REFLEX OSCILLATORS 553 



nection with automatic frequency control circuits in a manner too compli- 

 cated for treatment here. In some cases this necessitates a restriction on 

 the total frequency shift between extinction points for a given repeller mode. 

 The relationship between half power and extinction electronic tuning has 

 been discussed in Section \TI. 



In addition to the electrical requirements which have been outlined, 

 military applications dictate two further major objectives. The first is the 

 attainment of simple installation and replacement, which will determine, in 

 part, the outward form of the oscillator. The second is low voltage opera- 

 tion, which fundamentally affects the internal design of the tubes. In some 

 instances military requirements conflict with optimum electronic and circuit 

 design, and best performance had to be sacrificed for simplicity of construc- 

 tion and operation. In particular, in some cases it was necessary to design 

 for maximum flexibility of use and compromise to a certain extent the 

 specific requirements of a particular need. 



In the following section we will describe a number of reflex oscillators 

 which were designed at the Bell Telephone Laboratories primarily to meet 

 military requirements. These oscillators are described in approximate 

 chronological crder of development in order to indicate advances in design 

 and the factors which led to these advances. 



The reflex oscillators which w'ill be described fall into two general classi- 

 fications determined by the method employed in tuning the resonator. In 

 one category are oscillators tuned by varying primarily the inductance of the 

 resonator and in the other are those tuned by varying primarily the capaci- 

 tance of the resonator. The second category includes two types in which 

 the capacitance is varied in one case by external mechanical means and in 

 the second case by an internal means using a thermal control. 



B. A Rejiex Oscillator With An External Resonator — The 707 



The Western Electric 707A tube, which was the first reflex oscillator 

 extensively used in radar applications, is characteristic of reflex oscillators 

 using inductance tuning. It was intended specifically for service in radar 

 systems operating at frequencies in a range around 3000 megacycles. Fig. 58 

 shows a photograph of the tube and Fig. 59 an x-ray view showing the inter- 

 nal construction. A removable external cavity is employed with the 707A 

 as indicated by the sketch superimposed on the x-ray of Fig. 59. Such 

 cavities are tuned by variation of the size of the resonant chamber. Such 

 tuning can be considered to result from variation of the inductance of the 

 circuit. 



The form of this oscillator is essentially that of the idealized oscillator 

 shown in Fig. 58. The electron gun is designed to produce a rectilinear 

 cylindrical beam. The gun consists of a disc cathode, a beam forming elec- 



