418 REGULATORY CIRCUITS 



In designing an AGC loop, particular attention should be paid to the 

 following four areas of performance which are of primary importance: 



1. The steady-state or d-c regulation of the video voltage 



2. Attenuation of amplitude fluctuations 



3. Fidelity with which intelligence is transmitted 



4. AGC loop stability 



The steady-state regulation determines the degree to which the AGC loop 

 compensates for slow variations in the average signal level caused by 

 changes in target aspect and range. As noted above, such variations can be 

 as great as 100 db. The AGC loop is often required to reduce slow variations 

 of the output level to only a few decibels. For instance, in an angle tracking 

 loop, the loop gain is proportional to the output signal level so that varia- 

 tions in this output produce corresponding variations in loop gain. When 

 the average output varies by 2 to 1, or 6 db, the angle track loop gain will 

 also vary by the same factor, and this may have a serious effect on the 

 overall angle track loop stability and performance. 



Fluctuations in the strength of radar echoes reflected from aircraft 

 targets have been discussed in Paragraph 4-8, and typical spectra of this 

 amplitude noise for two types of aircraft are shown in Figs. 4-23 and 4-24. 

 In Fig. 4-23 the amplitude noise spectrum for a propeller-driven aircraft 

 illuminated by X-band radiation is shown. Very predominant propeller 

 modulation peaks at harmonics of about 60 cps persist to over 300 cps. In 

 Fig. 4-24, the amplitude noise spectra generated by a B-45 jet bomber 

 illuminated by several wavelengths are shown. With no propeller modu- 

 lation, the spectra all fall off within a few cps. In general, it is desirable for 

 the AGC loop to remove amplitude noise whose frequency components fall 

 within the pass band of the angle tracking loop. Otherwise, modulation of 

 an angular lag error by amplitude fluctuations in the receiver output can 

 produce excessive angle noise. 



Besides removing noise modulation from the receiver signal, the AGC 

 loop must also transmit intelligence modulatibn without appreciable 

 distortion. This is a critical problem in systems which employ sequential or 

 conical scan lobing to generate angle error signals. For instance, in a 

 conically scanned system, the angle error is contained in the amplitude and 

 phase of a sinusoidal error signal at the scan frequency which may vary 

 because of poor scan rate generator regulation. Generally, the AGC loop 

 must be able to transmit this signal with negligible phase shift or change in 

 amplitude. 



Since the AGC circuit is a feedback loop, stability questions are im- 

 portant and servomechanism design techniques are applicable. These 

 techniques are applied to a linear small-signal approximation to the 

 nonlinear loop which will be derived in the following paragraph. Adequate 



