MICROWAVE PARAMAGNETIC RESONANCE ABSORPTION 463 



from the random frequency variations of the microwave source or from 

 the random \-ariations of the resonant frequency of the cavity (e.g., 

 rising helium bubbles at 4°K or just any microphonics). 



From the equivalent circuit of a reflection cavity (see Fig. 2) we can 

 write for the voltage standing wave ratio 



VSWR = |i:L = ^ + /^, (cL - 4;) 

 Rou^ Ron- Ron- \ coC/ 



Ron' 



\ CO /J Ron- 



where 5 == Qo(2Aa)/co) and Ao? is the frequency deviation from the 

 resonant frequency of the cavity. The reflection coefficient T is then 

 given by: 



Ron^ j2 Ron . 







r = '-^ = To + 2 —-^5 -i- 2.7 



ft«= ^ , 



r-7 



where To is the reflection coefficient at the resonant frec^uenc}^ of the 

 cavity. The other two terms giving the changes in T for a given fre- 

 quency deviation Aw. The changes of Aw (or AT) having ac components 

 near the modulation frequency will thus represent noise terms which 

 will pass together with the signal through the detection system. The 

 shde screw tuner (see Fig. 1) which is used to buck out part of the micro- 

 wave power will introduce an additional reflection coefficient r« + jTn'. 

 Thus the total reflection coefficient will be given by 



i^o^^ .2 Ron , 

 



r = To - Ts + 2 y^^4 X-, - JT/ + j2 y^ ^ (31) 



(?f + ij ' " ■' ^R^^ ,J 



We are interested only in the magnitude of V (i.e., | F |) reaching the 

 detector. Tuning to the dispersion mode (x') the slide screw tuner is 

 adjusted such that T^' » To — Fr . Under these conditions the output 

 noise voltage will be given by 



Ron . 



Tuning to the absorption (x"), the condition Fo — F^ » Fr will be 



