424 REGULATORY CIRCUITS 



4. Adequate AGC bandwidth (fast AGC) to ensure isolation from 

 amplitude fluctuation in the received echo and minimization of closed-loop 

 angle tracking noise in the practical employment of the radar system 

 (particularly including recognition of situations where angle tracking errors 

 will exist). Estimates of the half-power frequency of amplitude noise for 

 some target tracking problems are as high as co = 10 (a gain of +20 db or 

 1-^2^21 = 10 at CO = 20 may be taken as a practical design objective). 



5. Adequate AGC bandwidth to ensure suitable transient response. This 

 requirement is another aspect of system demands consistent with item (4) 

 above. 



6. The phase of the quantity [1 /(I + K2G2(s))] should not vary exces- 

 sively over the range of angle tracking modulation frequencies surrounding 

 the scan frequency to limit crosstalk effects between the azimuth and 

 elevation angle tracking axes. Phase shifts of up to 5° or 10° can be allowed 

 in most systems although some applications may require this phase shift 

 to be maintained less than a few degrees. As will be shown in Paragraph 

 9-9, with phase shifts of more than 10°, the antenna has a tendency to spiral 

 or circle, and with even larger shifts, it will become unstable. This phase 

 shift must often be maintained in the face of uncontrolled variations in the 

 incremental loop gain K2 of as much as 10 db and uncontrolled variations 

 in scanning frequency of up to ±5 per cent. 



7. In order to ensure adequate loop stability and transient response, a 

 minimum gain margin of 6 db should be maintained for all possible varia- 

 tions of the incremental loop gain. Similarly, a phase margin of from 40° to 

 50° should be maintained. 



8. In pulsed systems, it is necessary to provide a minimum gain margin 

 of at least 6 db at one-half the repetition rate to ensure stable operation. 

 This is particularly important in monopulse systems where rapid AGC 

 action is desired. 



8-19 THE MODULATION TRANSMISSION REQUIREMENT 



When large fluctuations in the AGC loop gain are possible or very small 

 scanning frequency phase shifts are required, special design techniques must 

 be used to maintain the phase of the intelligence signal being transmitted 

 through the system. Two such techniques are available. In the first, 

 additional high-frequency lag and lead terms are incorporated into the 

 AGC filter to provide an open-loop phase shift of 180° at and near the 

 lobing frequency. The phase shift of the closed loop is then zero at the 

 lobing frequency and insensitive to variations in loop gain and lobing rate. 

 A second approach is to attenuate K2G2(s) with a null over the required 

 frequency band so that the maximum closed-loop phase </>« will be limited 

 to a small value regardless of the phase of KoGiis). 



