THE RADAR RECEIVER 709 



where: 



p , 



G = ~ , by definition the "gain" of the network. 



or we may write: 



Fj,u = FKTBG watts. 

 It is common practice to express F in decibels given by the relation 10 \ogu^F. 

 For the case of two generalized networks in tandem following the signal 

 generator and having the same effective bandwidth we may similarly write: 



P„„,., = FaG,Gt.KTB + {Fo - DG^KTB 



= U\, + tt^jG^G,, KTB watts. 



where the subscripts refer to networks a and b. 



The effective noise hgure of such a system is given by: 



/.' - (f + ^''" ~ ^ 



' system — I ' « T , , 



This expression indicates the im{)ortance of the gain of the lirst network in 

 the over-all system noise performance. 



As an illustration, a noise figure of 11 db and a loss of 6 db may be con- 

 sidered as acceptable performance for a typical silicon crystal converter 

 operating at M)i)() mc. If the following input circuit of the IF amplifier 

 has an effective noise performance represented by a noise figure of vSdb , the 

 over-all system noise figure will be found to be 13 db. The reduction in sys- 

 tem performance due to the noise contribution of the input stage of the 

 IF am])lifier here is ai)proximately 2 db. If the system performance must 

 be improved by increasing the power of radiation, the importance of this 

 secondary noise contribution is apparent. 



A comparison of the noise figures of a point-contact crystal converter 

 element and the ()L-2C4() Lighthouse vacuum tube used as an amplifier 

 and as a converter element is given in Fig. 5. At frequencies below lOOO mc 

 there is a definite advantage in employing a \acuum tube as a radio- 

 frequency (Rl*) amplifier preceding the nonlinear clement. 



2.13 1000 MC Radio-Frequency Amplifier Design 



In the design of military radar systems for the lOOO mc operating range 

 the GL-2C4() vacuum tube has been employed rather universally in con- 

 verter circuits.'' The essential design features of this special purpose triode 



""The Lighthouse Tul)e," IC. D. McArthur and IC. F. l^cterson, Rroc. of Natiotuil 

 Electronics Conference, Vol. 1, 1044. 



