US3869638A

US3869638A - Triangular dither-tuned microwave tube

Info

Publication number: US3869638A
Application number: US412526A
Authority: US
Inventors: William Allen Gerard
Original assignee: Varian Associates Inc
Current assignee: Varian Medical Systems Inc
Priority date: 1973-11-02
Filing date: 1973-11-02
Publication date: 1975-03-04
Anticipated expiration: 1992-03-04

Abstract

A dither-tuned microwave tube apparatus having approximately triangular frequency variation with time includes a movable, frequency controlling tuning element for an electromagnetic cavity. The tuning element is reciprocally moved between two extremes of travel in response to a single rotation of a shaft. A gear train means drives the shaft in a manner such that the tuning element rapidly reverses as it approaches each extreme of travel. The gear train means has two cycles of speed variations during each reciprocation cycle of the tuning element. In one embodiment the gear train means comprises an identical pair of meshing centrally driven elliptical gears, while in a second embodiment the gear train means comprises an identical pair of elliptical gears driven at their foci and a 2:1 gear reduction.

Description

United States Patent 11 1 [111 3,869,638

Gerard Mar. 4, 1975 1 TRIANGULAR DITHER-TUNED Primary Examiner-James W. Lawrence MICROWAVE TUBE Assistant E.\'aminerSaxfield Chatmon, Jr. Attorney, Agent, or FirmStanley Z. Cole; D. R. Pressman; Robert K. Stoddard [57] ABSTRACT A dither-tuned microwave tube apparatus having approximately triangular frequency variation with time includes a movable, frequency controlling tuning element for an electromagnetic cavity. The tuning element is reciprocally moved between two extremes of travel in response to a single rotation of a shaft, A gear train means drives the shaft in a manner such that the tuning element rapidly reverses as it approaches each extreme of travel. The gear train means has two cycles of speed variations during each reciprocation cycle of the tuning element. In one embodiment the gear train means comprises an identical pair of meshing centrally driven elliptical gears, while in a second embodiment the gear train means comprises an identical pair of elliptical gears driven at their foci and a 2:1 gear reduction.

12 Claims, 12 Drawing Figures 3g; la n, I 6

I i p]? M RESOLVER PAIENTEM 419-75 sum 1 a; 2

T T I 0 w n V AEL DU 0 7 m T. 0 I I M GK NN NM w W 2 2 G W R l E A8 V SN IL NA 0 NSECL s W CL 0 \|\R C 6 Mu l T2 m A. m l M 0 FM M DI TUNER L CRANK MEANS COMPENSATING 3 TRIANCUL- ARIZING MEANS S 24 MOTOR TRIANGULAR DITHER-TUNED MICROWAVE TUBE FIELD OF THE INVENTION BACKGROUND OF THE INVENTION Heretofore, dither-tuned microwave tubes, particularly magnetrons, have frequently employed a substantially constant speed motor driven crankshaft reciprocating a tuning plunger between two extremes within a cavity of the tube. The plunger reciprocates substantially sinusoidally in time, typically at 100 to 200 hertz, thereby producing a sinusoidal variation in the output carrier frequency of the tube. Such a dither-tuned microwave tube is used for providing a band of carrier frequencies for transmitted pulses in a frequency agile radar. Because of the sinusoidal carrier frequency variation, there is an undesirable increased .carrier frequency density at the extreme carrier frequencies which are derived at the travel extremes of the tuning plunger. Since the pulse repetition rate of such a radar is constant, more pulses have carrier frequencies near the extremes, where the sinusoidally varying frequency is changing slower in time, than in the frequency band centered between the extremes. Thus, adjacent pulses at the extreme carrier frequencies do not differ from each other sufficiently to be considered as having independent carrier frequencies, with a tendency to preclude the expected advantages of frequency agile radar. Further, countermeasures against such a radar can be more effective because the frequency spectrum of the radar is concentrated at the frequency extremes. These undesirable characteristics inhere in the sinusoidal frequency variation of the present generally available dither-tuned microwave tubes of the type employing a crankshaft.

While the crankshaft type system has these disadvantages, it has been found to be particularly adaptable to precise plunger position control. Additionally, the instantaneous angle of the rotatable crankshaft is easily and directly read out with an electromechanical resolver that exhibits a sinusoidal voltage envelope variation with crankshaft angle. The resolver provides an output voltage proportional to instantaneous frequency which is also a substantially sinusoidal function of crankshaft angle.

A triangular variation of the operating frequency in time (Le, a linear rise and fall) does not present these objectionable characteristics because the undesirable increase of pulses at the carrier frequency extremes is eliminated. However, a triangular frequency variation can only be approached because it implies infinite deceleration and acceleration of the tuning plunger at the extremes of travel to reverse the plunger direction, i.e., to instantaneously change the plunger from a positive velocity to a negative velocity.

OBJECTS OF THE PRESENT INVENTION It is an object of the present invention to provide a new and improved dither-tuned microwave tube exhibiting a substantially triangular variation of microwave output frequency with time.

It is another object of the present invention to provide a dither-tuned microwave tube having a crankshaft for reciprocating a tuning plunger between two extremes, thereby varying the operating frequency of the tube between two extremes, with means for driving the crankshaft to produce a substantially triangular variation of the output frequency with time.

It is another object of the present invention to provide a dither-tuned microwave tube having a crankshaft for reciprocating a tuning plunger between two extremes, thereby varying the operating frequency of the tube between the two extremes, with means for rapidly decelerating the tuning plunger at the extremes.

SUMMARY OF THE PRESENT INVENTION These objects are fulfilled by providing a non-linear gear train. to drive a crankshaft for a tuning plunger of a dither-tuned microwave tube. The gear train causes a more rapid rotation of the crankshaft at the extremes of tuning plunger travel than midway between the extremes. The gear train thus functions as means for producing two cycles of crankshaft speed variation for each cycle of the tuning plunger reciprocation to provide maximum speed at the two extremes, and minimum speed at each of two crossings through a point midway between the extremes. The gear train, in one embodiment, comprises a pair of meshing identical center rotated elliptical gears. which produce two cycles of speed variation per complete gear rotation. The pair of elliptical gears has equivalents, such as other non-linear or eccentric gear arrangements for producing a different number of speed variations per gear rotation combined with an appropriate gear ratio for producing two speed variations for each complete cycle of crankshaft reciprocation.

Other objects, features and advantages of the present invention will become apparent from the following de tailed description of the invention, taken in conjunction with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a mechanical schematic diagram of a prior art dither-tuned microwave tube system having a crank driven tuning plunger;

FIG. 2 is a mechanical schematic diagram of the dither-tuned microwave tube of the invention having triangularizing means for driving the tuning plunger to ob tain a substantially triangular variation of tube operating frequency;

FIG. 3 is a cross-sectional view of a preferred embodiment of the dithentuned microwave tube of the invention schematically illustrated in FIG. 2;

FIGS. 4, 5 and 6 are views of FIG. 3 along the lines 44, 5-5, and 6-6, respectively, with: FIG. 4 showing a pair of centrally driven elliptical gears forming the triangularizing. means, FIG. 5 showing a pair of focus driven eccentric gears, and FIG. 6 showing an eccentric bearing for cranking the tuning plunger;

FIG. 7 is a view of the centrally driven elliptical gears at a position displaced from the position shown in FIG. 4;

FIG. 8 is a schematic diagram showing an alternate embodiment for deriving a substantially triangular variation of tube operating frequency; and

sus time, tuning plunger crankshaft angle of rotation versus time, and the tube operating frequency versus time.

DETAILED DESCRIPTION OF THE INVENTION Referring to FIG. 1, there is shown schematically a prior art drive for a tuning plunger 19 of a dither-tuned microwave tube which is preferably a magnetron. The tuning plunger 19 is ultimately driven by constant speed motor 12 which directly drives input shaft 14. Compensating means is driven by input shaft 14 and drives tuning crank 18 through rotating crankshaft 22. Tuning crank 18 reciprocally slides tuning plunger 19 within microwave tube 10. Shaft 14 also drives a resolver 16, for providing electrical readout of the tube operating frequency. The constant rotational speed of shaft 14 produces a sinusoidal reciprocating movement of tuning plunger 19 that is approximately tracked by a sinusoidal output voltage of resolver 16.

Compensating means 20 drives tuner crankshaft 22 and plunger 19 to compensate for non-linearity of the relationship between the instantaneous position of the plunger and the microwave carrier frequency derived from tube 10. Compensation is necessary to produce a substantially pure sinusoidal operating frequency variation from the tube 10 for constant rotation rate of shaft 14 in order for the resolver output voltage to accurately track the operating frequency of the tube. The compensating means 18, invented by Richard C. Stoke, can be further studied in US. Pat. No. 3,590,313, issued June 29, 1971, although the following brief discussion of it suffices for the purposes of the present invention.

Non-linearity in the relationship between the position, of tuning plunger 19 and the operating frequency of tube 10 has a tendency to cause the time for the operating frequency to vary between one extreme and a median value between the extremes to be less than the time required to change from the median value to the opposite extreme. Thus, driving tuner crankshaft 22 at constant speed has a tendency to produce a microwave frequency asymmetry between alternate half cycles of the rotation of shaft 14. Compensating means 20 overcomes this tendency by driving the tuner crankshaft 22 slower during the half cycle of shaft 14 that is relatively short, while during the longer half cycle of shaft 14 the compensating means drives shaft 22 faster than driving the preceeding half cycle. Thus, the compensating means 20 drives shaft 22 so that the shaft has only one cycle of speed variation per rotation of tuner crankshaft 19.

Reference is now made to FIG. 2, a schematic diagram of a preferred embodiment of the invention wherein the device of FIG. 1 is modified by including triangularizing means 24,, driven by constant speed motor 12 via shaft 26, for driving input shaft 14 at a cyclically varying speed. Triangularizing means 24 rotates shaft 14 more rapidly at the extremes of travel of tuning plunger 19 than midway between the extremes in order to produce a nearly triangular variation with time, i.e., linear oppositely directed variations during adjacent half cycles, of both the operating frequency of tube 10 and the frequency tracking output voltage V of resolver 16. The triangularizing means 24 must therefore produce two cycles of speed variation for each complete rotation of tuner crankshaft 22 and each corresponding complete reciprocation of tuning plunger Reference is now made to FIG. 3, wherein there is shown-in cross section the operative parts of one embodiment of the dither-tuned microwave tube 10 of the invention. Tube 10 is a magnetron tube of the type generally described in US. Pat. No. 3,441 ,795, issued Apr. 29, I969. Tube 10 includes an annular resonant cavity 30, defined by an outer cylindrical wall 31, an inner cylindrical wall 36 and upper and

lower end walls

40 and 42. An array of vane resonators 32 is directed inwardly from inner cylindrical wall 36 and coaxially surrounds an axial cathode 34 to define an annular magnetron interaction region between the vane resonators and the cathode. Wall 36 is a boundary between the inner annular magnetron interaction region and the outer annular cavity resonator. An array of coupling slots 38 in wall 36 between alternate vane resonators 32 is provided for coupling the 1r mode of oscillation in the magnetron region to a-circular electric oscillating mode in the annular cavity resonator. The upper end wall 40 of the annular cavity is movable between upper limit 41 and lower limit 43 to effect tuning of the annular cavity 30 and to thus vary the operating frequency of the tube. Movable wall 40 is the lower surface of tuning plunger 19 which is vertically reciprocated by tuning plunger crank 18, thereby producing a cyclical operating frequency variation with time. i

The tuning crank 18 comprises a slider crank mechanism including a piston rod 44 having a lower end coupled to tuning plunger 19 via ball joint 46. The upper end of rod 44, as indicated in FIG. 6, forms a circular yoke 48 housing a ball bearing race 50. Within ball bearing race 50 is a circular eccentric portion'52 at one end of tuner crankshaft 22. It is a well known characteristic of a slider-crank mechanism that a constant rotation of the crankshaft produces a substantially sinusoidal reciprocation of the slider, in this case the tuning plunger 19.

Tuner crankshaft 22 is cylindrical and hollow enabling resolver input shaft 54 to pass coaxially through the crankshaft for driving resolver 16. The coaxial arrangement of

shafts

22 and 54 enable resolver 16 to be located for tight packaging of the entire dither-tuned microwave tube assembly. The rotor of resolver 16 is connected to shaft 54 which is driven from input shaft 14, via

gears

56 and 58 having a 1:1 ratio. It should be understood that the resolver position could be such that the rotor is directly attached to shaft 14.

.Compensating means 20, illustrated in FIG. 5, comprises a pair of meshing gears 60 and 62 having axes or

foci

61 and 63, respectively. Shafts 22 and 14 (FIG. 3), are fixedly connected in driving relation with

gears

60 and 62 for rotation of the gears about their respective axes. Since meshing gears 60 and 62 are identical, one

complete rotation of input gear 60 and shaft 14 provides a complete rotation of the output gear 62 and shaft 22, corresponding to an average gear ratio 1:1. Output gear 62 drives crankshaft 22 and hence eccentric portion 52 to cause tuning plunger 19 to move from one limit of its travel to the other limit of its travel as input shaft 14 rotates. Due to movement of plunger 19, there is a sinusoidal variation of operating frequency which is accurately read out by resolver 16.

Since it is desired to provide, as closely as possible, a triangular variation of operating frequency versus time, shaft 14 is not directly driven by a constantly rotating shaft 26; instead, triangularizing means 24 is interposed between

shafts

26 and 14. Triangularizing means 24 drives the input shaft 14 more rapidly when the tuning plunger 19 is at the

limits

41 and 43 of its travel than when the plunger is midway between these limits. The triangularizing means comprises an identical pair of meshing elliptical gears 70 and 72 (FIG. 4) which are fixedly and coaxially mounted to

shafts

26 and 14 respectively for rotation about the respective gear centers 74 and 76.

Shafts

26 and 14 are spaced apart a distance equal to the sum of the semi-major and semi-minor axes of the ellipses forming the gears. The distances from the point 82 where the gears mesh to the centers of

gears

70 and 72 are respectively represented by

lines

78 and 80. Since the ratio of the distances represented by

lines

78 and 80 changes for each angular position of shaft 26 and gear 72, there is a constant speed variation in output gear 72 when input gear 70 is constantly rotated. The average speed of output gear 72 equals the average speed of input gear since the gears are identical, whereby a complete rotation of the input gear produces a complete rotation of the output sitions (one shown in FIG. 4 and another shown in FIG.

7) where the major axis of one gear is aligned with the minor axis of the othergear. When the distance represented by line 78 is greater than the distance represented by 80, as shown in FIG. 4, there is an increased speed of output gear 72 over average. However, after 90 of rotation of

gears

70 and 72, as shown in FIG. 7, the distances represented by

lines

80 and 82 are reversed and the output gear 72 has a lower speed than average. Thus, there are two maxima and two minima in the rotation rate of gear 72 and shaft 14 for one revolution of

gears

70 and 72 as well as

shafts

26 and 14. The maximum rotation rate of gear 72 is determined by multiplying the constant speed rotation rate of shaft 26 by the ratio of the major axis to the minor axis of the identical

elliptical gears

70 and 72. In contrast, the minimum rotation rate of gear 72 is found by dividing the speed of shaft 26 by the same ratio.

It should be understood that there are numerous equivalents for triangularizing means 24. For example, as shown in FIG. 8, one cycle means 84, such as a pair of focus rotated elliptical gears, can be used to drive a 2:1 reduction gear which in turn drives the input shaft 14 whereby two revolutions of the focus rotated elliptical gears and consequently two speed cycles are produced for each revolution of input shaft 14.

.Referring now to FIG. 9a, there is a plot 90 relating operating carrier frequency, f, and the instantaneous angle 6 of input shaft 14. The frequency variation is a substantially pure sinusoidal function of the angular position of shaft 14, due in part to the provision of the compensating means 20 interposed-between shaft 14 and the tuner crank 18. As is apparent, a constant rotation rate of shaft 14 produces frequency the same sinusoidal function of time. Plot 92, in FIG. 9b, relates the instanteous speed, d0/a't, of input shaft 14 versus time as produced by the triangularizing means 24 where T is the pcriod for one revolution of the input shaft. Plot 92 is a periodic waveform with two cycles in the period T in which the undirectional dB/dt varies about an average value of d6/dt represented by horizontal line 110. d6/dt has a minimum value 112 occuring at zero time. Another plot, 94, in FIG. 9c shows the angle6 as ordinate versus time, where 0 is obtained by integrating plot 92 and initializing 0 equal to 0 and time O. 0 equal to 0 corresponds to a tuning plunger 19 location midway between the extremes of

travel

41 and 43. Plot 94 shows a two cycle periodic undulation of 6 about a straight line 114. This straight line 114 corresponds to a linear ascent of 0 in accordance with the average value 110 of d0/dt. Because of plot 92s two cycle periodicity, 6 corresponds to 180 at time equal to half of the period and 360 at time equal to the complete period. With this information, the resultant fre quency versus time plot 96 can be constructed. As can be seen, in the region of the frequency versus 0 near 0, 0 changes slowly, because of proximity to the minimum value 112, while in the region of the frequency versus 0 curve near 0 changes rapidly thereby producing the substantially triangular variation 96 which is characterized by generally straight rising and falling portions 98 between the short curved portions 100 at the maximum and minimum frequencies. The curved portions 100 have a radius of curvature substantially less than that of a sine wave of the same frequency excursion and period T. This radius of curvature is related to the turn-around deceleration of the tuning plunger; the smaller the radius of curvature, the higher the turnaround deceleration.

It has been found that the tuning plunger of a sinusoidally dither-tuned X-band magnetron normally exhibiting 3G peak turn-around deceleration can be driven with the present invention at decelerations up to 46 G5 with no bearing failure. This corresponds approximately to a decrease in the radius of curvature at the extremes of the frequency versus time curve by a factor of 15.

As should be apparent, having described the invention, numerous modifications are possible within its spirit and scope. For example the gear train means can also be used to drive the ceramic element in a magnetron of the type which is dithered by rotation of such an element in its cavity or to drive the band used to constrict the flexible wall of the cavity of a magnetron which is dithered by tightening and loosening such a belly band around its cavity. Also the gear train means can be used to drive the tuning elements of other microwave tubes. It is therefore intended that the spirit and scope of the invention be determined with reference to the appended claims.

What is claimed is:

1. In combination with a tunable frequency microwave tube apparatus having microwave circuit means, tuning means movable within the region of said circuit means for tuning said microwave circuit means, and thus the operating frequency of the tube; an input shaft; means for cyclically moving said tuning means between two extremes of travel in response to rotation of said input shaft dither the operating frequency; the improvement comprising first gear train means driving said input shaft from a drive shaft and producing with respect to rotation of said drive shaft a cyclical rotation rate of said input shaft such that maximum rotation rate occurs at said two extremes of travel of said tuning means and such that the operating frequency of said tube has a triangular dither characteristic with respect to time.

2. The apparatus of claim I wherein said cyclically moving means includes second gear train means cou pling said first gear train means to said inputshaft and producing, with respect to rotation of the output shaft of said first gear train means, rotation of said input shaft having one cyclic variation of rate per one cycle of motion of said tuning means.

3. The apparatus of claim 1 wherein one cycle of rotation of said input causes one cycle of motion of said tuning means and said first gear train means includes means for causing a two cycle speed variation in the movement of said input shaft for each cycle of movement of said tuning means.

4. The apparatus of claim 2 wherein one cycle of rotation of said input shaft produces one cycle of movement of said tuning means.

5. The apparatus of claim 1 wherein said first gear train means comprises an identical pair of meshing elliptical gears.

6. The apparatus of claim 5 wherein said pair of meshing elliptical gears has centers and wherein said first gear train means additionally comprises means for rotating the elliptical gears about their centers.

7. The apparatus of claim 2 including additional readout means coupled to said output shaft of said first gear train for indicating the instantaneous angle of said shaft and thus said operating frequency.

8. A microwave tube structure comprising a cavity, having a resonant frequency, tuning means within said cavity for varying the resonant frequency of said cavity, a shaft, means for cyclically moving said tuning means between two extremes of travel in response to rotation of said shaft, and means for driving said shaft with a cyclical rate of rotation, which rate is maximum at the two extremes of travel of the tuning means and such that the operating frequency of said tube has a triangular dither characteristic with respect to time.

9. The structure of claim 8 wherein said driving means comprises an identical pair of meshing elliptical gears.

10. The structure of claim 9 wherein said pair of meshing elliptical gears has centers and wherein said driving means additionally comprises means for rotating the elliptical gears about their centers.

11. The structure of claim 9 additionally comprising a magnetron interaction region coupled to said cavity.

12. The apparatus of claim 2 wherein said second gear train means comprises a pair of eccentric gears.

Claims

2. The apparatus of claim 1 wherein said cyclically moving means includes second gear train means coupling said first gear train means to said input shaft and producing, with respect to rotation of the output shaft of said first gear train means, rotation of said input shaft having one cyclic variation of rate per one cycle of motion of said tuning means.