386 THE RADAR RECEIVER 



7-16 GATING CIRCUITS 



Gating circuits are employed to improve the signal-to-noise and signal- 

 to-clutter ratios at the output of the receiver. A gating circuit consists of a 

 modulator to which the signal and the gating signal are applied. In most 

 applications the only output desired is the intermodulation between gating 

 signal and desired signal. To accomplish this, balanced modulators are 

 required. At video frequencies, such circuits are difficult to realize, the 

 dynamic range usually being small. At IF frequencies such circuits are 

 more easily provided, and dynamic ranges greater than 50 db are common. 

 The choice between the IF and video gating depends on the nature of the 

 signals to be encountered by the radar receiver. Typical gating circuits for 

 video and IF applications are shown in Fig. 7-14. Gating circuits are 



Gating 

 Signal 

 IF GATING CIRCUIT 



Fig. 7-14 Typical Gating Circuits. 



employed having gate lengths equal to the range displayed on an indicator 

 and also with lengths equal to or somewhat less than the transmitted pulse. 

 When a dynamic range greater than 50 db is required from a gating 

 circuit, component selection is required. This is a result of uncontrolled 

 cutoff characteristics of vacuum tubes that must be utilized. When gating 

 occurs in the IF amplifier, spurious signals are always encountered. These 

 spurious signals occur because it is difficult to suppress completely all of the 

 modulating signal (gate pulse) at the output of the gater. The gating pulse 



