458 THE BELL SYSTEM TECHNICAL JOURNAL, MARCH 1957 



microwave signal. This change in power level will ha^•e to be consider- 

 ably larger than kTAv before it can be detected.' The physical reason for 

 this is that the fluctuating fields a.s.sociated with the noise power com- 

 bine with the microwave fields to produce power fluctuations much 

 larger than kTAv. It is more straightforward to compare noise voltages 

 rather than powers, especiallj'' since the power changes are not neces- 

 sarily a constant of the system. In Fig. 1 for instance, the power change 

 in arm 4 is 



no 

 whereas the power change in arm 2 (for the same voltage change) is 



APo = 



Rn 



It also shows that one wants to maximize the change in output voltage 

 as was done in Section TV. 



The open terminal R^IS noise voltage of a system with an internal 

 impedance Ro is given b\' 



Frms = V-iR^kTAv 



If we terminate this system with a noiseless resistor Ro , the voltage 

 across it will be \/RokTAv. However, the terminating resistor is also 

 at temperature T, so that the total RMS voltage across it will be 

 V2 VRokTAv. 



Comparing this RMS noise voltage with the signal ^•oltage obtained 

 in (16), we get for the reflection cavitj-* 



AT" = V V^wx'-nQo = V2 VRokTAv (25) 



As an example let us consider the following typical value for a 3-cm 

 setup. Qo = 5 X 10^ Ap = 0.1 cps, Po = 10~' Watts 



= If 4 ^ 4F, 

 '^ Fe /dV- ~ 10 cm' • 



For this case 



(Xmin")(T^.) = ~2 X 10"" 



* In most cases the behaviour of the transmission and reflection cavity is 

 similar, so that they will not be treated separately. 



