MICROWAVE PARAMAGNETIC RESONANCE ABSORPTION 



469 



those assumptions (36) becomes: 



1 



// 



Xmin — 



/kTA 

 QorjT \ Po 



4 + NkG' 

 G , 



(43) 



a. Straight detection 



A block diagram of the microwave part of a simple barretter system 

 is shown in Fig. 6. The attenuator serves the purpose of preventing power 

 saturation of the sample or burn out of the bolometer at high powers. 

 By means of the slide screw tuner and magic T arrangement one makes 

 the system sensitive to either the real or imaginary part of the suscepti- 

 bility. 



The characteristics of a typical barretter (like the Sperry No. 821) 

 are: R = 250 Q; k = 4.5 U/niW; Pmax = 32 mW. We take the worst 

 generator noise figure reported, i.e., Nk = 5,000 Prf (see Section VID). 



The ratio of the minimum susceptibility xmin-obs that can be de- 

 tected with this system to the minimum theoretical value if one were 

 limited by thermal noise only becomes with the aid of (41) and (43) 



n 

 Xmin-obs 



// 

 Xmin-th 



^ 4 + NkG ^ 

 . G , 



1 -f 5 X WPuFk'- 



r rfl 



RF^ dc 



R'{1 - h'k) 



fc^ 



LRPL. 



(44) 



RF-I do 



PHI - Uk) 



ATTENUATOR 



STABILIZED 

 KLYSTRON 



ISOLATOR 



SLIDE 



:^^SCREW 



TUNER 



MAGIC 

 -- T 



't. Pa 



DETECTOR 



CAVITY 



— Ho + AH SINO^t 



Fig. 6 — Essential microwave parts of a simple barretter or crystal set-up. 



