354 THE RADAR RECEIVER 



The noise figure is defined as 



SJN,^N^ (7.4) 



So/ No N^G ^ ' 



where F is the noise figure (a power ratio) 



SilNi is the available signal to noise ratio at the input 



So /No is the available signal to noise ratio at the output 



G is the available power gain. 



For the case of the generator connected to a load, the noise figure of the 

 combination is then 



F =. I -\-t ^. (7-5) 



If both the generator and load are at the same temperature, then the 

 noise figure is merely the attenuation of the signal resulting from the 

 termination. 



In a radar receiver it is convenient to associate a noise figure with 

 various elements and then determine the receiver noise figure resulting from 

 their combination in cascade. 



Consider that a number of elements characterized by a noise figure Fj 

 and an available power gain Gj are interconnected as in Fig. 7-2. It is 



F,,G, 



Fig. 7-2 Noise-Equivalent Radar Receiver. 



assumed that all the noise sources are at a temperature T (a difference in 

 temperature may be included as a temperature ratio). Assume further 

 that all elements are linear, and that the effective noise bandwidth of each 

 element is 5„. 



The input noise is A^i = kTB„ 



The overall gain G is G1G2G3 - 



Output noise originating from the source is GkTB„. 



The additional noise at the output contributed by the first box is 



GkTBn{F\ — 1). The additional noise at the output contributed by the 



, , . GkTB„{F2 - 1) ^, r n • -k • AA 



second box is — -■ I he sum or ail noise contributions add up 



Gj 



