MICROWAVE PARAMAGNETIC RESONANCE ABSORPTION 



459 



This corresponds for an unsaturated Lorentz line^ to a static suscepti- 

 bility xn = \/Zx"{^^/^), where ^w is the hne width between inflection 

 points. For the free radical diphenyl picryl hydrazyl having a 2 oersted 

 line width this expression at i-oom temperature gives for the min. number 

 of spins 10'". A plot of the minimum RF susceptibility and minimum 

 number of electrons versus microwave power is shown in Fig. 5. 



VI. SIGNAL-TO-NOISE IN PRACTICAL SYSTEMS 



.4 . General Considerations 



1 . Why Field Modulation? 



From a design point of view it is instructive to consider the mini- 

 mum fractional voltage change corresponding to the above Xmin"T's of 

 2 X 10"'*. This turns out to be, see (16), 



V 



2 X 10 



-10 



From this figure one may safely conclude that it is not feasible to use 

 an}^ system in which the microwave carrier level reflected from the 

 cavitj^ has to be kept constant to this accuracy. Such systems would 

 include straight detection, the dc being bucked out and amplified or 

 systems employing amplitude modulation of the carrier. (Although 



ICROWAVE POWER FROM KLYSTRON, Pq [w] 



Fig. 5 — Minimum RF susceptibility aiid number of electrons which should 

 be observable under thermal noise limitations. The conditions for the minimum 

 number of spins correspond closeh" to those under which the experimental set-ups 

 were tested. 



