Measurement of loop gain in feedback systems†

In this design of a feedback system it is desirable to make experimental measurements of the loop gain as a function of frequency to ensure that the physical system operates as analytically predicted or, if not, to supply information upon which a design correction can be based. In high loop-gain systems it is desirable that the loop-gain measurement be made without opening the loop. This paper discusses practical methods of measuring and interpreting the results for loop gain of the closed-loop system by a voltage injection or a current-injection technique ; extension to the case in which the measurement can be made even though the system is unstable ; and extension to the case in which neither the voltage nor current-injection technique alone is adequate, but in which a combination of both permits the true loop gain to be derived. These techniques have been found useful not only in linear feedback systems but also in describing-function analysis of switching-mode converters and regulators.