MICROWAVE PARAMAGNETIC RESONANCE ABSORPTION 



457 



All the previous sensitivity expressions are proportional to x"- For 

 an unsaturated condition it may be replaced by x whenever the output 

 is sensitive to phase changes in the cavity. 



It should be noted that we maximized the output from the detector. 

 This will result in a maximum signal to noise ratio if the noise is a 

 constant independent of the microwave power. This, however, is in 

 general not the case and in the next section we will in\'estigate the signal 

 to noise ratio taking into account its dependence on the RF power. 



v. MINIMUM DETECTABLE SIGNAL UNDER IDEAL CONDITIONS 



The minimum signal is ultimately determined by the random thermal 

 agitation. Due to this cause the power fluctuates by an amount kT^v, 

 where k is Boltzmann's constant, T the absolute temperature and Aj/ 

 the bandwidth. The minimum detectable microwave power will be then 

 of the order of kTAv. This is the problem one faces when designing sensi- 

 tive microwave receivers. However, our problem is of a different nature. 

 We want to detect a small change in the power level of a relatively large 



8 6 4 212 4 6 8 



VOLTAGE STANDING WAVE RATIO 



Fig. 4 — Output versus V.S.W.R. for reflection and transmission cavity. 



