Measurements on and with non-linear systems: Problems and approaches

Whenever the relationship between two qualities is anything other than a straight line, time variations of one quality at one set of frequencies will result in variations of the other quantity at a different set of frequencies. Non-linear systems are frequency-creative. This is an inherent property of non-linear systems, and can, in fact, be used as a check on system linearity. This property of linear systems may be desired when new frequencies are to be created, such as in frequency multiplication, division or in modulators of all kinds. This property is undesired and can create havoc in those measuring systems in which any of the following criteria are to be met: (1) Frequency-content reproduction or spectral-content reproduction, for subsequent analysis as signatures, thumbprints, patterns, etc.; (2) Wave-shape reproduction with subsequent analysis of patterns; (3) Peak-to-peak reproduction of wave shapes, as in calibration signals with square waves of injected resistance changes (shunt calibration); (4) Integration of signals; and (5) Differentiation of signals.