Injection locked oscillators

Abstract: A high impedance circuit has radially disposed first vanes (26¹) and radially disposed second vanes (26²) interdigitating between the first vanes. The first vanes and the second vanes are each interconnected by a first toroidal strap (38) and a second toroidal strap (40), respectively. The first strap and the second strap are disposed co-axially on opposite sides of the vane structure (28). The vanes and straps are dimensioned so that the circuit has a single cavity impedance commensurate with a predetermined interaction impedance for the oscillator which is sufficient to sustain oscillation for a preselected injection locking bandwidth of the oscillator.

Description: The invention relates to injection locked oscillators and more particularly to magnetrons. The invention is especially advantageously applicable to the design of magnetrons comprising an anode having a cavity in which first and second groups of vanes are arranged coaxially to form an interdigitated set of vanes, the vanes being adapted to accommodate straps, with the straps contacting only respective groups, magnetrons of this kind being known from, for example, US 2,992,362 and US 4,056,756. A study of injection locking of non-coherent oscillators is described in Adler, "A Study of Locking Phenomenon in Oscillators," Proceedings of the IRE, June, 1946, pages 351-357. As can be derived therefrom, the coherent bandwidth, ΔF, of an injection locked oscillator is approximately given by the equation ΔF = 2F o (P i /P o ) ½ /Q e wherein F o is the frequency of the oscillator, P i is the injected coherent power, P o is the output power and Q e is the external quality factor of the oscillator. The study of injection locking by Adler was further developed by others. For example, see Huntoon & Weiss, "Synchronization of Oscillators," Proceedings of the IRE, December, 1947, pages 1415-1423. The Huntoon reference provides a strong theoretical basis for injection locking regardless of circuit configuration. One of the earlier articles relating to the injection locking of magnetron oscillators is given in David, "R. F. Phase Control and Pulsed Magnetrons," Proceedings of the IRE, June, 1952, pages 669-685. Although the theoretical concept of injection locking of magnetrons is known, the practical implementation in the prior art of injection locked magnetrons has not been realized until relatively recently. First, appropriate low cost coherent sources of RF energy with sufficient power to drive magnetrons have not been available. Secondly, the existing magnetron circiuts have an apparent limitation which limit the obtainable circuit bandwidth. The disadvantage resulting from this limitation is that the known magnetron circuits were insufficient for commercial exploitation. Recent advances in solid state oscillators have all but eliminated the first limitation of the prior art noted above. Power levels for magnetrons are now available in the 0.5 to 5.0 kilowatt level. With current devices, coherent gains of ten to thirteen dB are achievable over narrow bandwidths. The exploitation of these advances for magnetrons has, however, been limited by the ability of conventional magnetron circuits to present a sufficiently high impedance to the electron stream in the interaction region to sustain proper magnetron operation over a sufficiently wide bandwidth. In a known prior art magnetron with a conventional circuit configuration, manipulation of the coupling between the conventional circuit and its external load will reduce its external quality factor Q. The reduction of the external Q will achieve a wider injection locking bandwidth. Because of the fundamental relationship between the external Q and the loaded Q, this will cause the fields on the magnetron circuit to become lower and lower until a phenomenon called "sink" is reached. At this point the magnetron ceases to work. The reason is that the total RF impedance of the circuit becomes too low to sustain oscillation. According to the present invention there is provided an injection locked oscillator comprising an anode ring having an inner cavity, the inner cavity comprising a high impedance circuit operable over a preselected injection locking bandwidth thereof, said circuit comprising a plurality of vanes coaxially disposed in said inner cavity and at least one strap interconnecting a portion of said vanes, wherein said injection locking bandwidth, ΔF, is given by: ΔF = 2F o (P l /P o ) ½ /Q e wherein F o is the frequency of said oscillator, P o is the power out of said oscillator, P i is the injected coherent power, and Q e is the external quality factor of said oscillator; wherein said circuit has a single cavity impedance given by: (L/C) ½ wherein said oscillator has an interaction impedance, Z int , given by: Z int = Q l (L/C) ½ wherein Q 1 is the loaded quality factor of said circuit; and wherein Q 1 is given by: 1/Q 1 = 1/Q o + 1/Q e wherein Q o is the unloaded quality factor of the circuit, characterised in that said plurality of vanes includes first radial vanes interdigitally disposed in said cavity with second radial vanes to form a vane structure and said at least one strap includes two toroidal straps, being a first toroidal strap, coaxially disposed at a first side of said vane structure, for electrical connection to said first radial vanes and a second toroidal strap, coaxially disposed at a second side of said vane structure, for electrical connection to said second radial vanes, and in that, in order to sustain oscillation over said injection locking bandwidth, the vanes and the straps are dimensioned so as to provide a value of the single cavity impedance of the circuit such that the oscillator's interaction impedance is at least 5000 ohms. The present invention thus provides a high impedance circuit which satisfies the conflicting requirements of wide bandwidth and sufficient circuit impedance. Specifically, the single cavity impedance of the oscillator is increased by ensuring that the circuit, in lumped constant terms, has a very high inductance and a very low capacitance circuit. The invention thus takes account of the fundamental relationships which govern this sink phenomenon. These can be summarized as follows: Δ F = 2F o (P i /P o ) ½ /Q e Z int = Q l (L/C) ½ 1/Q l = 1/Q o + 1/Q e From the above equations, it can be seen that the interaction impedance is the product of the loaded quality factor Q l and the single cavity impedance of the magnetron. Because of the fundamental relationship between the loaded quality factor, which is related to the ability to maintain oscillation, and the external quality factor Q e , which is related to the ability to obtain large injection bandwidth, decreasing the external quality factor Q e for a fixed circuit decreases the loaded quality factor Q l . As a consequence thereof, the interaction impedance Z int is also decreased. In one embodiment of the present invention, each of the vanes is generally T-shaped. Each vane has a relatively wide high conductive first portion and a relatively high inductance second portion. The first portion is disposed proximate to an axis of the cavity with the second portion extending radially outward therefrom. Advantages attainable by appropriate design are the high-single cavity impedance of greater than 200 ohms in a 16 resonator configuration and a wide vane face which presents an adequate peak dissipation surface to the electron stream of the interaction space. This is an especially important advantage for high power applications. Other advantages attainable allow the independent control of the interaction impedance and the external quality factor by divorcing the single cavity impedance from the coupling circuit which controls the bandwidth. The simple shape of the vane allows it to be fabricated using conventional stamping operations. The toroidal strap can be easily made from available wire through a simple forming operation. The designs facilitate the manufacture of the circuit thereby reducing its cost. For a better understanding of the present invention and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings in which: Fig. 1 is a schematic diagram of a magnetron oscillator circuit; Fig. 2 is one view of a high impedance arrangement of a magnetron; Fig. 3 is a view taken along line 3-3 of Fig. 2; and Fig. 4 is an enlarged view of a portion of Fig. 3. Referring now to Fig. 1, there is shown a schematic diagram illustrating an injection locked magnetron 10. A source 12 of coherent microwave energy delivers low power energy to a circulator 14. The circulator injects the low power energy into the magnetron 10. The low power energy is amplified by the magnetron 10 as is well known in the art. The amplified energy developed by the magnetron 10 is redirected to the circulator 14. The high power microwave energy is then coupled to an antenna 16 to radiate the high power coherent output energy. Referring now to Figs. 2 to 4, there is shown a high impedance circuit 20 for an anode ring 22 in the magnetron 10. The circuit 20 is disposed within an inner cavity 24 of the anode ring 22. The high impedance circuit 20 includes a plurality of first radial vanes 26 1 and a plurality of second radial vanes 26 2 . The first radial vanes 26 1 are coaxially positioned within the cavity 24. The second radial vanes 26 2 are interdigital with the first vanes 26 1 to form a vane structure 28. Each of the first vanes 26 1 and second vanes 26 2 has a relatively wide, high conductance, first portion 30 and a relatively narrow, high inductance, portion 32, as best seen in Fig. 4. The second portion 32 extends radially outward from the first portion 30. The first portion 30 is radially proximate to an axis 34 of the cavity about which the magnetron cathode is disposed. The circuit further includes a first electrically conductive toroidal strap 36 and a second electrically conductive toroidal strap 38. Both the first strap 36 and the second strap 38 are arranged to be coaxial with the axis 34. The first strap is disposed along the first side of the vane structure 28. The second strap is disposed along the second side of the vane structure 28. The first strap interconnects only the first vanes 26 1 and the second strap 38 interconnects only the second vanes 26 2 . Each of the vanes 26 1 , and 26 2 , the first strap 36, and second strap 38 is dimensioned so that the circuit 20 has a single cavity impedance of at least 200 ohms commensurate with a predetermined interaction impedance, of at least 5000 ohms, which is sufficient to sustain oscillation for a preselected injection locking bandwidth, as is derived from the above references. More particularly, the relatively narrow second portion 32 concentrates rings of magnetic field, B, around the vane 26, as best seen in Fig. 4. The electric field between the vanes reverses direction between each of the first vanes 26 1 and the second vanes 26 2 . The straps, being of circular cross-section, minimize capacitance of the circuit, while giving sufficient mode separation. Where the straps 36, 38 are connected to the appropriate one of the vanes 26 1 , and 26 2 , a mounting portion 40 is provided therein with an arcuate channel 42. The second portion 32 of the vanes may be soldered to the anode ring 22. It will now be apparent that, for a given injection lock bandwidth, ΔF, a value for the interaction impedance, Z int can be selected so that oscillation is maintained. The shape of the vanes 26 is then structured so their inductance and capacitance satisfy the conditions set forth in the above equations to achieve the selected Z int . The T-shape of the vanes 26 1 , 26 2 has been found to satisfy these conditions.

Claims: An injection locked oscillator comprising an anode ring (22) having an inner cavity (24), the inner cavity comprising a high impedance circuit operable over a preselected injection locking bandwidth thereof, said circuit comprising a plurality of vanes (26) coaxially disposed in said inner cavity (24) and at least one strap (36, 38) interconnecting a portion of said vanes, wherein said injection locking bandwidth, ΔF, is given by: ΔF = 2F o (P l /P o ) ½ /Q e wherein F o is the frequency of said oscillator, P o is the power out of said oscillator, P i is the injected coherent power, and Q e is the external quality factor of said oscillator; wherein said circuit has a single cavity impedance given by: (L/C) ½ wherein said oscillator has an interaction impedance, Z int , given by: Z int = Q l (L/C) ½ wherein Q 1 is the loaded quality factor of said circuit; and wherein Q 1 is given by: 1/Q 1 = 1/Q o + 1/Q e wherein Q o is the unloaded quality factor of the circuit, characterised in that said plurality of vanes includes first radial vanes (26 1 ) interdigitally disposed in said cavity with second radial vanes (26 2 ) to form a vane structure (28) and said at least one strap (36, 38) includes two toroidal straps, being a first toroidal strap (36), coaxially disposed at a first side of said vane structure (28), for electrical connection to said first radial vanes (26 1 ) and a second toroidal strap (38), coaxially disposed at a second side of said vane structure (28), for electrical connection to said second radial vanes (26 2 ), and in that, in order to sustain oscillation over said injection locking bandwidth, the vanes and the straps are dimensioned so as to provide a value of the single cavity impedance of the circuit such that the oscillator's interaction impedance is at least 5000 ohms. An oscillator as set forth in claim 1, wherein said plurality of vanes are generally T-shaped. An oscillator as set forth in claim 2, wherein each of said first vanes (26 1 ) and said second vanes (26 2 ) has a relatively wide, high conductance, first portion radially proximate to an axis of said cavity and a relatively narrow, high inductance, second portion extending radially outward from said first portion An oscillator as set forth in claim 1, 2 or 3, wherein the single cavity impedance is greater than 200 ohms. An oscillator as set forth in any preceding claim, wherein each of said first and second vanes (26 1 , 26 2 ) has a first portion radially proximate an axis of the cavity which is wider than the portion of the vane extending radially towards the first portion, the first portion having a recess (40) to accommodate the strap (36, 38) to which it is connected and has, on its side opposite to the recess (40), an edge extending to the second portion and of concave form to remain spaced from the other strap. An oscillator as set forth in any one of the preceding claims, which is a magnetron.

Inventors: ENGLISH ROBERT C, THORNBER GEOFFREY, WALKER CHRISTOPHER M, ENGLISH, ROBERT C., THORNBER, GEOFFREY, WALKER, CHRISTOPHER M.
Applicants: LITTON SYSTEMS INC, LITTON SYSTEMS, INC.
Application year: 1991
Main classification: H01J23
IPC codes: H05B6, H01J23