MICROWAVE PARAMAGNETIC RESONANCE ABSORPTION 467 



will have to be modified as the "art", of detector manufacturing im- 

 proves. Also new systems might come into prominence in the near 

 future. An example would be the use of low noise travelling wave tubes 

 preceding the detector or even low noise solid state masers. 



1. Barretter {bolometer) detection. 



The resistance of a barretter is given by the relation 



R = Ro + kP"" (37) 



For practical purposes n may be taken as vmity. The instantaneous power 

 input to the barretter for a modulated microwave is given by 



1^ 

 R 



Vo sin fin 1 + ^ sin coi ) + T^dc | (38) 



where Vo is the amplitude of the microwaves, AV the change of the am- 

 plitude due to the absorption given by (16), fl the microwave fre- 

 quency, CO the field modulation frequency, and Vdc the bias on the 

 barretter. Expanding (38) and assuming that AV/Vo <$C 1 we get, after 

 throwing out the high frequency terms, 



R = Ro-h k (Pi,, + Pdc + ^^—^ sin con (39) 



where Prf is the power in the unmodulated carrier reaching the bar- 

 retter. It is of course smaller than Pq the microwave power from the 

 klystron because of the power splitting in the magic T and the reflection 

 from the cavity. Taking a reflection coefficient F ^^ 0.5 (see Section IVA), 

 Prf/Po ^^0.1. The desired voltage fluctuation associated with the re- 

 sistance change is: 8V = IqAR, where 



AR=^AP = ^ (AP«. -f- h'AR), = ^ 



,„ ^ , VoAV . , 



8V ~ hk 7- zr—r sin cot 



(1 — lo^k) 



1 



j2dR 

 ^'dP 



APi^r 



(40) 



7o is the current bias on the barretter which we want to keep constant 

 for a maximum voltage change 8V. 



The power gain of this device G is given by 



G = 



Signal Power from Barretter SV' / AV , lokP 



RF 



Power in the Sidebands 2R / 4R R(l - Io%) 



-7.2 PiXcPrF 



' R\i - m) 



