Stable solid dielectric microwave resonator and separable waveguide means

[57] ABSTRACT

A compact, stable microwave resonator employs a solid quartz cylinder coated on its surface with a material having good electrical conductivity at very high frequencies. The coating is made up of an inner layer of chromium much thinner than the skin depth of microwave currents and a thick outer layer of a metal of high conductivity for microwave currents. The coating has a pair of diametrically positioned apertures for coupling microwave energy to hollow wave guides extending radially from the cylindrical conductive surface. The waveguides have end faces matching the curved conductive surface and each has a mounting frame with arms extending partly around the cylindrical surface. Opposed flanges on the arms cooperate with resilient fasteners to clamp the waveguides in cooperative positions on opposite sides of the cylindrical surface.

Claims, 4 Drawing Figures

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PATENTED JAN 18 bt?

3,636,480

SHEET 1 OF 2

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ATTORNEY

PATENTED JAN 181372

3.636,480

SHEET 2 OF 2

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FIG. 3

INVENTOR

William N. Hoeck by

ATTORNEY

3,6:

STABLE SOLID DIELECTRIC MICROWAVE RESONATOR AND SEPARABLE WAVEGUIDE MEANS

BACKGROUND OF THE INVENTION .

1. Field of the Invention

The invention pertains generally to means for the calibration or stabilization of the operating frequency of highfrequency signals and more particularly to compact highfrequency or microwave resonators of the stable reference type suited for application as frequency controlling reference resonators, discriminators, frequency measurement devices, or the like.

2. Description of the Prior Art

An early approach to the problem of generating stable highfrequency or ultra-high-frequency signals was to employ a relatively iow-frequency oscillator whose frequency is controlled by a specially cut quartz crystal and then to use a succession of several frequency multiplier stages finally to generate the desired high-frequency signal. When such a system is employed to stabilize the output, for instance, of a velocity modulation oscillator, the number of components and consequently size, weight, cost, and power consumption become considerable. Further, reliability of the system is not always acceptable. Though a pure signal may have been produced by the crystal controlled oscillator, spurious signals appear in the multiplier stages which may represent interference signals in associated equipment and which certainly must be removed from the system output by application of suitably located filters. At high overall multiplication ratios, 30 the noise problem is further aggravated because the frequency modulation side band power increases as the square of the overall multiplication ratio.

A second known method of stabilizing high or ultra-highfrequency signal generators is to stabilize the generated signal directly by use of an ultrastable hollow cavity resonator. Stable hollow cavity resonators may be used in a variety of ways for the purpose; for example, when placed in the feedback loop of a velocity modulation oscillator tube, a stable cavity resonator will control the output frequency of the velocity modulation tube to quite close tolerances. Such stable hollow cavity resonators not only have utility as frequency controlling reference cavities, but also find application as discriminators, frequency meters, and filters.

The most common prior art approach to developing stable hollow cavity resonators is to employ a low expansion material such as an iron-nickel alloy like invar, along with high coefficient-of-thermal-expansion materials such as brass or steel, in a compensation configuration. However, it is found that the degree of compensation is exact only over a relatively narrow temperature range. As the operating temperature range is increased the action of the compensation configuration progressively degrades with one or more of the compensation members assuming the dominant role, overriding the effects of the remaining members. Also, it is found that consistently good compensation is not always achievable, since the behavior of some low expansion materials has been found to depend upon machining procedures and is certainly sensitive to the kind of heat treatment to which the material is subjected during its go original fabrication and subsequent use.

More recently, attempts have been made to use quartz, fused silica, or other glass or ceramic materials to form stable hollow cavity resonators. In one such device, a machinable ceramic material is employed to define a cylindrical hollow 65 cavity and its interior is coated several times with a silver paste, between which coating steps baking at an elevated temperature is required. Other attempts have involved use of glass or other substrates, the interior of the cavity being coated by a sputtering or by an electroplating process.

These attempts have not met with material success on several counts. The effect of moisture in the air on such cavity resonators is well known; generally, this is met by evacuating the cavity or by using an inert gas in its interior. Such requires the use of wave guide windows to provide a continuous pres6,480 sure or vacuum envelope. The effects of atmospheric pressure changes must be dealt with, especially if the cavity resonator is to be evacuated or pressurized. If the cavity resonator used an inert gas, temperature changes can strongly change the pres5 sure of that gas, again producing pressure-induced distortion of the resonator. The problems associated with fastening the hollow cavity parts together are increased if it is to be evacuated or pressurized. The resonator quality factor Q depends on the conductivity of the current-carrying walls of the cavity;

¹⁰ the latter will degrade in certain atmospheres.

Perhaps the most serious unsolved problem has been to achieve a practical design for coupling input and output transmission lines to the cavity resonator. Waveguides have with 15 difficulty been fastened to the cylindrical outer wall of the cavity resonator by using low expansion screws as fasteners to the ceramic material. Attempts also without remarkable success have been made to solder the guides directly to silver coated surfaces on the cavity resonator substrate. Such ar2q rangements have no apparent appeal for use in commercial equipment on the ground of their fragility and difficulty of manufacture. Furthermore, materials used in commercially available wave guides have relatively high expansion coefficients. Where temperature changes are not sufficient to break 25 the cavity resonator, serious distortion effects again contribute to alternation of its calibration.

SUMMARY OF THE INVENTION

The invention is a compact, stable, high-frequency resonator for high-frequency operation featuring the full use of the inherent capabilities of the stable materials of which it is constructed. The resonator itself is constructed as a separable unit and the hollow waveguide transmission lines associated therewith are formed as separable parts.

The resonator is solid, being formed of a material such as quartz, and is coated on its surface with an electrically conductive material. The coating is penetrated by oppositely located apertures for transmission of microwave energy. Being 4Q massive and solid, the resonator is more stable than resonators made in the form of cavities within such stable material. Its rugged nature also subdues adverse effects of atmospheric temperature and pressure changes and provides immunity to changes due to aging and moisture.

According to the invention, the mounting arrangements for the waveguides associated with the resonator consists of separable elements. They have end faces with coupling apertures matching the apertures provided in the coating on the resonator surface, and the end faces match the shape of the 50 adjacent surface of the coating. Clamped together by resilient means symmetrically about the coated surface of the resonator, they perform the function of transmission of electromagnetic energy into and out of the stable resonator without interfering with its stable properties.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a solid resonator employed in the present invention.

FIG. 2 is a perspective view of the waveguide and mounting system for the resonator of FIG. 1.

FIG. 3 is a cross-sectional view of the combined solid resonator and waveguide mounting system.

FIG. 4 is a fragmentary elevation view in cross section taken at the line 4 of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIGS. 1, 2, and 3 illustrate the invention in its preferred form and show the relationships of the separable parts thereof.

Referring especially to FIGS. 1 and 3, the high frequency resonator 10 takes the form, for example, of a polished right circular cylinder of a low loss, high-dielectric material of suitable mechanical and electrical properties such as fused or synthetic quartz. The quartz resonator cylinder may have a 75 circularly cylindrical wall surface 11 as seen in FIGS. 3 and 4

3,636,480 and a first flat end wall surface 1 as seen in FIG. 4, as well as a second similar flat end wall surface 2 (not shown) opposite to and substantially parallel to the first end wall surface 1. Wall surfaces 1 and 2 are preferably perpendicular to the axis of cylindric wall II.

While materials other than quartz, such as glass or certain ceramic materials, may be used, synthetic quartz is preferred for the resonator because of its stability, its low thermal expansion, and its high dielectric constant and low electrical loss factors. The use of quartz, for example, permits a significant reduction in the size of resonator 10 for a operating frequency over the size of resonator 10 for a given operating frequency over the size of the prior art cavity resonator of the type whose active volume is evacuated, because resonator of the type whose active volume is evacuated, because all lineal dimensions of the resonator 10 are reduced by the reciprocal of the square root of the dielectric constant of the material of the active volume. Thus, if a given dimension of the evacuated cavity is denoted by L, the corresponding dimension of the quartz resonator 10 is 0.514 L, which represents almost a 50 percent reduction in lineal dimensions with a corresponding almost eightfold reduction in volume and weight. A correlative benefit also derives because of the low loss properties of quartz.

A condition must be established at the wall surfaces 1, 2, and 11 of the solid resonator 10 to prevent electromagnetic energy from radiating through those surfaces so that substantial energy is stored per cycle within the resonator. This is accomplished by supplying the wall surfaces 1, 2, and 11 of resonator 10 with an electrically conducting layer, such as the cylindric layer 12 on the cylindric wall 11 and as the flat conducting layers 13 and 14 on the respective flat end walls 1 and 2 of the resonator 10.

While several ways of applying the conducting layers 12,13, and 14 are available, it is desired that the surfaces of the layers contacting the quartz or other dielectric material of resonator 10 be highly conducting to high frequency electromagnetic energy. By keeping ohmic losses in the walls low, the quality factor Q of the resonator is correspondingly improved, just as it is by use of a low loss factor dielectric material for the body of resonator 10.

The wall surfaces of resonator 10, including wall surface 11 and the two flat end wall surfaces 1 and 2 of the cylinder, are first highly polished by any of several known methods. A polish to a degree providing what is known in the industry as an optical polish is found to be adequate, though the more polished the surfaces, the higher will be the quality factor Q of the finished resonator. This conventional measure, in the convenient terms of a polish suitable for visible light devices, is known as the Rayleigh limit which permits a one-quarter wavelength optical perturbation in an optical surface. Perturbations of less than one-quarter wave of visible light are also readily established by standard polishing procedures.

It is desired to place on the wall surfaces 1, 2, and 11 of resonator 10 a layer of silver, gold, or other good electrical conductor, a layer sufficiently thick as to be mechanically and otherwise stable, and a layer tightly bound to the quartz material. Such is accomplished by first applying to the quartz surfaces a very thin layer of a material that bonds better to quartz than does silver, for instance.

A base layer of chromium is used to permit adhere silver layer to be adequately bound. Chromium is found to adhere strongly to quartz and silver, in turn, adheres strongly to chromium. The chromium is preferably vacuum deposited by sputtering on all wall surfaces 1, 2, and 11 of quartz resonator 10 by a conventional vacuum metal process. Other vacuum evaporation or metal distillation methods are also applicable.

A conventional method of thickness control of the chromium layer 3 and 4, for example, is employed, since the thickness of the layer must b held below an upper extreme value. Chromium has a relatively high ohmic loss to highfrequency currents; therefore, the thickness of the chromium layer is controlled to be a minor portion of the skin depth dimension for the wavelength of the operating high-frequency energy. If the chromium layer were so thick as to be a significant part of the depth to which the high-frequency energy penetrates into the chromium layer, excessive ohmic losses 5 would result. Therefore, the thickness of the chromium layer is held to a value not greater than 1,000 Angstrom units for operation, for example, at 15 GHz. Opposed apertures 15 and 16 are diametrically required for a purpose to be explained hereinafter on the cylindrical surface 11 of resonator 10 (see ¹⁰ FIG. 3). Chromium and the later-to-be-deposited silver are prevented from falling on these aperture areas by a suitable patch mask held during the plating operation by an adhesive to the resonator surface. Upon completion of the coating 15 steps, the mask is removed manually in the conventional manner.

Over the chromium plates surfaces 1, 2 and 11 is added a final layer of silver; a conventional electroplating or other method may be employed for the purpose. It is to be observed 2q that the thickness of the silver layer may be made great enough to afford good mechanical stability with relatively little possibility that the silver film thickness will cause distortion of resonator 10, the body of the resonator 10 being relatively massive and immune to such distortion. In FIG. 4, the final 25 relatively thick silver layer is represented by the respective continuous layers 5 and 6, layer 5 overlying chromium layer 3 to form composite layer 12, while layer 6 overlies chromium layer 4 to form composite layer 14, for example.

The drawing of FIG. 3, for the sake of convenience, illus30 trates the composite chromium-silver layer as a single layer 12 because the chromium layer is so very thin as to be very small in thickness compared to the silver layer. ί

It is seen that the inventive resonator makes full use of the stable quartz material, as it is used in a manner significantly 35 reducing the size of resonator 10. It is used as the resonator itself, not merely to bound the resonator; thus, the resonator is small and furthermore is stable over a range in excess of—100° C. to +250° C., the loss factor and the dielectric constant of quartz being substantially constant over that temperature ⁴θ range.

The resonator 10 is relatively easily manufactured there being no interior surfaces to shape or to plate the metal. Machining and other fabrication problems inherent in prior art reference cavity resonators are largely eliminated.

The conducting surface of the silver is an enclosed interior of metal-quartz interface surface, rather than an exterior surface, as in the prior art, and is inherently protected from the degrading effects of moisture, handling, and the like. Pressurized windows are eliminated, along with sensitivity to atmospheric effects, since evacuation or pressurization of a hollow cavity is eliminated.

Resonator 10 is seen to be devised so as to permit its characteristics to be determined primarily independently of the characteristics required by any usual kind of transmission line input or output system to be coupled to it. In a similar manner, it will be seen that the novel transmission line system, now to be discussed with reference to FIGS. 2 and 3, is devised so as to have characteristics primarily determined by requirements 6Q of such a system and, furthermore, so that it features full compatibility with the established characteristics of resonator 10.

As is seen in FIGS. 2 and 3, the separable parts of the wave guide transmission line system are substantially symmetrically placed about resonator 10 when in use. One portion of the 65 system includes a hollow waveguide 20 equipped in a conventional way with a coupling flange 21 for fastening to utilization equipment. At the end of waveguide 20 opposite flange 21 is a conducting diaphragm 22 having surfaces 24, 25 conforming to the shape of cylindric wall 11 of resonator 10. Diaphragm 70 22 is further provided with a centrally located aperture or coupling iris 23 adapted to cooperate with aperture 15 of resonator 10 for transmission of high-frequency energy.

Affixed to opposed sides of waveguide 20 are arms or brackets 26 and 27 which form part of a mounting frame, the arms 26 and 27 extending partly around the cylindrical sur3,636,480 face 12 of resonator 10. Opposed flanges 28 and 29 are provided on the respective arms 26 and 27; flanges 28 and 29 are equipped with holes 30 and 31 for purposes yet to be discussed.

Directly opposite the just-described separable waveguide element is a second similar wave guide element which is a mirror-image counterpart of the first. The second waveguide device includes a hollow waveguide 120 equipped with a conventional coupling flange 121. Guide 120 has a diaphragm 122 having surfaces 124,125 also conforming to the cylindric shape of wall 12 of resonator 10. Diaphragm 122 is also provided with a centrally located coupling iris 123 adapted to cooperate with the aperture 16 of resonator 10.

Affixed to the opposed sides of waveguide 20 are arms or brackets 126, 127 which form part of a mounting structure, the arms 126,127 extending partly around the cylindrical surface 12 of resonator 10 toward arms 26 and 27. Opposed flanges 128 and 129 are provided on the respective arms 126 and 127. Flange 128 lies generally parallel to flange 129. Flanges 128 and 129 are respectively equipped with holes 130 and 131; holes 30 and 130 are aligned and holes 31 and 131 are aligned.

The separable parts supporting waveguides 20 and 120 are mounted in symmetric relation about resonator 10 so that aperture 23 is aligned with aperture 15, and so that apertures 16 and 123 are similarly aligned. The said separable parts are held in position by resilient means including bolts 60 and 61, nuts 62 and 63, and springs 64 and 65. Bolt 60 passes through holes 30 and 130 for this purpose, while bolt 61 passes through holes 31 and 131.

As seen from FIG. 3, the wave guides 20 and 120 are maintained in cooperative relation for high-frequency signal propagation with resonator 10. The metal parts of the system supporting guides 20 and 120, when subjected to temperature change, are free to move by virtue of springs 64, 65 without distorting or damaging the dielectric material of resonator 10. The resilient mounting system including springs 64 and 65 permits expansion of the metal parts relative to the low expansion quartz resonator without stressing the quartz.

In operation, wave guide 20 may be coupled to a source of high-frequency energy which flows through aperture 23. The wall of aperture 23 is seen to form a continuous conducting surface across boundary 70 between apertures 23 and 15 and along the interior of conducting layer 12 at wall 11. Consequently, energy fed into resonator 10 may build up a field therewithin at the resonant frequency of the system. Energy may be similarly coupled out of wave guide 120, since the wall of aperture 16 forms a continuous conducting surface across boundary 170 with the wall of aperture 123.

It is seen that the novel stable resonator system avoids the serious deficiencies of the prior art by providing a stable solid dielectric resonator with a highly conducting surface substantially independent of temperature, pressure, and aging effects. The device takes a form that is readily adaptable to manufacture and is also relatively immune to damage. The mounting arrangement for the associated transmission lines permits use of conventional waveguide materials. Furthermore, in the form illustrated, the invention may be used with interchangeable cylindrical resonators of substantially the same diameter (where the length of the resonator, rather than its diameter, determines the center frequency of the resonator). For example, in certain higher order mode resonators, the coupling apertures 15 and 16 may be located near one of the flat end walls of the resonator. Specifically, resonator systems according to the invention have been operated in the TE_O1I mode and the the TE_Oh mode, through many other modes may be selected.

While the invention has been described in its preferred embodiments, it is to be understood that the words which have been used are words of description rather than of limitation and that changes within the purview of the appended claims may be made without departure from the true scope and spirit of the invention in its broader aspects.