THE RADAR RECEIVER 739 



2.23 Interstage Circuit Design 



The design of the IF amplifier interstage circuit is basically concerned with 

 achieving the greatest possible gain per stage while not compromising the 

 frequency characteristic and stability, or complicating the structure to an ex- 

 tent where it will be difficult to realize the designed measure of performance 

 under military operating conditions. The problem can be resolved into the 

 choice of the vacuum tube and the selection of an interstage coupling 

 network. 



The gain of a pentode operated into a 2-terminal parallel resonant network 

 which exhibits a maximum impedance Ro at resonance is given by G^Ro 

 under the restriction Ro < < Rp . If a band width AF is defined as that 

 band of frequencies where the magnitude of the impedance Z of the network 



Ro . 



is equal to or greater than ^^ it can be shown that 



The gain-band width product of an amplifier stage is a measure of perform- 

 ance of the stage and serves as a criterion for tube performance if a standard 

 load impedance as above is adopted. Then the band merit Bg for this con- 

 dition will be given by 



B, = AFG„, Ro = r-^ (On. Ro) = ^. 



The band merit Bo of a tube has the dimensions of frequency and may be 

 interpreted as the frequency at which the voltage gain of the vacuum tube is 

 at unity with a plate load impedance restricted solely by the sum of the plate 

 and grid tube capacitances. The ideal IF amplifier vacuum tube will 

 exhibit a high band merit figure, stable operating characteristics over the 

 life of the tube, uniform characteristics during the production period, small 

 size for compact amplifier use and for resulting mechanical rigidity, and 

 finally low-power consumption, desirable from both the power supply stand- 

 point and heat dissipation considerations. The actual types of vacuum 

 tubes generally employed for radar IF amplifiers during the past military 

 program in order of their availability were the 6AC7, 717A, and finally the 

 6AK5. The improvement in performance achieved through this succession 

 of developments can be observed by reference to Table I and the band merit 

 and power consum])tion figures for these tube types. 



Figure 27 illustrates three types of interstage coupling networks which 



have been commonly employed in radar IF amplifiers for military purposes. 



The synchronous single-tuned network design has an advantage of simplicity 



I of construction and permits relatively simple realignment procedures under 



