US2998582A - Temperature compensated microwave cavity - Google Patents

Description

United The present invention relates in general to temperature compensation and more particularly concerns means for stabilizing a frequencysensitive dimension of a microwave cavity in a prescribed manner over a wide temperature range. When employed as a frequency-controlling resonant cavity, the frequency-sensitive dimension remains constant in the presence of temperature variations. As a result, a microwave signal characterized by exceptional frequency stability over a wide temperature range may be generated. The properties of such a cavity are also advantageously employed in microwave circuits, such as frequency discriminators, where frequency response characteristics independent of temperature are desired.

The theory of temperature compensation of microwave cavities is fully discussed in Patent No. 2,790,151 eutitled Temperature Compensated Cavity Resonator, issued April 23, 1957 to Henry J. Riblet. In that patent, there is disclosed a cavity resonator capable of yielding an exceptionally accurate frequency indication of a microwave signal coupled thereto over a relatively broad frequency band and despite relatively Wide temperature variations. This is accomplished by selectively changing the temperature coefficient of expansion of the member controlling the cavity length as the cavity length changed from one range to another.

The present invention contemplates and has as a primary object the extension of the range over which accurate temperature compensation of a frequency-sensitive dimension of a microwave cavity may be obtained. This is accomplished by controlling a frequency-sensitive dimension, generally length, of the cavity with different means over diiferent temperature ranges, each of the means changing length by a different increment in re- Spouse to an incremental temperature change of the same magnitude. In one form of the invention, each means comprises a multiplicity of members of approximately the same length, but of different material having unlike temperature coeflicients of expansion. In another form, each means comprises different lengths of the same material.

, In a specific embodiment of the invention, the cavity is defined by a generally hollow cylindrical conducting structure having a fixed end wall and an oppositely disposed axially movable confronting end wall. A plurality of axially expansible sections are supported within the cylindrical structure. Resilient means urge the movable end wall and at least one of the expansible sections in opposite axial directions. The cylindrical structure and expansible sections have different temperature coefficients of expansion. As the temperature varies, the fractional change in length in response to an incremental change in temperature differs for the cylindrical structure and each section. This difference is utilized to cause a corresponding change in the axial force applied to the movable end wall whereby it tends to move axially in a direction opposite to the change in length of the cylindrical structure. However, in any one temperature range, a selected one of said sections controls the magnitude of this restoring axial force applied to the end wall.

In one form, the sections comprise a plurality of concentrically arranged annular cylinders having different temperature coefiicients of expansion. In each temperature range, one of the annular cylinders is longer than the others and its ends separate a plug fixed in the end Patent of the cylindrical structure from a slideable member for transmitting axial restoring forces to the movable end wall. Accordingly, expansion and contraction of the longest cylinder in a particular temperature range controls the magnitude of the restoring force.

In another form, the sections comprise difierent lengths of the same cylinder having an annular ridge adapted to fit loosely within an annular groove inside the cylindrical structure. In a first temperature range, the inside edge of the ridge, that is, the edge nearest the movable end wall, abuts the inside edge of the groove and the restoring force is determined by length changes of the section of the cylinder between its inside end and the inside edge of the ridge. In a second temperature range, the ridge contacts neither groove and length changes of the entire cylinder governs the magnitude of the restoring force. In a third temperature range, the outside edges of the ridge and groove meet and length changes of the cylinder section from the outside ridge edge to its outside end controls the restoring force.

Other objects, features and advantages of the invention will become apparent from the following specification when read in connection with the accompanying drawing in which:

FIG. 1 shows an embodiment of the invention wherein the length adjusting sections comprise a pair of concentrically arranged annular cylinders having different temperature coefiicients of expansion;

FIG. 2 illustrates an alternative form of the invention with the length adjusting member comprising a cylinder having an annular ridge loosely engaging an annular ridge inside the generally hollow cylindrical structure; and

FIG. 3 shows an embodiment of the invention, especially suitable for large scale production because of the relative ease of fabrication.

Similar elements are designated by the same reference symbol throughout the drawing.

With reference now to the drawing and more particularly FIG. 1 thereof, a first form of the temperature compensated microwave cavity is illustrated. The cavity 11 is defined by the generally hollow cylindrical structure 12, shown in cross-section, having a fixed end wall 13 and a movable end wall formed of piston 14 opposite the latter and free to slide axially within cylindrical structure 12. Piston 14 is formed with a central collar section 15 and neck section 16 outside the cavity 11. A resilient diaphragm 17 surrounds neck section 16, is in surface contact with collar section 15, and is secured at its outer periphery by welding or other suitable means to the wall of cylindrical structure 12 whereby an axial force is imparted upon piston 14 urging it to the left.

Neck section 16 is integral with or rigidly attached to a bolt 21 at the flat disc-shaped head 22 thereof. The stem 23 of bolt 21 is centrally supported within end plug 24 in a manner permitting the bolt 21 to slide axially but not radially. End plug 24 is screwed into the right end of cylindrical structure 12 after the remaining internal elements have been inserted. v

A threaded ring 25 is screwed inside cylindrical structure 12 and holds diaphragm 17 firmly in place. A resilient annular disc 26 is locked between ring 25 and a threaded cylindrical shell 27. The radially inward edge of disc 26 rests in shoulder 31 of head 22 and urges bolt 21 to the right.

Annular cylinders 32 and 33 coaxially surround stem 23. The edges of at least one of these cylinders, depending on the temperature, contact head 22 and end plug 24.

Having described the structural arrangement of a temperature compensated microwave cavity, its mode of operation will be discussed. Preferably, all the rigid structural elements except annular cylinders 32 and 33 are made of Invar having a low temperature coeflicient of expansion. Cylinder 32 is made of Invar having a higher temperature coefficient of expansion than cylindrical structure 12 and cylinder 33 is formed of brass having a still higher temperature coefficient of expansion. Thus, as the temperature changes, the fractional change in length of cylindrical structure 12 and the other elements made of like Invar, of Invar cylinder 32 and of brass cylinder 33, differs.

If no temperature compensation were provided, a change in temperature would be accompanied by a change in the length L of cavity 11 as a result of contraction or expansion of cylindrical structure 12. In accordance with the inventive concepts, piston 14 is moved axially in a direction tending to counteract the change in length of cylindrical structure 12 and the cavity length L accordingly remains substantially constant, regardless of temperature. In a first temperature range, a corrective displacement is supplied by changes in length of cylinder 32 While in a second range such corrections are in response to changes in length of cylinder 33. This will be better understood by considering the following example.

At a specified temperature, typically centigrade, cylinders 32 and 33 are of the same length and in edge contact with head 22 and end plug 24. Since Invar cylinder 32 has a lower temperature coefficient of expansion than brass cylinder 23, the fractional change in length of the former for a given change in temperature is less than that of the latter. Accordingly, at temperatures below 0 centrigrade, only cylinder 32 is in edge contact with head 22 and end plug 24. As a result, below this temperature, contractions and expansions thereof are transmitted through head 22 to piston 14 rigidly connected thereto and move the piston axially to compensate for changes in length of cylindrical structure 12 due to temperature variations in this region. Above 0 centigrade, compensatory movement occurs in response to length changes of brass cylinder 33.

Resilient diaphragm 17 and resilient annular disc 26 center the rigidly connected piston 14 and bolt 21 and impart axial forces thereon in opposite directions to aid in overcoming any static friction between slideable elements and cylindrical structure 12. Thus, slight compensatory changes in length are immediately followed.

With reference to FIG. 2, there is illustrated an alternative embodiment of the invention. Elements corresponding substantially to those in FIG. 1 are identified by like reference numerals. Cavity 11 is defined by the generally hollow cylindrical structure 12 having a fixed end wall 13 and accommodating piston 14, the axially slideable end wall of the cavity 11. Resilient diaphragm 17 surrounds neck section 16, is in surface contact with collar section 15, and is secured at its outer periphery by welding or other suitable means to the wall of cylindrical structure 12 whereby an axial force is exerted upon piston 14 urging it to the left. Cylindrical structure 12 and neck section 16 have annular ridges 34- and 35, respectively, extending radially inward and outward respectively. A resilient diaphragm 36 surrounds neck section 16 and presses against the right edge of ridge 34 and the left edge of ridge 35 at its outer and inner peripheries, respectively, thereby urging piston 14' to the right.

In the right section of cylindrical structure 12, there is an annular groove 38 extending radially outward from the inner surface of cylindrical structure 12. A brass cylinder 39 is supported within the right end of cylindrical structure 12 and has as an annular ridge 41 extending radially outward adapted for loose mating relationship with annular groove 38. At the left end, cylinder 39 has an axially aligned conical groove 42 separated from a similarly aligned conical groove 43 by ball 40. The right end of cylinder 39 also has an axially aligned conical groove 48 facing a similar groove 44 in cup-shaped end cap 45 and separated therefrom by ball 46. The inner tangential edge 47 of end cap 45 slides over the annular ridge 51 of cylindrical structure 12. A resilient spring 52 between end cap 45 and the radially inward extending annular ridge 53 at the right end of cylindrical structure 12 exerts a force toward the left on cylinder 39 through ball 46 and end cap 45, and also serves to keep the latter elements in place.

Temperature compensation is obtained in this embodiment in the following manner. Initially, it is convenient to assume that the temperature is in the lowest range and brass cylinder 39 contracted. At this time, the force exerted by resilient spring 52 toward the left is sufficient to overcome oppositely directed forces from other resilient members and force cylinder 39 to the left until the left or inner edge 55 of annular ridge 41 contacts the corresponding edge 54 of groove 38. Compensation in this temperature range is then effected by expansion and contraction of the left section of length A between the left or inner end of cylinder 39 and the left or inner edge 55.

In the next higher temperature range, the force exerted by resilient diaphragm 36 to the right overcomes other oppositely-directed forces and the left edge 55 moves away from left edge 54. In this range, neither edge of annular ridge 41 contacts a radial edge of groove 38 and length changes of the entire cylinder length B govern the corrective displacement imparted to piston 14.

Finally, the highest temperature range is reached when cylinder 39 has moved toward the right with right edge 57 of annular ridge 41 contacting the corresponding edge 56 of annular groove 38. Compensation is then determined by expansion and contraction of the section of length C extending from the left end of cylinder 39 to the right or outer edge 57.

Since the incremental change in length is given by:

AL=ozLAT where L is the length of the expansible section, a is its thermal coefficient of expansion, and AT is the incremental change in temperature, in the lowest range, the compensating length change is AL=aAAT In the intermediate temperature range, half the incremental length change of the cylinder is imparted to the left to correct the position of piston 14 and the other half moves resilient spring 52 to the right. Accordingly, in this range, the compensating length change is AL (12A T Finally, in the third range the compensating length change is AL=0LCAT Cylinder 39 may be constructed of sections of material having different thermal coefiicients of expansion bolted together to obtain different degrees of compensation. There may be several annular ridges and mating grooves. Various different combinations of spring tensions may be employed.

With reference to FIG. 3, there is illustrated still another embodiment of the invention for obtaining precise temperature compensation. The advantages of the other embodiments are retained, yet fabrication in large quantities is made still easier. The axially movable end wall is a resilient conducting disc 61. The peripheral edges of disc 61 and piston 14 are brazed together at 60. A hollow unitary structure 62 is made of Invar having a temperature coeflicient of expansion different from the Invar used for the cylindrical housing 12. The unitary structure 62 is formed with a stem 63 and a hub 64, all coaxially surrounding a brass rod 65. The right ends of rod 65 and stem 63 are soldered together. The remaining portion of rod 65 is free to expand and contract within the hollow chamber 66. A resilient diaphragm 17 is welded to cylinder 12 and stem 63 at its outer and inner circumferential edges, respectively. The right end of stem 63 seats in the mating opening within end plug 24. In FIG. 3 stem 63 is shown having a shoulder 67 held in abutting relationship with end plug 24 by the action of resilient diaphragm 17. Thus, the cavity length L may be initially adjusted by rotating end plug 24 to impart axial motion to structure 62 without rotation. Alternatively, where no adjustment of cavity length is desired, stem 63 may be rigidly secured to end plug 24 by soldering or other suitable means.

Over the low temperature range, disk 61 remains in contact with hub 64 by virtue of its own resiliency, and changes of length of structure 62 determine the compensation for temperature variations. In the high temperature range, the left end of brass rod 65 extends beyond the surface of hub 64 to exert an axial force on disk 61 to cause a temperature compensating displacement thereof.

There has been described a temperature compensated microwave cavity capable of retaining a desired dimension over a wide temperature range. It is apparent that those skilled in the art may now make numerous modifications of and departures from the specific embodiments described herein without departing from the inventive concepts. Consequently, the invention is to be construed as limited only by the spirit and scope of the appended claims.

What is claimed is:

l. A temperature-compensated microwave cavity comprising a hollow conductive cylinder conductively closed at one end thereof and formed with rigid supporting means at the other end thereof, a movable conductive wall disposed in said cylinder, the portion of said cylinder between said closed end and said movable wall defining the resonant volume of said cavity, resilient means urging said movable wall toward said supporting means, a plurality of temperature-sensitive control elements, said cylinder and each of said control elements having a different thermal coefficient of expansion, said plurality of control elements being interposed between said movable wall and said supporting means and arranged to limit the displacement of said movable wall under influence of said resilient means toward said supporting means, said control elements having lengths such that below a predetermined temperature one of said elements determines the position of said movable wall by alone obstructing movement of said movable Wall toward said supporting means while above said predetermined temperature another one of said control elements determines the position of said movable wall by alone obstructing movement of said movable wall toward said supporting means.

2. A temperature-compensated microwave cavity in accordance with claim 1 wherein said plurality of temperature-sensitive control elements comprise first and second concentric cylindrical spacers interposed between said movable wall and said rigid supporting means, said spacers being of such aXial length that for temperatures below a predetermined temperature one of said spacers alone obstructs the movement of said movable Wall toward said supporting means while for temperatures above said predetermined temperature the other of said spacers alone obstructs the movement of said movable wall toward said supporting means.

References Cited in the file of this patent UNITED STATES PATENTS 2,183,215 Dow Dec. 12, 1939 2,533,912 Bels Dec. 12, 19-50 2,667,623 Martin Ian. 26, 1954 2,716,222 Smullin Aug. 23, 1955

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