' 1967v I. KAUFMAN ETAL ULTRA HIGH FREQUENCY GENERATING APPARATUS Filed March 12, 1963 FREQUENCY INPUT 45 O I O 0 2w OUTPUT GUIDE FREQUENCY w \N PUT EN ERGY INVENTORS //?v//ve KAUFMAN ALLAN 5. R/SLEY United States Patent C 3,296,519 ULTRA HIGH FREQUENCY GENERATING APPARATUS Irving Kaufman, Woodland Hills, Calif., and Allan S.

Risley, Boulder, Colo., assignors, by mesne assignments, to TRW Inc., a corporation of Ohio Filed Mar. 12, 1963, Ser. No. 264,543 4 Claims. (Cl. 32169) The present invention relates to apparatus for generating ultra high frequency energy, and more particularly to a microwave cavity device for frequency multiplying.

It is a primary object of the present invention to provide an improved apparatus for producing microwave energy by means of harmonic generation. It is a further object of our invention to provide an improved apparatus for producing microwave energy by means of frequency mixing.

It is a further object of our invention to provide a microwave cavity device adapted to simultaneously satisfy the conditions that the device is resonant to both an input fundamental frequency signal, to an output harmonic signal and the condition that a ferromagnetic material element contained within the cavity is ferromagnetically resonant at the frequency of said harmonic signal.

It is a further object of the present invention to provide an improved doubly resonant microwave cavity device which permits efficient energy conversion and also uses larger ferromagnetic material elements than has heretofore been possible. It is an additional and related object of our invention to provide an ultra high frequency harmonic generator of substantially improved energy conversion efficiency.

A more specific object of our invention is to provide a microwave cavity device of the type described which includes means for tuning the cavity to fundamental frequency resonance without altering the figure of merit of the cavity or the resonance of the cavity to a harmonic signal which is to be generated.

In accordance with a preferred embodiment of our invention, we achieve the foregoing objects and purposes by providing a microwave cavity structure which allows eificient coupling of power to the frequency converting element at either a fundamental frequency or two frequencies to be mixed and which is resonant at a harmonic frequency, or the sum or difference frequency respectively. In a particular embodiment, a microwave cavity structure is resonant in one mode at a fundamental frequency and is also resonant in a different mode at a harmonic frequency. The cavity comprises first and second cavity portions with the first portion being dimensioned so that it is, by itself, resonant at the harmonic frequency. This first cavity section has disposed therein a ferromagnetic material element, preferably in the form of a thin disc, supporting on one wall of the cavity which element is operative to generate magnetic fiux at the harmonic frequency when subjected to both a substantially constant or DC. magnetic flux and a fundamental frequency alternating magnetic flux. The first and second .cavity portions are coupled together in a manner to permit propagation of fundamental frequency energy from the second cavity portion into the first cavity portion and in a manner to substantially exclude harmonic frequency magnetic fields from the second cavity portion. More specifically, the first and second cavity portions taken together are arranged and dimensioned so that, jointly, they form a single cavity which is resonant at the fundamental frequency.

The foregoing and other objects and features of this invention will be apparent from the following description taken with the accompanying drawings, throughout Ice which like reference characters indicate like parts, which drawing forms a part of this application and in which:

FIGURE 1 is a graphical illustration of certain operational characteristics of the invention useful in explaining the advantages of same;

FIGURE 2 is a partially broken away perspective view of a cavity structure in accordance with our invention;

FIGURE 3 is a perspective view of a complete system in accordance'with our invention and embodying a cavity structure similar to that of FIGURE 2; and

FIGURE 4 is a cross-sectional view taken along the lines 44 of FIGURE 3.

It has been shown in the prior art that when a microwave field is impressed on an insulative ferromagnetic material in a manner such that the microwave magnetic field has a component normal to the direction of a DC. magnetizing field, an asymmetric precession of the magnetization of the material can be achieved and will produce harmonics or second order components of magnetic flux. This harmonic frequency magnetic flux can be utilized to generate and produce microwave output signals having frequencies which are integral multiples of the fundamental input microwave energy. The prior art treatment of the theory of frequency multiplication by means of ferromagnetic materials will be found, for example, in (1) an article by J. H. Melchor, W. P. Ayres, and P. H. Vartanian, Proc. Inst. Radio Engrs. 45, 644 (1957), and (2) in an article by I. Kaufman, A. S. Risley, and D. D. Douthett, Bull. Am. Phys. Soc. 5, 297 (1960). In the prior art, the foregoing frequency multiplying effect has been largely a laboratory phenomenon and has not been utilized to produce practical magnitudes of microwave power with energy conversion efliciencies high enough to enable commercial exploitation of the phenomenon. That is, ferrite frequency multipliers which have been experimentally operated heretofore have been of little practical use for the reason that they have required'rnany kilowatts of input power in order to achieve high efficiency. At lower power levels, the efliciencies achieved have been extremely low. For example, one such frequency multiplier previously reported in the literature has required watts of 9000 megacycle input power to produce 0.1 watt of output energy at 18,000 megacycles.

The prior art treatment of the theory of frequency mixing'by means of ferromagnetic materials will be found, for example, in a report by R. L. Iepsen, entitled Harmonic Generation and Frequency Mixing in Ferromagnetic Insulators, Contract No. AF 19(604)1084, Harvard University, May 25, 1958.

In contrast with previous ferrite harmonic generation devices, we have found that apparatus in accordance with the present invention incorporating specifically selected materials and dimensional parameters within a certain critical range is capable of producing as much as about 18 watts of harmonic output energy in response to fundamental frequency input power levels of about 300 watts. For the special case of frequency doubling with ferromagnetic materials in a circuit simultaneously resonant to frequencies and 2,, the second harmonic output power P is The foregoing Equation 1 and its derivation is found in our previous publication entitled Microwave Harmonic Generation by Ferrimagnetic Crystals, Journal of Api the ferrite element, and the microwave circuitry (through the symbol R Equation 1 states that in harmonic generation by a volume of nonlinear material (as a ferrite, for example), the output power is a function of:

( 1) Input power P,,,

(2) Figure of merit of the nonlinear material, F

(3) The coupling impedance R The choice of material is governed by P,,, and F and is independent of the cavity structure. The coupling impedance is determined by the cavity structure. With the cavity structure of our present invention it is possible and practical to obtain a conversion efliciency of 5% with input power at the 300-watt level, as contrasted to prior devices which have required power levels of the order of one kilo watt to achieve that efiiciency.

To fully understand the criteria for selection of the cavity parameters, we refer to the definition of R To minimize R in accordance with (1), we should dimension the cavity so that the ratio is a minimum. Now, in general, U is proportional to the product V h where V is the effective cavity volume at 2w. Therefore R is proportional to This means that a cavity should have the minimum volume that allows resonance, while yet maintaining a high Q. From a practical point of view, the cavity containing the ferrite must allow introduction of fundamental frequency power, must be resonant at and must permit tuning of the w-circuit without detuning the 2w-circuit. This is accomplished in the cavity which we disclose herein.

By using a cavity structure having two distinct portions and 27 as shown in FIGURE 2, the 2m fields are confined to the lower cavity portion 25 so that V is minimized. Since the 2w fields in the portion 27 of the cavity are negligible, the latter may be moved without altering the resonance of the 2w portion 25 of the cavity. Accordingly, the requirements mentioned above have been satisfied. For fundamental to second harmonic energy conversion efilciencies smaller than five percent the figure of merit F of the ferromagnetic material element is given by the following expression:

In Equation 2, mks units with B H+M are used. M is the saturation magnetization, e is the ratio of the two transverse components of the magnetization and is minimized by using a thin disc-like ferrite element positioned so that 11,, and H are in its plane, and which has Here 17,, is the absorbed power per unit volume; H and h,, are the intensities respectively of the dc and the fundamental frequency magnetic fields. As stated heretofore, we define is the loaded Q of the 2w circuit and the U is the energy stored therein at field intensity k Equation 1 assumes that the ferrite element is uniformly excited, that the 2m circuit is matched to its load, and that operation is at ferromagnetic resonance at frequency to. That is, Equation 1 is valid so long as the foregoing three conditions are substantially satisfied.

Combining Equations 1 and 2, we get an expression for the energy conversion efficiency, 1;

4 Equation 3 can be solved for the value of X required for particular values of 7; and P,,. Moreover, since for a ferrite element having a specified volume V, these chosen values of 7 and P also specify h by the definition of X (above). The relation between h,, and X," at various values of input power P is given by the straight line curves 13 and 15 of FIGURE 1, for two values of the conversion efiiciency 1;, a specific value of R of 4.77 10 ohm-m. and a specific ferrite element having a diameter of 0.20 inch and a thickness of 0.10 inch.

To determine the actual efficiency attainable with a given material, we plot its X vs. h, characteristic, or, rather, the inverse relation h,, vs. X on the graph of FIGURE 1. In general, a particular efficiency can be achieved with a given material if the line corresponding to that efficiency intersects or is tangent to the h,, vs. X curve of that material. For example, if the h, vs. X characteristic of the given material intersects the curve 13, an etficiency' of 5% can be had with that material.

The three curves of FIGURE 1 correspond to three different materials. More specifically, the curve 17 of FIGURE 1 is a plot of 11,, vs. X," for general ceramics type R-l polycrystalline ferrite. Curve 19 is a plot of the same variables for single crystal yttrium-iron-garnet and curve 23 is a graph of the same relationship for single crystal manganese ferrite. The straight line curves 13 and 15 of FIGURE 1 indicate the relationship between h and X for various fields of fundamental frequency input power and for conversion efficiencies of 0.5% (curve 15) and 5.0% (curve 13). From examination of the curves of FIGURE 1, we have selected manganese ferrite as being the material having optimum efliciency of energy conversion from the fundamental frequency of 8.5 gc. to the second harmonic of 17.0 gc.

In FIGURE 2, the-re is shown a microwave cavity structure in accordance with one embodiment of the present invention which is operative in accordance with the theoretical considerations discussed above. Specifically, the cavity structure of FIGURE 2 comprises a first cavity section 25 in the form of a generally rectangular structure having side walls 29 and 31, a front wall 33, a corresponding rear wall 45, a generally planar bottom wall 39 and a top wall comprising portions 35 and 37 to which are joined a second cavity portion 27. A cylindrical wafer 30 of ferromagnetic material, preferably manganese ferrite, is mounted on the bottom wall 39 substantially at the center thereof. The ferromagnetic element 30 is subjected to a substantially constant or DC. magnetic field provided by any oneof a number of conventional magnetizing devices (not shown). The means for applying a DC. magnetization to the element 30 is represented schematically in FIGURE 2 by the arrow 32 which indicates that the static magnetization h is generally in the direction of the y-axis and in the plane of the ferromagnetic material element 30. For excitation of asymmetric precession in the ferromagnetic element, it is additionally subjected to an alternating magnetic field intensity at the frequency w of the input microwave energy. This magnetic field intensity is applied to the wafer 30 substantially in the plane of the wafer and in a direction normal to that of the DC. magnetization. That is, the cavity structure is preferably arranged so that the fundamental frequency magnetic flux is applied to the wafer 30 in the direction of the x-axis. In accordance with the preferred embodiment the ferromagnetic material element 30 is about 0.2 in diameter and about 0.01" thick.

The first cavity portion 25 is constructed and dimensioned so that it is resonant at the harmonic frequency which is desired to be generated. Specifically, in this embodiment, the first cavity portion 25 is resonant at 17.0 gc. The 17.0 gc. harmonic field pattern in the first cavity portion 25 is in the TE mode so that the harmonic frequency magnetic flux 11 is in the direction of the yaxis and therefore, generally in the plane of the ferromagnetic material wafer 30. That second harmonic magnetic flux field is indicated by the line 36 in FIGURE 2. The electric field of the second harmonic energy is, of course, normal to the magnetic field and thus the electric field B extends within the cavity generally normal to the front and rear walls 33 and 45.

Fundamental frequency input energy is applied to the cavity structure of FIGURE 2 from a source of microwave energy which propagates energy downwardly through the second cavity portion 27. The means for applying the input fundamental frequency microwave energy is represented by the arrow 41 in FIGURE 2 and will be described in further detail hereinafter in connection With FIGURE 3. The second cavity portion 27 is generally rectangular comprising side walls 47 and 49, a front wall portion 43 and a rear wall 45. In a preferred embodiment, the dimensions of the second cavity portion are generally the same as a standard X-band waveguide, so that the second cavity portion 27 functions as means for propagating 8.5 gc. microwave energy into the first cavity portion 25. Because the walls 47 and 49 of the second cavity portion are relatively closely spaced, the second harmonic energy which exists in the first cavity portion 25 is prohibited from propagating upwardly along the second cavity portion 27. That is, the restricting dimensions of the aperture between the first and second cavity portions serves to confine the second harmonic fields to the first cavity portion 25 while simultaneously permitting fundamental frequency energy to be fed thereto from the second cavity portion 27.

In FIGURE 3, there is shown a complete frequency multiplying apparatus in accordance with the present invention which includes a doubly resonant cavity structure as described heretobefore in connection with FIGURE 2. Specifically, the first cavity portion 25 is shown at the left hand end of the apparatus of FIGURE 3 and the second cavity portion 27 extends to the right therefrom. At its right hand end, the second cavity portion 27 is provided with a conventional waveguide tuning means which won prises essentially a tuning plunger within the end of the waveguide. The tuning plunger is operated by means of a thumb screw 52, to position it optimumly within the waveguide for tuning the entire system, including the first cavity portion 25 and the second cavity portion 27, to the frequency of the fundamental input energy. That fundamental frequency energy is applied to the system by means of a crossed waveguide coupling element 53 positioned generally perpendicular to the waveguide cavity portion 27 and contiguously adjacent thereto. Coupling of energy from the crossed Waveguide 53 to the waveguide portion 27 is controlled by means of a coupling probe 55 the details of which are more clearly shown in FIG- URE 4 and described hereinafter. Second harmonic energy is coupled out of the first cavity portion 25 through a small aperture in the hidden side wall thereof. An output coupling waveguide 50 is connected to that side wall and is utilized to propagate second harmonic energy outwardly from the system to any desired microwave utilization means as represented schematically by the arrow 56. In the embodiment shown in FIGURE 3, the output waveguide 50 is returned generally parallel to the waveguide portion 27 in order to facilitate mounting of the cavity structure 25 between the poles of a magnet as indicated schematically by the arrow 32.

In the harmonic generator of our invention, as shown in FIGURES 2 and 3, the fundamental frequency fields fill the entire volume of the cavity portions 25 and 27, while the second harmonic fields are confined substantially to the first cavity portion 25. By isolating the 2w harmonic fields in this manner, we have kept R small by minimizing U and We have permitted tuning of the to circuit without affecting the resonance of the first cavity portion to the 210 energy. In one apparatus which we have constructed in accordance with our invention, the 2w cavity 6 was matched to its load and had a Q of 3800. About 90% of the power entering the input guide was absorbed in the ferrite element 30. In this particular embodiment, there was no need for tuning the first cavity portion 25, since the fundamental frequency source was tunable, for adjustment of the input frequency and therefore the generated harmonic to the natural resonant frequency of the cavity portion 25. It will be appreciated that when a fixed frequency fundamental signal source is to be used, it will generally be necessary to provide means for tuning the first cavity portion 25 to the generated harmonic signal. In such a case, conventional tuning screws, not shown, placed generally parallel to the direction of the 2w electric field in the cavity portion 25 may be utilized.

The apparatus Was driven with a magnetron (2,12 sec., 40 p.p.s., 8.5 gc.). A number of different elements 30 of single crystal MnFe O were tested. In the most efiicient case, the cavity input power was 334 watts and the output 18.4 watts. The other sample elements 30 were somewhat poorer but still of about the same order. From FIGURE 1, we could expect higher eificiencies at higher input powers.

The detailed structure of the crossed waveguide input energy feeding means 53 is best illustrated in FIGURE 4 which is a cross-sectional view taken in the plane which includes the longitudinal axis of the second cavity section 27 and the axis of the movable probe 55. Specifically, the feeding means comprises a dielectric cylinder 57 which extends through the walls of the crossed waveguide 53 and inwardly through one side wall 47 of the Waveguide cavity portion 27. The dielectric cylinder carries at its inner end a metallic coupling stub 59 which absorbs fundamental frequency energy from within the crossed waveguide 53 and radiates that energy to the interior of the second cavity portion 27. The dielectric cylinder 57 is movable axially by means of thumb screw 61 to controllably vary the amount of input power which is coupled into the waveguide cavity portion 27 Thus the crossed waveguide 53 and the movable probe 55 serve to control the amount of fundamental frequency power which is coupled into the first cavity portion 25 for excitation of the ferromagnetic material wafer 30.

In accordance with a further embodiment of our present invention, we contemplate that a cavity structure of the type shown in FIGURE 2 may be utilized for heterodyne conversion to obtain the sum or difference frequency of two separate input microwave signals. That is, two separate input signals may be applied to the wave guide portion 27 by means of first and second coupling devices similar to the crossed waveguide 53 as shown in FIGURE 3. These two separate input signals are propagated simultaneously along the waveguide portion 27 to the first cavity portion 25 and jointly excite the ferromagnetic element 30. By virtue of the nonlinear characteristics of the element 30, a heterodyne mixing of the two input signals may be accomplished so that either the sum or the difference frequency may be extracted from the first cavity portion 25 by means of an output circuit generally'similar to the output waveguide 50 of the apparatus of FIG- URE 3.

While the present invention has been shown in one form only, it will be obvious to those skilled in the art that it is not so limited but is susceptible to various changes and modifications without departing from the spirit and scope thereof.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A frequency-multiplying microwave cavity device comprising in combination:

a cavity resonant in one mode at a fundamental frequency and resonant in a different mode at a harmonic frequency, said cavity having a generally T- shaped cross-sectional configuration with the crossbar of the T being a first cavity portion and the leg of the T being a second cavity portion having a longitudinal axis intersecting the crossbar back wall, said first cavity portion being dimensioned to be alone resonant at the harmonic frequency and said second cavity portion being dimensioned to propagate fundamental frequency energy into said first cavity portion but to substantially exclude harmonic frequency energy generated in said first cavity portion from said second cavity portion to provide maximum coupling of fundamental frequency energy to said first cavity portion,

a ferromagnetic material element disposed entirely within said first cavity portion on said longitudinal axis and crossbar back wall, said element being capable of generating magnetic flux at the harmonic frequency when subjected to a DC. magnetic flux in the plane of said ferromagnetic material element and a fundamental frequency magnetic flux substantially in the plane of said element in a direction normal to that of the DC. magnetic flux,

energy input means for exciting said cavity to resonate at the fundamental frequency so that said ferromagnetic material element is caused to produce magnetic flux at the harmonic frequency in the direction of the DC. magnetic flux and in the plane of said ferromagnetic material element,

and tuning means associated with the outer end of said leg for tuning said cavity to said fundamental frequency without altering the harmonic frequency resonance of said first cavity portion.

References Cited by the Examiner UNITED STATES PATENTS 3,041,524 6/1962 Karayianis et a1 321-69 3,044,021 7/1962 Poole et a1. 330-4.8 3,054,042 9/1962 Weiss 321-69 3,076,941 3/1963 Yariv 330-4.9 3,229,193 1/1966 T. Schaug-Petterson 321-69 FOREIGN PATENTS 833,130 4/1960 Great Britain.

OTHER REFERENCES Microwave Frequency Doubling From 9 to 18 KMC in Ferrites by Melchor, Ayres and Vartanian; Proceedings of IRE (1957); pages 643646.

JOHN F. COUCH, Primary Examiner.

G. GOLDBERG, Assistant Examiner.

Claims (1)

1. A FREQUENCY-MULTIPLYING MICROWAVE CAVITY DEVICE COMPRISING IN COMBINATION: A CAVITY RESONANT IN ONE MODE AT A FUNDAMENTAL FREQUENCY AND RESONANT IN A DIFFERENT MODE AT A HARMONIC FREQUENCY, SAID CAVITY HAVING A GENERALLY TSHAPED CROSS-SECTIONAL CONFIGURATION WITH THE CROSSBAR OF THE T BEING A FIRST CAVITY PORTION AND THE LEG OF THE T BEING A SECOND CAVITY PORTION HAVING A LONGITUDINAL AXIS INTERSECTING THE CROSSBAR BACK WALL, SAID FIRST CAVITY PORTION BEING DIMENSIONED TO BE ALONE RESONANT AT THE HARMONIC FREQUENCY AND SAID SECOND CAVITY PORTION BEING DIMENSIONED TO PROPAGATE FUNDAMENTAL FREQUENCY ENERGY INTO SAID FIRST CAVITY PORTION BUT TO SUBSTANTIALLY EXCLUDE HARMONIC FREQUENCY ENERGY GENERATED IN SAID FIRST CAVITY PORTION FROM SAID SECOND CAVITY PORTION TO PROVIDE MAXIMUM COUPLING OF FUNDAMENTAL FREQUENCY ENERGY TO SAID FIRST CAVITY PORTION, A FERROMAGNETIC MATERIAL ELEMENT DISPOSED ENTIRELY WITHIN SAID FIRST CAVITY PORTION ON SAID LONGITUDINAL AXIS AND CROSSBAR BACK WALL, SAID ELEMENT BEING CAPABLE OF GENERATING MAGNETIC FLUX AT THE HARMONIC FREQUENCY WHEN SUBJECTED TO A D.C. MAGNETIC FLUX IN THE PLANE OF SAID FERROMAGNETIC MATERIAL ELEMENT AND A FUNDAMENTAL FREQUENCY MAGNETIC FLUX SUBSTANTIALLY IN THE PLANE OF SAID ELEMENT IN A DIRECTION NORMAL TO THAT OF THE D.C. MAGNETIC FLUX, ENERGY INPUT MEANS FOR EXCITING SAID CAVITY TO RESONATE AT THE FUNDAMENTAL FREQUENCY SO THAT SAID FERROMAGNETIC MATERIAL ELEMENT IS CAUSED TO PRODUCE MAGNETIC FLUX AT THE HARMONIC FREQUENCY IN THE DIRECTION OF THE D.C. MAGNETIC FLUX AND IN THE PLANE OF SAID FERROMAGNETIC MATERIAL ELEMENT, AND TUNING MEANS ASSOCIATED WITH THE OUTER END OF SAID LEG FOR TUNING SAID CAVITY TO SAID FUNDAMENTAL FREQUENCY WITHOUT ALTERING THE HARMONIC FREQUENCY RESONANCE OF SAID FIRST CAVITY PORTION.

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