368 THE RADAR RECEIVER 



where Bi is 3-db bandwidth of one network consisting of n poles. Then as 

 an example for three staggered pairs of one-poles 



Jo (1 + .v^) 



Noise bandwidth ^ Jo (1 + •V)' ^ 1 r (i)r(3 - \) _ 

 3-db bandwidth (2"^- l)'/^ 0.714 r(4)r(3) " ^•^-• 



(7-29) 



7-8 CONSIDERATIONS OF IF PREAMPLIFIER DESIGN 



The IF amplifier is frequently divided into two units, an IF preamplifier, 

 and the main IF amplifier (postamplifier). This arrangement allows the 

 input stages of the IF amplifier to be physically located near the mixer. 

 When long cables are used between the mixer and IF amplifier, bandwidth 

 and noise figure must usually be compromised. Principal considerations 

 in the design of the IF preamplifier are the noise figure, signal-handling 

 capability, selectivity, and gain. 



Triode tubes are almost always required for the first two IF amplifiers. 

 They are used because they exhibit less shot noise^ than pentodes. (Shot 

 noise is the noise resulting from fluctuations of the currents in a vacuum 

 tube.) The input IF amplifier may be used in a grounded-cathode or 

 grounded-grid arrangement. For the ordinary AI radar with broad band 

 mixer, the grounded-cathode amplifier is usually employed for the first 

 tube. To minimize input admittance variation caused by feedback from 

 grid to plate this amplifier stage is neutralized. 



The equivalent circuit representing the sources of noise associated with 

 an IF amplifier stage is shown in Fig. 7-7. From this arrangement of noise 

 generators and signal generator the noise figure as defined by Equation 

 7-4 becomes 



1 + ^" + ^'^ + ''' + '''' + ^L + g. +,. + Kf + Sv. +/ 



= ]+^^ + ^|y, 



(7-30) 



where ^ is the total susceptance appearing at the input terminals. The 

 additional parameters involved are defined in Fig. 7-7. This expression 

 yields the single-stage spol noise figure of an amplifier which is defined 

 as the noise figure at a specific point of input frequency. When the noise 

 figure of a radar receiver is measured, a noise source is generally employed. 

 If the spot noise figure is constant over the bandpass of the overall receiver, 

 then the noise figure that is measured will be independent of bandwidth 



5B. J. Thompson, "Fluctuation Noise in Space Charge Limited Currents at Moderate High 

 Frequencies," RCA Rev. 4, 269 (1940). 



