Patent ID: 8291750

Claim:
An apparatus for measuring characteristics of a system that can be made to vibrate, comprising: (a) an oscillator adapted to produce a resonator excitation signal having an excitation frequency; (b) a resonator, that resonates in response to said excitation signal, and is adapted to be placed in a medium; (c) a sensor for sensing resonator motion and that produces a sensor signal representative of said motion; (d) timing circuitry adapted to control said excitation and sensing so that said excitation occurs during repeated excitation periods and said sensing occurs during repeated sensing periods that are mutually exclusive to and interleaved with said excitation periods; (e) an amplifier responsive to said sensor signal to produce an amplified sensor signal; (f) a phase detector adapted to measure phase relationship between said resonator excitation signal and said amplified sensor signal; (g) a controller, responsive to said phase detector to adjust said excitation frequency of said excitation signal so as to create a phase lock loop; (h) a DC and low frequency integrator, adapted to receive said amplified signal during periods that are mutually exclusive to and interleaved with said excitation periods and to otherwise have an open circuit input that maintains its latest voltage while open, and having an output; (i) said DC and low frequency integrator being adapted to produce, at its output, a signal that is the negative of an integrated DC and low frequency component of said amplified signal; and (j) wherein said DC offset and low frequency integrator output is connected to said input of said amplifier so that said integrated DC and low frequency component is subtracted from said input to said amplifier.