570 BELL SYSTEM TECHNICAL JOURNAL 



made it possible to present to the oscillator at each frequency the admittance 

 into which the oscillator delivered maximum power output. The solid 

 lines show the power output and electronic tuning over the band with the 

 oscillator connected to a load presenting a resistive 50 ohm impedance to 

 the coupling unit of Fig. 68. The problems involved in obtaining such 

 performance will be outlined in the next section. 



E. Broad- Band Reflex Oscillators— The 2K25 



As experience with the design of radar systems and components developed 

 and as a better understanding of the operation and limitations of the in- 

 dividual components was achieved, a great deal of effort was directed 

 toward simplifying and making more reliable the number of adjustments 

 required to optimize the performance of a system. As an illustration of 

 this problem as related to the beating oscillator, the development problem 

 of the 2K25 will be described. As the number of radar systems in the three 

 centimeter range increased it became apparent that to avoid self-jamming 

 it would be desirable to assign frequencies to various sets operating, for 

 example, in a fleet unit. Secondarily, the over-all band of the three centi- 

 meter range was widened to cover from 8500 to 9660 Mc/s. Prior to this 

 the 723A/B had been essentially a spot frequency oscillator and had been 

 primarily tested as such. As was so frequently the case with tubes for 

 military requirements, it was desired that the ultimate tube be interchange- 

 able with an existing tube, in thjs case the 723A/B, and hence the im- 

 provements had to be effected within its framework. 



Changes in the electronic design from that of the 723A/B produced an 

 improved performance in the 2K25. These were a mcdification of the 

 electron gun which increased the effectiveness of the electron stream and 

 the elimination of a resonance of the region containing the electron gun 

 which coupled with the resonant cavity and in some cases impaired the 

 performance over the wide band. Beyond this the problem concerned the 

 determination of an output coupling system which would provide the de- 

 sired properties. This will be described in detail. 



One of the most serious difficulties which occurred in early radar receivers 

 arose from the general failure to appreciate the effect of the load impedance 

 on the performance of an oscillator. This j)roblem has been discussed in 

 Section IX. In early radar receivers the method of coupling the beating 

 oscillator to the crystal was dictated mainly by mechanical convenience 

 rather than electrical considerations, and as a result most of the discontinu- 

 ities of performance due to bad load conditions which are discussed in 

 Section IX occurred at one time or another in most of the systems. For 

 instance, users were much surprised to hnd that a beating oscillator which 

 could be tuned to frequencies both above and below that required for 



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