Active notch filters for extending frequency response of accelerometers

Abstract

The useable frequency range of lightly damped accelerometers is limited to 0.2 of their mounted natural frequency for amplitude distortions of less than 5 percent, while phase distortion is not a major problem until near resonance. There have been situations where the measured motion contains unforeseen high-frequency components which are distorted due to the accelerometer transfer function. There are several ways to overcome amplitude distortion of the higher-than-anticipated frequency components: (1) change instrumentation to an accelerometer which has a higher natural frequency, (2) establish data-analysis techniques which will account for the amplitude distortion, or (3) set up a notch filter circuit which has a transfer function that is the reciprocal of the accelerometer transfer function. The objective of this paper is to present several such circuits and to discuss what happens when the transfer functions are mismatched as to natural frequency of the accelerometer vs. the center frequency of the filter and accelerometer damping vs. filter damping. The results show that the useful frequency range of the accelerometer can be extended to near resonance if (a) the accelerometer-mounted natural frequency and the filter center frequency are matched within ±2 percent and (b) the damping ratios are matched within a factor of two.