MICROWAVE PARAMAGNETIC RESONANCE ABSORPTION 473 



in the linear region. Values of S and C for the 1N23C were found to be 

 S c^ 500 Watt"' and C ~ 0.3. 



a. Simple straight detection 



If one does not make use of the bucking possibilities of the magic T 

 (i.e., eliminate the slide screw turner in Fig. 6) one has the simplest 

 possible set-up sensitive to x" • Under those conditions the microwave 

 power reaching the crystal will be identical to the reflected power from 

 the cavity. Equation (36) becomes: 



Xmin-obs _ ( GNk + Fx^ip 4~ ^ ~ 1 

 Xmin-th 



With the aid of (48), (49), (50), and (51), this relation reduces for the 

 1N23C in the square law region to: 



;;•■- (^ ■^^v.^r.HP+.-i j (52) 



" /i 1 r V, ir»9] 



Xmin-obs /I + 5 X lOPo X" /^on 



- -I (o3) 



Xmin-th 



50Po 



and for the linear region to: 



'^^^^i^ = (3 X lO^Po)' (54) 



Xmin-th 



A plot of (53) and (54) is shown in Fig. 9. As before the assumption was 

 made that Prf/A ^^0.1 (see barretter case). The noise figure of the 

 amplifier Famp was taken as unity which again can be closely ap- 

 proached by means of a step-up transformer. The field modulation 

 frequency was assumed to be 1,000 c.p.sec, although (47) shows that 

 from a point of view of noise one would like to go to as high a frequency 

 as possible. However practical consideration such as power require- 

 ments for getting a given modulation field, pick-up problems, skin 

 depth losses in the cavity wall usually set an upper limit. The modula- 

 tion frequency may be also dictated at times by the relaxation times 

 of the investigated sample. 



h. Straight detection with optimum microwave bucking 



From Fig. 9, we see that the straight crystal detection scheme suffers 

 at low powers because of the poor conversion gain of the crystal and at 

 high powers because of excess crj^stal noise. This situation can be 

 greatly improved by adding some microwave power to the crystal when 

 the reflected power from the cavity is low (to be referred to as positive 

 bucking) or subtracting some of the power in the other case (negative 

 bucking) . In this section we will find the improvement over the unbucked 

 system and the amount of bucking required to effect it. 



