Envelope transfer function analysis in A-C servo systems

IN THE ANALYSIS of a-c servo systems for stability and performance, attention is centered upon the carrier ¿envelope,¿ i.e., the signal proper. Extension of the convenient transfer function description used in direct-signal (low-pass) servo systems thus requires that the dynamic behavior of each element in the a-c system be described in terms of its effect on the carrier envelope by ¿envelope transfer functions¿ which are functions of signal frequency. In general, three types of elements are encountered in a-c servo systems: type-1 elements in which both input and output are modulated carriers; type-2 elements which have inputs at signal frequencies and outputs which are modulated carriers, and are called ¿modulators¿ and type-3 elements which accept modulated carriers as inputs, produce outputs at signal frequencies, and are called ¿demodulators.¿ Both type-2 and type-3 elements readily lend themselves to simple envelope transfer function description if their dynamic behavior occurs on the signal side. Type-1 elements, however, have known transfer functions in terms of actual frequency, rather than signal frequency, and the envelope or signal transfer function must be derived.