Zero Frequency Response from AC Transducers

Piezoelectric and certain other transducers are not suitable as displacement monitors for very-low-frequency phenomena because the sustained signal resulting from a steady input dies away. The gradual disappearance of the generated voltage essentially partially differentiates a signal. Good low-frequency response down to zero frequency is reconstituted by mixing with a signal some of its own integral. The detailed conditions and circuits are given. A relevant theorem on the possible perfection of diodes in integrators is demonstrated. Drift limits response to a few minutes, and static pressure or displacement measurements are presently limited to this period. Experiments on simultaneously cancelling several cascaded time constants, e.g., pneumatic and electric, are cited.