US 3748528 A
A high-power, X-band microwave burst generator having an input coaxial line feeding a post extending into a waveguide. The center conductor of the coaxial line has a first switching gap therein and the post terminates at an end spaced, by another switching gap, from the interior wall of the waveguide. An RF block is disposed around the post adjacent the first switching gap. The gaps are dimensioned so that energy switched by the first gap can pass the block, but oscillations caused by discharge at the second gap cannot pass the block.
Description (OCR text may contain errors)
United States Patent 1191 Cronson [4 July 24, 1973 [S4] MICROWAVE GENERATOR 3,076,157 1/1963 Manwarren 315/39 X 2,769,909 11/1956 Radmacher [751 invent CNN", Lexmgwn Mass- 3,078,424 2/1963 Carter et a1 315/39 x [73 Assignee: lkor Incorporated, Burlington, Mass.
Primary Examiner-Rudolph V. Rolinec  led: 1972 Assistant Examiner-Saxfield Chatmon, Jr. 2 APPL 237,273 Attorney-Robert J. Schiller et a1.
 I US. Cl .3. 315/39, 333/13, 331/101, ABSTRACT 307/106 331/127 A high-power, X-band microwave burst generator hav- 51 Int. Cl H0lj 25 12 1161 25/16 an input line feeding a Post extending mm  Field of sea'rch 3 5 333/13. a waveguide. The center conductor of the coaxial line no 127.'307/106 has a first switching gap therein and the post terminates at an end spaced, by another switching gap, from the  References Cited interior wall of the waveguide. An RF block is disposed around the post adjacent the first switching gap. The UNITED STATES A N gaps are dimensioned so that energy switched by the 3,434,619 12/ 1969 Proud, Jr. 315/39 first gap can pass the block but oscillations caused by 4 27 discharge at the second gap cannot pass the block. 3:360:678 12/1967 Kerns 315/39 x 8 Claims, 3 Drawing Figures MICROWAVE GENERATOR This invention relates to pulse generating devices, and more particularly to pulse generators useful for producing microwave signals.
It is known to produce bursts of microwave energy by switching power across a gap in the central conductor of a coaxial line, as exemplified in US. Pat. No. 3,521 ,l2l issued July 2l, 1970 to J. M. Proud, Jr., and then feeding the resulting steep-rise time pulse into a filter circuit as suggested in the article by G. F. Ross in IEEE Transactions on Microwave Energy and Techniques, Sept. 1965, pp. 704-706. While this foregoing method provides a compact, simple microwave generator, it does not lend itself readily to generation of high power bursts of microwave energy at very high frequencies such as X-band.
A principal object of the present invention is to provide a device for generating bursts of microwave energy of both high power and very high frequency. Another object of the present invention is to provide such a device which is simple to-manufacture, requires but a few simple parts and is rugged and compact.
Other objects of the invention will in part be obvious and will in part appear hereinafter. The invention accordingly comprises the apparatus possessing the con struction, combination of elements, and arrangement of parts which are exemplified in the following detailed disclosure, and the scope of the application of which will be indicated in the claims. For a fuller understanding of the nature and objects of the present invention reference should be had to the following detailed description taken in connection with the accompanying drawings wherein:
FIG. 1 is a diagram in cross-section showing the structure of a preferred embodiment of the invention;
FIG. 2 is a schematic diagram illustrating an operating system for using the invention; and
FIG. 3 are exemplary idealized waveforms of signals generated in operation of the invention.
Generally, the foregoing and other objects of the present invention are realized with a device comprising a coaxial line joined to a waveguide, the coaxial line having therein an RF block intended to block passage of microwave energy originating in the waveguide, and a switch for shaping an input pulse to the block so that the pulse has sufficiently low frequency components to pass through the block relatively undistorted. The center conductor of the coaxial line following the block constitutes a post extending into the waveguide to just short of the far wall of the latter, so that the end of the post and the wall constitute a very high speed spark gap switch.
Turning now specifically to FIG. I, the invention is embodied in a pulse burst generator formed of a coupled resonant cavity formed by rectangular waveguide and coaxial input line 22. Line 22 comprises the usual center conductor 24 separated by dielectric material 26 from an outer cylindrical conductor 28. In the form shown, material 26 can be a ceramic orother good dielectric substance capable .of supporting the inner and outer conductors in the desired coaxial relation. At one end of line 22, outer conductor 28 is conn'ected and preferably sealed to a surface of waveguide 20. At a portion of inner conductor 24 before line 22 joins waveguide 20, there is provided a hollow space 29 wherein the dielectric surrounding conductor 24 is gaseous. Center conductor 24'-is provided with a discontinuity or shaping gap 30 disposed within space 29. Gap 30 is defined by two separated spark electrodes 32 and 33 formed of a highly refractory, electrically conductive material such as tungsten. The facing surfaces of electrodes 32 and 33 are preferably substantially flat and parallel to one another.
The gaseous dielectric in space 29 should be reasonably chemically stable upon passage of an are therethrough between electrodes 32 and 33, i.e., will not permanently break down, or form products which .will attack the electrodes, or inner or outer conductors to any substantial extent, or be explosive, and will not materially have its electrical properties changed after prolonged arcing. ln a typical embodiment, space 29 is filled with a gas such as air, pure nitrogen, neon, argon, krypton and the like, including mixtures. The gas preferably is at superatmospheric pressure, e.g., nitrogen at 200 psi, argon at 400 psi and the like.
That portion of conductor 24 having one end terminating at or connected to electrode 33, forms post 36 extending outwardly from the center of line 22 into the interior of waveguide 20 so as to be symmetrically located in the latter parallel to the electric field of energy propagated down the waveguide. The axis of line 22 is thus substantially normal to the broad face of waveguide 20. The other end of post 36 terminates a short distance from the wall of guide 20 at electrode 38. Preferably the wall of waveguide 20 is provided with another electrode 40 facing electrode 38, the spacing between electrodes 38 and 40 forming waveguide gap 42. Electrodes 38 and 40 are preferably formed also of a refractory electrical conductor such as tungsten and having substantially flat faces opposed to one another. Waveguide 20 may be filled with a gas at superatmospheric pressure in a manner: similar to that described for space 29.
Mounted around post 36 intermediate its ends is a conductive disk or RF block 44 having a diameter much greater than that of post 36 and only slightly less than the inside diameter of outer conductor 28 so as to define a thin annulus of dielectric material between disk 44 andconductor 28. If desired, waveguide 20 can have a sliding short 45 emplaced therein.
The operation of the device of FIG. 1 can be advantageously described in connection with the schematic showing of FIG. 2 and the exemplary idealized waveforms of FIG. 3. One can assume for example that waveguide 20 is rectangular and is dimensioned to propagate microwave energy in the X-band region, although other resonant cavity configurations for other modes and frequencies can also be employed. For reasons appearing hereinafter, gap 30 is dimensioned to be about 20 to 30 mil inches and gap 42 is assumed to be less than about 3 to 4 mil inches. As shown in FIG. 2, to operate the device it is desirable to provide means for introducing a wavefront of a traveling wave or pulse into line 22. To this end, FIG. 2 includes a source 46 of high voltage DC in series with a charging resistor 47 leading to a length of coaxial line or a pulse-forming section 48 terminating at one terminal of primary switch 50. The other terminal of switch 50 is connected to central conductor 24 of line 22 at the end of the latter remote from waveguide 20.
The structure thus described will be recognized as a simple pulse generator for launching a wavefront into line 22 and is only one of several forms of known apparatus for accomplishing this end. Where switch 50 (which can be a spark gap, solid state switch or the like) is open, section 48 will charge up to some voltage determined by source 46 and resistor 47. When switch 50 is closed, section 48 will be discharged into line 22, the steepness of the wavefront being determined largely by the switching speed of switch 50. Typically, one can readily obtain an input pulse through switch 50 of 12-20 kv amplitude and about a l nanosecond risetime such as is shown in FIG. 3A. This pulse then propagates into the section of line 22 extending between switch 50 and gap 30.
Gap 30 is intended to operate as a switch which serves two purposes. First, the switch should hold off the input voltage, i.e., not switch or leak substantially, until the latter has risen to a desired large value. Secondly, the switch, when it switches should do so in a very short switching time, i.e., change from a substantially infinite impedance to a dead short (ideally) in a very short time. If the wavefront is steep enough, the section of line 22 between switch 50 and gap 30 can charge to high overvoltage i.e., to a voltage well beyond the voltage at which the dielectric in gap 30 will break down to initiate a discharge. This occurs because there is a short delay before the breakdown will occur due to the statistical processes incurred in the dielectric breakdown. Thus, gap 30 being between 20 to 30 mil inches and filled with a dielectric gas at superatmospheric pressure, will momentarily hold the wavefront form from the pulse launched by switch 50, even though the section of line 22 between switch 50 and gap 30 becomes charged to an overvoltage, whereupon finally the dielectric in the gap breaks down abruptly. The switching time of gap 30 is set by the gap spacing, the characteristics of the dielectric gas, and the form of electrodes, and is intended to serve to steepen the leading edge of the pulse originally introduced through switch 50. A pulse with a l nano second risetime can be steepened in this manner to give a new pulse with a leading edge having a risetime of typically around 200 picoseconds. It is preferred that the risetime of this new pulse be sufficiently long so that it can readily be propagated through the RF block provided by disk 44 without substantially being distorted. If for example, one intends to generate X-band microwaves, the dimensions of disk 44 and inside diameter of outer conductor 28 can be selected as known in the art, to provide effectively a short circuit between the inner and outer conductors of line 22 to X-band energy, but nevertheless cause little if any degradation in the 200 picosecond wavefront propagating down the line.
The steepened pulse provided by switch or shaping gap 30 then charges the stray capacitance of post 36. As with gap 30, gap 42 should not switch until post 36 is charged to some overvoltage with respect to gap 42, yet should switch with a switching time which is a fraction of the period of the microwave frequencies to be generated. For example, the period of X-band frequency of gigahertz is 100 picoseconds. Gap 42 should switch in less than this 100 picosecond period, hence has a dimension of less than 3-4 mil inches between the electrodes.
The switching speed of gap 42 is important inasmuch as the faster the switching the more efficiently is energy stored in post 36 converted into microwave power. If, for example, the resistance across gap 42 dropped linearly over several milliseconds from its highest to its lowest value, the energy stored in post 36 would tend to dissipate in a DC pulse with an exponential decay. Switching speed is insured by selection of gap spacing and gas pressures as above noted. Post 36 (at least between electrode 38 and disk 44) is a resonant element that, upon abrupt switching at gap 42, rings at a frequency dictated by the dimensions of the waveguide. Hence, the discharge across gap 42 causes the current flow across the gap to oscillate at the resonant micro wave frequency. Current flow back toward gap 30 is blocked by disk 44 as above described. The ringing will continue for a few nanoseconds to produce thereby a burst of microwave oscillations such as are shown in FIG. 3B, typically as high 10 to kw peak power. To obtain X-band outputs, one would employ, for example, a waveguide having cross-section dimensions of about 1% inch X 1 inch. With appropriate selection of waveguide dimensions, one can generate bursts of microwave energy at other microwave frequency bands.
Since certain changes may be made in the above apparatus without departing from the scope of the invention herein involved, it is intended that all matter contained in the above descriptoin or shown in the accompanying drawing shall be interpreted in an illustrative and not in a limiting sense.
What is claimed is:
l. A pulse generator comprising, in combination, a pair of coaxial conductors forming a coaxial line, the inner conductor thereof having a first gap therein between an input section of said inner conductor and an output post;
a resonant cavity having a lateral opening therein to which the outer conductor of said coaxial line is connected so that said post extends into said resonant cavity and terminates at a second gap adjacent the inner wall of said resonant cavity, and
an RF block positioned around said post adjacent said first gap.
2. A pulse generator as defined in claim 1 including means for launching a dc pulse into said coaxial line.
3. A pulse generator as defined in claim 1 wherein said first gap has a substantially greater gap spacing than said second gap.
4. A pulse generator as defined in claim 1 wherein said resonant cavity is dimensioned to provide a cavity resonant at a frequency within a microwave energy band,
said second gap is dimensioned to have a switching time less than the period of said frequency, and
said first gap is dimensioned to have a switching time substantially in excess of said period.
5. A pulse generator as defined in claim 4, said block is selected to pass substantially most of the energy launched by breakdown of said first gap and substantially a minimum amount of the energy launched by breakdown of said second gap.
6. A pulse generator as defined in claim 1 wherein said resonant cavity is a waveguide, said waveguide being dimensioned to porvide a cavity resonant to microwaves,
said first gap is substantially between about 30 to 40 mil inches wide, and
said second gap is substantially between about 3-4 mil inches wide.
7. A pulse generator as defined in claim 1 wherein said gaps each contain a gaseous dielectric.
8. A pulse generator as defined in claim 7 wherein said gaseous dielectrics are at superatmospheric pressure.
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