Broadband output circuit for klystron amplifier
The present invention relates generally to amplifiers utilizing linear beam tubes and, more particularly, to a tunable klystron amplifier having an improved bandwidth. 
     Klystron amplifiers usually have fairly narrow bandwidths because their output circuit conventionally consists of a high Q cavity that is coupled directly to the load through either a loop or iris. The instantaneous bandwidth of such an amplifier may, therefore, be only one-half percent or less. 
     For these narrow bandwidths, the output gap impedance is usually high enough to permit good beam coupling and efficiencies. However, as the requirement for bandwidth increases, the output cavity-to-load coupling must also be increased, that is, the Q of the loaded cavity must be reduced. Such a reduction results in a comparatively large decrease in the gap impedance and, thus, the klystron efficiency is appreciably lowered. Ideally, the load impedance should be about 1.5 times the beam impedance for optimum efficiency. However, other klystron design parameters, such as the electron beam perveance, normalized drift tube diameter, etc. will affect this optimum ratio. 
     One technique for improving the impedance match between the beam and the output circuit is to decrease the beam impedance by increasing its perveance, k. This method, however, has limited application because of the difficulties involved in electron gun design. 
     The generator-to-load impedance match may also be improved by resorting to a filter-loaded output circuit which increases the output cavity gap impedance over the operating band. In such a circuit, an RF filter cavity is placed between the load and the RF output cavity. The filter cavity must be designed to yield the required Q and resonant frequency. The difficulty in the design of such a broadband filter-loaded circuit is due to the high degree of coupling required between the load and the filter cavity. Loop coupling cannot be used for this purpose since the large loop dimensions necessary to achieve this amount of coupling introduce unwanted loop modes. A waveguide output circuit and waveguide coupling iris may be used in place of the loop and coaxial output line; however, this combination would not only be large and heavy, but would require a relatively large vacuum tight RF window. This is especially true at the lower microwave frequencies and U.H.F. frequencies. 
     It is, accordingly, a primary object of the present invention to provide a tunable, wideband, output circuit for use with linear beam devices. 
     Another object of the present invention is to provide a klystron amplifier of the type employing a single-gap output cavity which is capable of operating over a wide bandwidth. 
     Another object of the present invention is to provide a small, lightweight tunable, wideband output circuit for a klystron amplifier which consists in part of a single filter cavity that is capacitively tuned by a coaxial probe assembly which forms part of the output means. 
    
    
     Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings wherein: 
     FIG. 1 is a cross-sectional view of one preferred embodiment of the output cavity resonator circuit taken along a longitudinal axis of symmetry of the cavity resonators; and 
     FIG. 2 is a cross-sectional view taken at line 2--2 in FIG. 1; 
     FIG. 3 illustrates one simplified arrangement for tuning the output cavity resonator circuit. 
     Briefly, and in somewhat general terms, the above objects of invention are accomplished by coupling to the output cavity of a kylstron, a second cavity which performs as a filter resonator. This coupling is achieved by a suitable inductive iris which forms part of a common wall structure between the two cavities. The filter resonator has a coaxial probe which both capacitively tunes this cavity and serves as a means for extracting the output signal in a mode-free manner. The combination of the two cavities results in an overall improvement in bandwidth response accompanied by a higher output cavity gap impedance for the klystron amplifier than would be the case with either of the individual cavities alone. Specifically, the bandpass response is flatter in the passband and the band edges drop off much steeper. 
    
    
     Referring now to FIG. 1 of the drawings, it will be seen that the overall output apparatus for the klystron amplifier, which provides the improved bandwidth, includes a single-gap cylindrical cavity 10 of conventional design, which interacts with the electron beam 11 in the usual manner to extract radio frequency energy therefrom. For simplicity sake, the structural details of the klystron tube are not shown. Reference may be had to U.S. Pat. No. 2,944,187 for the details of such a high power multi-cavity klystron amplifier tube. However, the upstream and downstream drift tubes 12 and 13, respectively, which are required in the performance of this tube and are supported from the top and bottom walls of cavity 10, are schematically illustrated in order to identify the function of this particular cavity. 
     A second cavity 15, whose dimensions may be similar to that of cavity 10, and which serves as the filter resonator previously mentioned, is coupled to cavity 10 by an inductive iris 14 which is formed in a plate 23 that fits within a suitable recess in the rim portion of cavity 10. This component is best shown in FIG. 2. 
     Filter cavity 15 has a coaxial probe 16 positioned along its vertical center line so as to coact with the electrical field present at this location. This probe, which behaves in this regard as a fixed tuning slug, as best shown in FIG. 1, consists in one form of an inner cylindrical conductor 17 having an enlarged diameter lower disc portion 18. Conductor 17 is, in fact, an extension of the inner conductor of a coaxial line whose outer conductor, in its lower end, takes the form of an upstanding flange 19 affixed to the top wall of cavity 15. The enlarged disc portion 18, it will be appreciated, functions as the stationary plate of a tuning capacitor. A similar disc 21 aligned therewith and supported from the lower wall portion serves as the displaceable plate of this device. This lower disc 21 is moveable in the axial direction, and this movement is permitted by bellows 22 which is affixed at one end to the lower rim of disc 21 and at the other end to the lower wall of cavity 15. Bellows 22 is securely affixed at both ends to maintain the vacuum integrity of the output circuit. 
     A simplified arrangement for tuning cavity 15 which allows controlled axial movement of the moveable tuning element is shown in FIG. 3. Here, again, a bellows 23 is secured to the lower rim of disc 21 and a confronting inner wall portion of cavity 15 to preserve the vacuum condition within the assembly. A lower body portion of the tuning slug where it extends beyond the wall portion of the cavity is threaded, and a captive nut 25 held in place by supporting element 26 upon rotation, displaces this slug in an axial direction to vary the distance between it and its companion disc 18. 
     An annular window 20 positioned within flange 19 supports inner conductor 17. This window also establishes a vacuum tight seal for isolating the interior of the cavities from the surrounding environment. The external load 22 is connected to the coaxial line by any suitable means. 
     The introduction of coaxial coupler 16 within filter cavity 15 permits this cavity to be tuned by adjusting the gap between discs 18 and 21. This capacitive loading reduces the resonant frequency of this cavity and, as a consequence, its size may be reduced. It would also be pointed out that the use of a coaxial type coupler for this purpose permits a comparatively small vacuum window 20 because of the area occupied by inner conductor 17. This tends to reduce the incidence of window failure due to thermal stresses arising from RF dissipation in the di-electric window material. Cavity 10 may be tuned in the conventional manner, i.e., with a diaphragm located in the cavity wall. 
     The combination of the two cavities shown in FIG. 1 provides a flatter response than that attainable with either the single-gap cavity 10 alone or the single filter cavity 15 alone. 
     Although the output cavity 10 for the klystron amplifier is shown as having a single-gap, it should be appreciated that the output circuit disclosed can be utilized with multi-gap structures. Likewise, it would also be pointed out that with the output apparatus disclosed, a comparatively wide bandwidth can be achieved with a minimum decrease in the gap impedance. Thus, the improved bandwidth performance is attained with only a relatively small decrease in efficiency. Moreover, the lower gap impedance allows a greater collector depression, and this tends to offset this decrease in efficiency.