370 THE RADAR RECEIVER 



practice, careful measurement of the actual input admittance of the tube 

 under operating conditions and with feedback effects removed by neutral- 

 ization gives an input conductance which, when employed as ^gt in the 

 amplifier design, results in measured noise figures in close agreement with 

 calculated values. 



Fig. 7-8 shows a simplified equivalent circuit for a grounded-grid stage. 

 The noise figure is given by 



^Am + 1/ I I 



l+^ + |J + ^-:^{-T-r)|i^^l (7-32) 



1\ i h 



Nsi 



Fig. 7-8 Simplified Equivalent Circuit for a Grounded Grid Stage. A^^; in- 

 cludes noise due to grid loading and network loss. Ngi is therefore taken as a noise 

 current generator ^^^KTypgi, where gi is total conductance between cathode and 

 grid and i/' is an effective temperature ratio. The admittance seen to the right 

 of aa', is 



gli} + m) 

 1 + girp 



where m is the amplification factor of the tube. In determining the overall 

 noise figure of the IF amplifier, the available power gain of the amplifier 

 stage must be considered. When losses in the interstage coupling network 

 are not included, the available power gain for the grounded-cathode and 

 grounded-grid stages is given by 



e) (7-33) 



(grounded grid) (7-34) 



where 



grr, 



The most frequently used IF preamplifier circuit in airborne radar re- 

 ceivers is the cascade circuit. This circuit consists of a grounded-cathode 

 triode stage followed by a grounded-grid stage. Such a configuration results 

 in a lower noise figure than can be obtained with a cascade connection of 

 two neutralized grounded-cathode triode stages. This is because the 

 interstage bandwidth is obtained by virtue of the loading incident to the 



