MICROWAVE PARAMAGNETIC RESONANCE ABSORPTION 



471 



dependent (unlike in crystals) this procedure improves the conversion 

 gain without increasing the noise. Since a balanced mixer is used the 

 noise from the signal generator is also cancelled. A necessary precaution 

 in this set-up is to include extra isolation between the second magic T 

 and the mixer in order to prevent any microwave power from leaking 

 through the balanced mixer into the cavity. 

 For this arrangement (44) becomes: 



Xmin-obs 

 Xmin-th 



V2|/| 



(45) 



The factor of \/2 arises from the fact that we had to split the power 



Po in the first magic T . vSince in this scheme we are at liberty to vary 



the input power to the barretter we want to maximize G with respect 



to Fi . For a fixed total power to the barretter given by its burn-out 



ratings (i.e., Fi + Pdc = constant) (41) is a maximum for Ft ^^ Pdc ^^ 

 p 

 ^]^ - Taking again the data for the No. 821 barretter we get for Gmax 



0.5 and for 



Xmin-obs 



// 

 Xmin-th 



1-^^ 4 



(46) 



Since the value of Fi can be held constant irrespective of the power 

 in the cavity, this ratio will be a constant (see Fig. 7). 



It should be pointed out that in this S3'stem a wrong phasing of arm 

 Fi will result not only in a reduction of the signal, but also in an ad- 

 niLxtui-e of x' and x"- Therefore after changing the power by means of a 



STABILIZED 

 KLYSTRON 



PHASE 

 ATTENUATOR SHIFTER 



BALANCED 

 MIXER 



/ Pn 



ISOLATOR 

 T-l 



SLIDE SCREW 

 TUNER / /^ 



T-2 r^ 



rt 



ISOLATOR X ,' 



BARRETTERS 



-^ ._---CAVITY 



Hn+AH SIN CoX 



Fig. 8 — Barretter system with balanced mixer. 



