MICROWAVE PARAMAGNETIC RESONANCE ABSORPTION 



481 



vi (this being also the reference frequenc}') in a band width Avo will con- 

 tribute to the output noise. This is because the beat between 2 noise 

 components like Vn and Vm (see Fig. 11) is too far removed from vi to 

 produce an output voltage. (This statement implies the condition that 

 Ai'i < vi otherwise the beat between 2j'i and vi could come through.) 

 Thus in this system the band width of the amplifier is immaterial as long 

 as the noise voltages are not so large as to saturate it. 



A more serious situation may arise in the absence of a reference volt- 

 age. In this case the noise components within the band width Aj^i can 

 beat with each other and produce a noise output which would increase 

 with the band width. This could become especially detrimental in a 

 superheterodyne scheme in which the IF band width can be a million 

 times larger than the output band width. It can be shown, however, that 

 if the carrier voltage Vc at the output of the IF is large enough the IF 

 bandwidth AFip does not enter into the noise consideration^^ the cri- 

 terion essentially is that 



TV > G'2kTZAF 



IF 



(60) 



where G is the IF amplifier gain, and Z the input impedance. Condition 

 (60) means that we want the noise which beats with the carrier to be 

 greater than the beat between 2 noise terms. Since the former is propor- 

 tional to the carrier, its predominance can be easily ascertained experi- 

 mentally by increasing the IF carrier and noting whether the noise 

 output increases proportionally. If it does, (60) is fulfilled. 



(b) 



y,-y-* 



Fig. 11 — Effective band width of a phase sensitive detector Sveit = Aj-.. 

 Note that the band width of tlie amplifier does not enter as long as ^vi < vi . 



