372 THE RADAR RECEIVER 



^input tube =15 MMl 

 Cgp = 1 .5 MMf 



AC =1.5 MMf due to space charge effect 



Cout = 3.0 mmF 



Cin of third = 7.0 ^iixi 

 tube 



gn of input = 10~^ mho 

 coil 



Problem Solution. Requirements are that the overall bandwidth 

 between the mixer and third tube in which the attenuation is less than 1 db 

 shall be 3 Mc, the noise figure of the amplifier shall be as low as possible, 

 and a cascode circuit employing two of the tubes having the specified 

 characteristics is to be used. 



For minimum F, the source conductance from Equation 7-31, 



^^^ ^10_2_+_5(10_^ = 6.3 X 10-4 mho is required. 



A source bandwidth (see Equation 7-35) of 

 6.3 X 10-4 

 ^ = .V2 20.5X10- = ^-^^^^ 



is obtained, assuming 1.0 MMf stray capacitance and a wide-band neutraliza- 

 tion of Cgp. 



At 3-Mc bandwidth the input coupling network produces an attenuation 

 (see Equation 7-22) 



10 login fl +[zl??^ =0.15db. 



The network between the second and third tube can therefore exhibit 0.85 

 db attenuation at a bandwidth of 3 Mc. A single-tuned circuit of 6.45-Mc 

 bandwidth is adequate: 



10 log, of 1 + I T^ 1 = 0.85 db. 



'"^"O+felV"- 



Total capacitance between the second and third tube is 3.0 -f 7.0 = 10 



If a step-down transformer is employed, the gain bandwidth product 

 could be improved and the conductance presented to the second tube output 

 is lowered. 



Ine step-down must be n = -\/t^ = \/t7j' 



