Equivalent circuit of a magnetic sensor coil and a simple filter for rejection of 60Hz man-made noise.

Abstract

A magnetic sensor coil is shown to behave as a 2nd order Butterworth low pass filter for induced e. m. f. if the coil's output ends are terminated with a set of parallel resistor and capacitor matched to the sensor. The conditions for matching are theoretically derived, assuming an equivalent circuit of the sensor coil. An experiment to check the Butterworth filter behavior agrees well with the theory. It demonstrates the validity of the assumed equivalent circuit of the sensor. The corner frequency of the filter can be changed, selecting different matched values of the resistor and capacitor. In the experiment, the corner frequency is set at 6Hz which rejects 60Hz man-made noise 40 db ca. without distorting magnetic pulsation signals up to the highest frequency range (Pc 1). This technique for rejection of man-made noise is more advantageous than another common practice, putting a twin-T filter between the sensor and the head amplifier. A twin-T filter used this way is shown to give rise to ringing depending on the characteristics of the sensor, the twin-T filter and the input impedance of the head amplifier.