US3020500A - Coaxial cavity tracking means and method - Google Patents

Description

Feb. 6, 1962 BEISER 3,020,500

COAXIAL CAVITY TRACKING MEANS AND METHOD Filed May 20, 1960 HIS ATTORNEY Unit Sttes 3,020,500 COAXIAL CAVITY TRAQKING MEANS AND METHOD Leo Beiser, Flushing Manor, N.Y., assignor to Polarad Electronics Corporation, Long Island City, N.Y., a corporation of New York Filed May 29, 196i Ser. No. 30,527 3 Claims. ((31. 33382) The present invention relates to coaxial resonators, and more particularly to new and novel tracking means for incrementally tuning the coaxial cavity to comply with a given error function.

Many microwave apparatus, in particular, field intensity meters and receivers, include a plurality of separate tunable circuits, and it often becomes important to be able to tune these various circuits simultaneously while maintaining them in some predetermined relationship. For example, in a tuned high frequency type of receiver, various tuned circuits must be maintained with equal resonant frequency. In staggered-tuned circuits, such tuned circuits must be ideally maintained with a constant difference between the resonant frequencies. Also, in the superheterodyne type of microwave receiver, a cm oscillator tuned circuit and a preselector tuned circuit must be tunable over the entire range of reception for the receiver, While simultaneously maintaining a relatively constant frequency difference between the resonant frequencies of the respectively tuned circuits. In the microwave field these tuned circuits generally are of the cavity resonator type wherein the resonant frequency of a cavity is varied by changing the effective electrical length of the transmission line defined by the cavity dimensions. Thus a typical microwave receiver might include an external cavity klystron local oscillator, a tracking preselector employing two resonant coaxial cavities coupled by means of apertures to yield both the desired bandwidth and the desired rejection of spurious signals, and other tracked circuit components such as dial mechanisms, a potentiometer to vary the klystron repeller voltage, etc.

- In many such applications as above described the tuned circuit consists of a coaxial resonator wherein the tuning is accomplished by the axial translation of the moveable center conductor with respect to the outer conductor of the cavity. In such cases the variation in resonant frequency is a non-linear function of the displacement of the control member, i.e., the moveable center conductor, and moreover this frequency vs. displacement relationship may differ for the various tuned circuits to be simultaneously controlled. Accordingly, this gives rise to a problem known as tracking, by which is meant the maintenance of the respective tuned circuits in their proper resonant frequency relationship over the entire range of variation in tuning. Thus, in the case of the cavity resonators employed in the klystron local oscillator and the preselector of the typical microwave receiver abovementioned, where the respective resonators are tuned to different frequencies requiring difierent movement of their controlling members for the same frequency change, the relationship of frequency vs. center conductor displacement for each cavity is a complexly'variable one over'the range of frequencies involved. Therefore relatively elaborate means must be provided for tuning the various resonant cavities and other tracked elements in the microwave apparatus in order to ensure that the desired relationshipover the tuning range exists between the respective frequencies, tuning dial indicators, repeller potentiometer, etc. One such means for accomplishing this tracking embraces the use of an initial or coarse cam for adjusting the displacement of the center conductor in the resonant coaxial cavity to approximately tune the device. To

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obtain optimum performance it then becomes essential that the resonant frequency of the cavity be adjusted by a fine correction cam which has many individual ad,- justment points and which superimposes an individual corrective effect upon the tuning of the cavities by the coarse cam in acting upon the displacement of the center conductor with respect to the outer conductor. The inclusion of this last named cam, which accomplishes the fine tune ing of the coaxial cavity to comply with a given error function in order to maintain the proper tracking relationship, is a technique in general use today in microwave equipment wherein this tracking problem is present. Mechanical tracking of this type suffers from many disadvantages among them the requirement that the drive assembly be critically assembled to close dimensions and tolerances with resultant costly machining and alignment procedures. Also the complicated mechanical linkages involved in transferring the compensation adjustment from the main drive to the incremental adjustment of the tuning plunger in the coaxial cavity inherently leads to residual backlash and dead spots. Further, such mechanical mechanisms are relatively expensive to fabricate because of the large number of components required. In many applications such as airborne equipment the great weight and size of the mechanical tracking adjustment. means is a serious factor. v

The present invention provides a novel electrica tracking compensator for a coaxial cavity, such as those used inpreselectors and other staggered-tuned circuits, to incrementally adjust the resonant frequency of the cavity to comply with a given error function, i.e., to accomplish the fine frequency tuning of a tracking tuned circuit. The coarse or major resonant tuning is first effected by a variation of the active length of the center conductor inside the coaxial cavity by means of a conventional mechanical displacement linkage; The tracking compensation is then achieved by the use of a plurality of individually adjustable members staggered in space relationship along the outer conductor wall of the coaxial cavity in conjunction with a modification of the center conductor or tuning plunger such that the electrical field configuration in the vicinity of the end point of the tuning plunger is influenced by the depth of penetration of the particular adjustable member in register with the end point of the center conductor to thereby provide an incremental variation of the cavity field geometry and thus of its resonant frequency. The invention provides an electrical tracking compensator which has the advan-' tages of simplicity; economy; high resolution; and very significantly, a relative independence of adjustment and effect of each discrete fine tuning member. The present invention is particularly useful in aligning and correcting the tracking of suchtuned circuits which are mass produced in quantity, wherein unavoidable variations in ,machining tolerances and electrical characteristics may be found, and which may thus be readily compensated for by the present invention.

It is therefore an objective of the invention to provide, a new and novel means for tracking a coaxial cavity resonator.

' It is another object of the invention to provide a new and novel means for incrementally tuning a resonant, co-

axial cavity to comply with a given error function. It is yet another objective of this invention to provide a tracking compensator which is particularly suitable for use in multi-stage preselectors and other staggered micro wave tuned circuits where it is desirable to maintain a predetermined frequency relationship between the nant frequencies of the respective tuned circuits.

Still another objective of the present invention-is-toprovide such a tracking compensator which is accurate;

economical to fabricate, simple in' construction, and of 3 much smaller size and weight than conventional mechanical tracking compeusators.

And a still further object of the present invention is to provide a new and novel tracking compensator for tunable coaxial cavities having a plurality of individually adjustable tuning corrections which are relatively independent of each other.

Further other objects and advantages of the present invention will become apparent upon consideration of the following drawings wherein FIGURES l, 2, and 3, are representative plots of certain curves which will be useful in explaining the theory of operation of the invention;

FIGURE 4 is a front view, partially sectional, of a coaxial cavity resonator incorporating the present invention; and

FIGURE is a cross-sectional view as seen substantially along the plane of the line 55 in FIGURE 4.

The problem of the tracking of a coaxial cavity resonator so that it may be incrementally tuned to comply with a given error function may be understood if reference is made to the following illustrative curves of FIGURES 1, 2, and 3. FiGURE l is a representative plot of the resonant frequency f vs. the active length l of the center conductor of a typical coaxial cavity resonator. As the tuning plunger or center conductor is axially translated with reference to the outer conductor of the cavity the resonant frequency thereof varies in the manner shown from a relatively high resonant frequency for a small active length of the plunger inside the cavity to a relatively low resonant frequency as the center conductor is extended to its fullest penetration inside the coaxial cavity. It is to be noted that although the curve is non-linear; it is nevertheless smoothly continuous and represents an inverse frequency function, as may be seen.

FIGURE 2 is a representative curve of the error function A vs. frequency which arises in other tracked components of the microwave system due to reference miscalibration; i.e., the incremental error which arises in the alignment of the tracked components of the system, such as the multiple tuned circuits having different tuning characteristics, the variation of the potentiometer adjustment controlling the klystron repeller voltage, the dial mechanism, etc. Thus FIGURE 2 is representative of the tracking error which might result in a microwave system having a number of tracked components after the assembly of components and their gross alignment, by means of coupled mechanical translating linkages (such as the cam and follower mechanisms above-described), is made. The error function A vs. frequency f for the microwave system may not follow any determinable relationship but may vary randomly around the desired operating point Af=0; i.e., perfect tracking.

FIGURE 3 is a super-position of the error function M on the curve of resonant frequency f vs. active length l of the center conductor inside the coaxial cavity resonator. A combination of the two yields a plot of the desired variation in the center conductor travel vs. frequency for the coaxial cavity resonator which would result in a perfect tracking of the system. Thus FIGURE 3 is suggestive of the prior art or mechanical method of providing tracking for a tuned cavity-incrementa variation of the active length of the center conductor to achieve frequency correction by means of a fine correction cam having many individual adjustment points or other similar mechanical means for achieving individual corrective effect upon the gross displacement by the main cam of the tuning plunger or center conductor with respect to the outer conductor of the coaxial cavity resonator.

The present invention, however, provides an electrica tracking means, whereby the equivalent of corrective motion of the center conductor is accomplished by an incremental variation of the cavity geometry, defined by the center conductor-to-wall spacing, insofar as the electrical field configuration in the vicinity of the end point of the tuning plunger is concerned. Incremental tuning correction and hence tracking compensation are achieved by the resulting change in the capacity of the resonant cavity as the center conductor-to-wall spacing is varied. The error correcting information is applied directly to the electrical field within the cavity such that a correction is accomplished at the frequency of interest.

Referring again to the drawings, and particularly to FIGURES 4 and 5 thereof, there is shown therein a coaxial cavity resonator, designated generally as 10, having an outer conductor 11 with an end wall 12 and a center conductor or tuning plunger 14. Apertures 13 are provided for inserting and extracting energy from the resonator in a conventional manner as is well understood by those cognizant of the art. Arranged along a line substantially parallel to the longitudinal axis of the outer conductor 11 and projecting therethrough, are a series of non-metallic members, designated as 16a 1611, made of some suitable material such as nylon, in spaced relationship. Each of these members 16a 1612, which are shown, for example, in the form of a threaded set screw having a charnfered tip portion 17 and a slotted head 18, are capable of adjustment such that the depth of penetration of each threaded member into the cavity may be individually varied with relative facility by rotation of a member 16 through means of its slotted head 18. The tuning plunger or center conductor 14 has secured thereon spring loaded metallic finger 20 which is affixed thereto by suitable means such as rivets 22. Spring loaded finger 20 has a portion 24 which is in contact with the tip portion 17i of particular non-metallic member 161' in such a manner that said portion 24 is constantly urged thereagainst by reason of the retarding force exerted on the spring finger 20. An axial displacement of the center conductor 14 within the cavity by gross tuning means (not shown) accomplishes the major tuning of the resonant frequency of the resonator 10. As such displacement takes place, it may be seen that spring loaded finger 20 substantially terminating the center conductor 14 rides along the plurality of nonmetallic members 16a 1611 by virtue of the spring engagement of the portion 24 and the tips 17a 1711. Thus the spring loaded finger 20 is positioned in a direction normal to the axis of the center conductor 14 as a direct function of the depth of penetration of the particular non-metallic member 16i in immediate contact with the finger 20 at the point of contact 24. The metallic spring loaded finger 20 afiixed near the termination of the center conductor 14- distorts the field configuration of the electromagnetic energy mode propagated through the cavity resonator 10.

This distortion of the electrical field in the vicinity of the end of the tuning plunger may be described in terms of the end loaded capacity of the center conductor 14 to the other conductor wall 11. Any variation of the distance between the metallic finger 20 and the outer wall 11 of the cavity 10 would vary the electrical field configuration in the immediate vicinity of the finger, and hence slightly affect (in comparison to the major tuning control) the resonant frequency f of the cavity, as will be readily understood by those conversant with the art. It may therefore be seen that a fine tuning for the resonator frequency can be achieved by adjustment of the depth of penetration of the various individual non-metallic members 16a 16n.

Non-metallic members immediately adjacent to the particular member 16i in contact with the spring finger 20 at portion 24, such members being designated for purposes of generality as 16i-1 and 16i+1, may have some effect on the actuation of the incremental tuning at the point of interest depending upon the relative flatness of the portion 24 of the spring finger 20. This slight degree of carryover of adjustment from point to point may be necessary in order to avoid a scalloping effect between nonmetallic members due to discontinuous carryover from one to the next as the tuning plunger 14 is axially translated.

Thus the invention provides a means of tracking compensation over a plurality of points throughout the frequency range in interest with only a small degree of interdependence of adjustment from point to point. By selective adjustment of each individual non-metallic member 161', in contact with the spring finger 20 at the particular resonant frequency t at which it is desired to have resonant cavity track the microwave system in which it is incorporated, it is possible to virtually reduce the error function Af to zero (corresponding to perfect tracking) at a number of frequency points equal to the quantity 11, corresponding to the number of nonmetallic members 16. This result is accomplished by a slight controllable distortion of the electrical field geometry in the vicinity of each one of the individual members 16a 16n, in the manner above-described, thereby to superimpose a fine tuning frequency correction on the major tuning of the coaxial cavity iii, as determined by the active length l of the center conductor 14 within the cavity walls.

It is thought that the foregoing description, if taken in conjunction with the annexed drawings, will be sufiicient to enable anyone practiced in the art to understand and construct a coaxial cavity tracking compensator in ac cordance with the principles of this invention. Accordingly, it is desired that the invention be taken to embrace any variation and embellishment of the particular embodiment shown which is within the spirit thereof and that the scope of this invention be limited solely by the following appended claims.

What is claimed is:

1. A tracking compensator for incremental adjustment of the resonant frequency of a tunable coaxial cavity comprising, a coaxial cavity having an outer conductor and a center conductor relatively moveable thereto along their common longitudinal axis, a metallic mass afiixed near the end point of said center conductor, and a plurality of individually adjustable members spaced in substantially axial progression along said outer conductor and projecting therethrough such that at a plurality of operating positions of said center conductor with respect to said outer conductor at least one of said individually adjustable members in the vicinity of the end point of said center conductor cooperates in non-shorting fashion with said metallic mass, substantially independently of the remainder of said individually-adjustable members, to vary the effective electrical field configuration in said vicinity to provide thereby a relatively fine adjustment in 6 the frequency tuning of said cavity at said plurality of operating positions.

2. A tracking compensator for incremental adjustment of the resonant frequency of a tunable coaxial cavity comprising, a coaxial cavity having an outer conductor and a center conductor relatively moveable thereto along their common longitudinal axis, a spring loaded metallic finger member afiixed near the end point of said center conductor within said cavity, a plurality of individually adjustable non-metallic members spaced in substantially axial progression along said outer conductor and projecting therethrough to a controllable depth of penetration into said cavity, such that at least one of said individually adjustable members in the vicinity of the end point of said center conductor cooperates, substantially' independently of the remainder of said individually-adjustable non-metallic members, with said spring loaded member at a plurality of operating positions to vary the end loaded capacity of said center conductor to said outer conductor wall to provide thereby a relatively fine adjustment in the frequency tuning of said cavity at said plurality of operating positions.

3. A tracking compensator for incremental adjustment of the resonant frequency of a tunable coaxial cavity comprising, a coaxial cavity having an outer conductor and a center conductor relatively movable thereto along their common longitudinal axis, a conductive laterallyextending member affixed to and movable with said center conductor within said cavity and having a relatively short longitudinal dimension, a plurality of individually adjustable members axially spaced in progression along said outer conductor and projecting therethrough to a controllable depth of penetration into said cavity to cause the end loaded capacity of said center conductor to said outer conductor wall to be substantially individually controlled by each adjustable member as said laterally extending member assumes respective ones of a plurality of operating positions corresponding to the axial position of a respective adjustable member, whereby a fine adjustment in the frequency tuning of said cavity is provided at said plurality of operating positions.

References Cited in the file of this patent UNITED STATES PATENTS 2,566,759 Clark Sept. 4, 1951 2,600,278 Smullin June 10, 1952 2,639,406 Crawford May 19, 1953 2,851,666 Kach Sept. 9, 1958

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