THE RADAR RECEIVER 751 



design of IF amplifier interstage networks, the video amplifier performance at 

 high frequencies can be improved by the use of more complex 2- and 4-termi- 

 nal interstage networks. In military radar systems, how^ever, the added 

 performance realized is more than offset by the undesirability of the addi- 

 tional circuit elements required and the more complex maintenance problems 

 that arise, so that usually only the simple resistance-coupled interstage 

 design is encountered in radar systems. 



At the low-frequency extreme of the video band the gain performance of 

 the simple video amplifier is related to the product of the series interstage 

 plate-grid coupling capacitance and the input resistance of the following 

 grid circuit. The low-frequency cut-off of a video amplifier is again defined 

 as the frequency where the gain has fallen 3 db over the value at midband 



1 



frecjuencies and is given by/^ = . The highest value of Rg that can 



be employed is related to the grid current characteristics of the vacuum tube 

 chosen. The use of a large value of Cc is undesirable for two reasons. 

 First, the interstage shunt parasitic capacitance increases as a physically 

 larger condenser is employed, which results in poorer high video frequency 

 performance. Second, the use of large coupling capacitances is undesirable 

 from the standpoint of increase in susceptibility to blocking or paralysis in 

 the presence of large signals or enemy jamming. The low-frequency gain 

 response can be improved by certain proportioning of the plate, screen, and 

 cathode by-pass elements also resulting in somewhat less possibility of un- 

 desirable feedback through the common power supply impedance. 



In certain military radar system designs, multistage negative feedback 

 \ideo amplifiers have been employed. Here considerably greater trans- 

 mission band width may be realized with the simple interstage network 

 design and an order of improvement in stability results. The feedback 

 amplifier design in these cases usually involves common cathode feedback 

 impedance between the first and third stages. 



2.32 Gain- Amplitude Considerations 



The use of nonlinear gain versus amplitude characteristics in a video 

 amplifier is a condition peculiar to the radar system and represents a con- 

 siderable departure from established television practice. The factors that 

 indicate the desirability of this treatment of the signal involve the behavior 

 of the amplifier under the extreme range of received radar and jamming 

 signals encountered and the electro-optical characteristics of certain radar 

 indicator cathode-ray tubes. 



Definite amplitude limiting of the video radar signal is commonly included 

 in military radar systems. By introducing amplitude limiting at an early 

 part of the video amplifier, complete amplifier paralysis is avoided when 



