The design of automatic-gain-control systems for auto-tracking radar receivers

A survey is presented of the automatic-gain-control requirements for target-tracking radars of the conical-scanning type. A simple analysis of linear closed-loop a.g.c. systems is given, from which the required performance can be related to circuit parameters. Practical considerations regarding methods of controlling i.f. amplifier gain, number and position of controlled stages, and means of extracting the conicalscan modulation are briefly reviewed. The frequency-response characteristics of the a.g.c. loop, as governed by the conflicting requirements of stability, speed of response, and faithful reproduction of the scan modulation, are discussed from the Bode and Nyquist-diagram points of view. A simple graphical method of allowing for the sampling process in a pulsed radar system is given. The design methods are illustrated by a description of the development of an a.g.c. system for a particular naval fire-control radar. This system employs suppressor-grid i.f. amplifier control, the control voltage being developed by a special d.c. amplifier having an exponential characteristic obtained by negative feedback via a series of biased diodes. A filter embodying a twin-T network is employed to achieve maximum speed of response consistentAwith minimum modulation phase shift. Target-tracking trials with this system demonstrated that a significant increase in aiming error can be produced by fast-acting automatic gain control unless the receiver noise level is sufficient to keep the loop closed during deep fades.