470 THE BELL SYSTEM TECHXICAL JOURNAL, MARCH 1957 



10 



10 



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1 



10"'' iO"6 iO-S 10"'* <0"^ 10-2 )0-< 



MICROWAVE POWER FROM KLYSTRON, Pq [w] 



Fig. 7 — The ratio of the minimum detectable susceptibility to the minimum 

 theoretical value versus microwave power for 2 different barretter schemes. 

 Full lines correspond to the predicted sensitivity and dots indicate experimental 

 values. 



Equation (44) is plotted in Fig. 7. From this plot we see that the system 

 is extremely poor at low powers (which is due to the low conversion 

 gain of barretters) and also starts getting worse at high powers (due to 

 the signal generator noise). The latter point is not of great importance 

 since one can alwaj^s buck down the microwave power by means of the 

 slide screw tuner to the desired level. By using the evacuated barretter 

 as mentioned earlier, the curve in Fig. 7 would be shifted to the left 

 corresponding to the increased conversion gain. 



b. Balanced mixer detection 



An improved barretter scheme is shown in Fig. 8. It eliminates the 

 poor conversion gain at low powers by employing a balanced mixer 

 into which a large amount of microwave power P2 can be fed from the 

 same signal generator. Since the barretter noise should not be power 



