380 THE RADAR RECEIVER 



In the early stages of the receiver, care must be exercised in applying 

 AGC. When a cascode type input amplifier is employed, relatively large 

 voltages may appear at the input grid and also the third tube grid in 

 receivers which must provide target tracking at very short ranges. An 

 AGC voltage is therefore applied to the first tube in these cases. However, 

 in order that the output signal-to-noise ratio of the receiver shall not be 

 seriously degraded, this AGC is usually not applied at the same input level 

 as the AGC on the other gain-controlled stages but is delayed until the 

 input signal-to-noise ratio is about 20 db. The AGC voltage delivered to 

 the cascode is selected so as to minimize the third-order coefficients of the 

 tube transfer characteristic. The effective cascode transfer characteristic is 

 somewhat superior to that of a single tube because of the d-c series connec- 

 tion which allows control of the current of both tubes. Controlling the 

 current of two tubes in a cascode arrangement has the advantage that the 

 stability is not impaired at low gain. When only one tube is controlled, the 

 grounded grid section may become unstable because of the reduced source 

 conductance which drives it. A disadvantage of controlling the current of 

 two tubes exists; not only does the conductance of the output of the first 

 tube decrease, but the input conductance of the grounded grid section also 

 decreases, thus narrowing the intercascode coupling bandwidth. 



Plate and screen grid control for AGC is attractive but reduces the 

 dynamic range of the amplifier stage for large signal input. The operating 

 point can be maintained at a value which minimizes the third-order 

 coefficient, but signal suppression occurs when the signal peaks drive the 

 control grid into cutoff and into grid current. 



Suppressor grid control is very attractive, since the third-order curvature 

 can be minimized without sacrificing dynamic range. One difficulty is that 

 the power dissipation of the screen grid is usually exceeded under strong 

 signal conditions. 



For a high-performance system the AGC voltage will be staggered, i.e., 

 the amount of AGC voltage applied to the various controlled tubes of the 

 amplifier will be different. This is required to obtain minimum envelope 

 distortion. The AGC decoupling circuits must be designed with the 

 precautions noted in Paragraph 7-12. In particular, the transmission of the 

 IF amplifier at low frequencies must not be significant — i.e., it must 

 operate only as a carrier amplifier. 



7-14 PROBLEMS AT HIGH-INPUT POWER LEVELS 



In an airborne radar set strong signals are obtained from short-range 

 targets, clutter, and other radar signals. Two situations occur in the 

 receiver. In one case the receiver may be operating at maximum gain and 

 be required to furnish output from signals having an input power of the 



