BROAD-BAXD MICROWAVE NOISE SOURCE 611 



Equation (9) will now be used to illustrate one method of measuring noise 

 figures. In this method, the network output noise power is measured for two 

 known values of the temperature of the generator resistance, Tn and Ti. 

 When the generator is hot, the output noise power is, by equation (5): 



I\0H = GkT,B + A';v (10) 



When the generator is cool, the output noise power is: 



P:,oc = GkT.B + Ns (11) 



Calling the ratio of these two noise powers F: 



_ Pnoh _ GkT^B + Nn /J2') 



iW ~ GkT.B + Nn 



Substituting for Nn the value given in equation (9), we have for the 

 noise figure: 



{m - - K^ - 



p _ .-- . (13) 



In practice Ti is often near enough to 290 degrees so that the second 

 term in the numerator of equation (13) is negligible. Setting Ti equal to 290 

 degrees, equation (13) becomes: 



F = 1^ (14) 



The determination of noise figure by this method is independent of the 

 gain of the network, the degree of mismatch and the bandwidth, provided 

 that the band of the noise source is broad compared with the overall RF 

 band of the network and the output power meter. 



The Noise Source 



The limitations at microwaves of a noise source such as a heated wdre will 

 now be discussed. In particular we are interested in measuring amplifiers 

 which have noise figures between 10 and 100 (10 db to 20 db) and band- 

 widths up to 200 mc. If a hot wire could be matched to the impedance of a 

 waveguide over a wide enough band, and raised to a temperature of 10 X 290 

 degrees our F factor would be (rearranging eq. 14): 



T 



1 



y = ??C> ^ , (,5) 



