558 BELL SYSTEM TECHNICAL JOURNAL 



C. A Reflex Oscillator With An Integral Cavity— The 723 



The need for higher definition in rachir systems constantly urges eperation 

 at sliorter wavelengths. Thus, while radar development proceeded at 3CC0 

 megacycles, a program of development in the neighborhccd cf lO.COO 

 megacycles was undertaken. Although waveguide circuit techniques were 

 employed to some extent at 3000 megacycles, the cumbersome size cf the 

 guide made its use impractical in the receiver and hence coa.xial techniques 

 were employed. The \" by h" guide used at 10,000 megacycles is con- 

 venient in receiver design and also desirable because the loss in coaxial con- 

 ductors becomes excessive at this frequency. Hence, one of the first 

 requirements on an oscillator for frequencies in this range was the adaptabil- 

 ity of the output circuit to waveguide coupling. 



In considering possible designs for a 10,000 megacycle oscillator the simple 

 scaling of the 707A was studied. This appeared impractical for a number 

 of reasons. The most important limitation was the constructional diffi- 

 culty of maintaining the spacing in the gap with sufficient accuracy with 

 the glass seaUng technique available. Also, variations in the capacitance 

 caused by variations in the thickness of the seals caused serious difficul- 

 ties in predetermining an external resonator. Contributing difficulties 

 arose from the power losses in the glass within the resonant circuit and the 

 problem of making the copper to glass seals close to the internal elements. 



Consideration of these factors led to a new approach to the problem, in 

 which the whole of the resonant circuit was enclosed within the vacuum 

 envelope. This required a different mechanism for tuning the resonator, 

 since variation of the inductance of a cavity requires relatively large dis- 

 placements w^hich are difficult to achieve through vacuum seals. The 

 alternative is to vary the capacitance of the gap. Since the gap is small a 

 relatively large change in capacitance can be achieved with a small dis- 

 placement. This sort of tuning permits the use of metal tube construc- 

 tional techniques, and these were applied. 



As a matter of historical interest an attempt at this technique made at the 

 Bell Telephone Laboratories is shown in Fig. 62. This device was held to- 

 gether by a sealing wax and string technique and was net tunable in the first 

 version. It oscillated successfully on the pumps, however, and a second 

 version was constructed which was tuned by means of an adjustable coaxial 

 line shunting the cavity resonator. Adjustment of this auxiliary line gave 

 a tuning range of 7.5%. Such a tuning method is fraught with the com- 

 plications outlined in Section IX. 



An early reflex oscillator tube of the integral cavity type designed at the 

 Bell Telephone Laboratories was the Western Electric 723A/B. 



