An Analog Circuit Technique to Improve a Geophone Frequency Response for Application as Vibration Sensors

Vibration sensors find use in monitoring and measuring vibrations of buildings, bridges and in seismological sciences. Geophones are one of the commonly used sensors for such applications. However, geophones have a natural frequency response like that of a high-pass filter. In the past, several innovations have been introduced to extend the 3 dB corner frequency of the geophone to capture the natural frequencies of a building around 1 Hz. These involved modifying the physical construction of the geophone, and/or introducing digital signal conditioning which is cost intensive. We investigated several analog circuit techniques such that the overall electrical response of the geophone approximates to a low-pass filter response. In one approach the device is followed by a cascade of an ideal integrator and a lossy integrator. This eliminates the zeros of the device while preserving the natural low-frequency pole of the device. It is desirable to create a low -frequency pole independent of the natural pole of the geophone. In order to achieve this goal we used multi-loop feedback method which affords to a low-pass characteristic where the pole frequency becomes different from the natural pole frequency of the geophone. In the following theoretical foundations for two techniques are presented. Validity of the multi-loop feedback technique has been established by numerical simulations and verified by lab-bench experiments.