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BELL SYSTEM TECHNICAL JOURNAL 



converter. From these data and other circuit constants, the designer may 

 calculate* expected receiver performance. 



For operation as converters,^ crystal rectifiers are employed in suitable 

 holders. These may be arranged for use with either coaxial line or wave 

 guide circuits, depending upon the application. Figure 7 shows a converter 

 for wave guide circuits installed in the radio frequency unit of an air-borne \ 

 radar system. A typical converter designed for use with coaxial lines is [ 

 shown in the photograph Fig. 8. A schematic circuit of this converter '. 

 is shown in Fig. 9. In such circuits the best signal-to-noise ratio is realized 

 when an optimum amount of beating oscillator power is supplied. The 

 optimum power depends, in part, on the properties of the rectifier itself, 

 and, in part, on other circuit factors as the noise figure of the intermediate 





Fig. 8 — Converter for use at 3000 megacycles. The crystal rectifier is located 

 adjacent to its socket in the converter. 



frequency amplifier. For a well designed intermediate frequency amplifier 

 with a noise figure of about 5 decibels, the optimum beating oscillator 

 power is such that between 0.5 and 2.0 milliamperes of rectified current 

 flows through the rectifier unit. Under these conditions and with the unit 

 matched to the radio frequency line, the beating oscillator power absorbed 

 by the unit is about one milliwatt. For intermediate frequency amplifiers 



■" The quantities L and Ni? are related to receiver performance bj' the relationship 

 F^ = Z.(N/? - 1 + FiF) 

 where Fr is the receiver noise figure and Fip is the noise figure of the intermediate fre- 

 quency amplifier. All terms are expressed as power ratios. A rigorous definition of 

 receiver noise figure has been given l)v H. T. Friis "Noise Figures of Radio Receivers," 

 Proc. L R. E., vol. 32, pp. 419-422; July, 1944. 



* C. F. Edwards, "Microwave Converters," presented orally at the Winter Technical 

 Meeting of the /. R. E., January 1946 and submitted to the /. R. E. for publication. 



