560 BELL SYSTEM TECHNICAL JOURNAL 



a frequency within one or two parts in 10,000. In addition, in order 

 to avoid field installation it was desired to have the tuning mechanism 

 cheap enough to be factory installed and discarded with each tube. 



4. The oscillator was required to be compact and light in weight to 

 facilitate its use in airborne and pack systems. 



Figure 63 shows a cross-section view of the final design of the 2K25 reflex 

 oscillator. The resonant cavity is formed in part by the volume included 

 between the frames which support the cavity grids and also by the 

 volume between the flexible vacuum diaphragm and one of the frames. 

 This diaphragm also supports a vacuum housing containing the repeller. 

 The electron optical system consists of a disc cathode, a beam electrode and 

 an accelerating grid. These are so designed as to produce a slightly con- 

 vergent outgoing electron stream. The purpose of this initial convergence 

 is to offset the divergence of the stream caused by space charge after the 

 stream passes the accelerating grid and to minimize the fraction of the elec- 

 tron stream captured on the grid frame on the round trip. The repeller 

 is designed to provide as nearly as possible a uniform retarding field through 

 the stream cross-section. 



Power is extracted from the resonant circuit by the coupling loop and is 

 carried by the coaxial line to the external circuit. The center conductor of 

 the coaxial line external to the vacuum is supported by a polystyrene in- 

 sulator and extends beyond the outer conductor to form a probe. Coupling 

 to a wave guide is accomplished by projecting this probe through a hole in 

 that wall of the wave guide which is perpendicular to the E lines so that 

 the full length of the probe extends into the guide. The outer conductor 

 is connected to the wave guide either metallically or by means of an r.f. 

 bypass or choke circuit. A more detailed section on such coupling methods 

 will be given later. 



The tube employed a standard octal base modified to pass the coaxial line. 

 Thus if a standard octal socket is similarly modified and mounted on the 

 wave guide it is possible to couple the oscillator to the wave guide and 

 power supply circuits simply by plugging it into the socket, just as with 

 any conventional vacuum tube. 



The tuning means for this type of oscillator tube presented a serious 

 problem. This will be appreciated when it is realized that the mechanism 

 must permit setting frequencies correctly to within one megacycle in a device 

 in which the frequency changes at the rate of approximately 200 megacycles 

 per thousandth of an inch displacement of the grids. In other words, the 

 tuner was required to make possible the adjustment of the grid spacing to 

 an accuracy of five miljionths of an inch. The design of the mechanism 

 adopted was originated by Mr. R. L. Vance of these Laboratories. The 

 operation of the tuner can be seen from an examination of the cross-section 



