8-18] AGC TRANSFER CHARACTERISTIC DESIGN CONSIDERATIONS 423 



mean square tracking error is plotted versus the lag error for no AGC, a 

 1-cps AGC, and a relatively fast AGC of 12 cps. With small lag errors, 

 less noise is produced with no AGC because of the correlation between 

 amplitude and glint noise noted above. For typical tracking conditions, 

 though, in which appreciable lag errors exist, the tracking noise and no or 

 slow AGC greatly exceeds that associated with fast-AGC designs. 



Other factors to be considered in the design of the low-frequency response 

 of the AGC loop are the transient recovery time of the receiver from deep 

 fades (as great as 60 db) which should be such that the angle error is not 

 blanked for longer than the angle tracking loop response time. This can be 

 achieved by providing a high enough velocity constant for the AGC loop 

 and allowing sufficient dynamic range in the output. 



The amplitude noise spectrum from most aircraft targets falls off with 

 frequency approximately as if it were filtered by a single section, low pass, 

 RC filter (see Paragraph 4-8). In order that no particular noise frequencies 

 be emphasized in the output, it is desirable, although not absolutely 

 necessary, that the AGC filter match this spectrum; that is, it should fall off 

 with frequency with a — 1 slope in the frequency region covering the angle 

 tracking pass band. 



The significant factor in determining the quantitative effects of the AGC 

 on received modulations is the transfer characteristic 



AGC transfer characteristic = :; — ; — j^ ^ , . - (8-15) 



1 -(- KiKj2\S) 



The required behavior of this function and the open-loop characteristic 

 KiGii^s) will be examined in more detail. 



8-18 AGC TRANSFER CHARACTERISTIC DESIGN 

 CONSIDERATIONS 



From the discussion in Paragraphs 8-14 through 8-17 of factors signifi- 

 cant to the design of a radar receiver automatic gain control, several basic 

 specifications emerge as AGC transfer function desiderata in sequential lobe 

 comparison radars, namely: 



1. High gain at low frequencies to provide adequate static regulation. 

 Some system specifications contemplate allowing only ±1 db variation 

 in the modulated envelope output for a range of input signal levels of 

 approximately 100 db. 



2. An initial transfer function slope of zero from dc to as high a frequency 

 (approaching the angle tracking bandwidth) as possible. 



3. Gain drop-off with a —1 slope on a db-versus-log frequency plot to 

 ensure, in view of the established nature of radar noise, that the receiver 

 output shows no noise emphasis at any particular frequency. 



