Sensor system with tunable narrow band filter

The sensor system comprises a sensor or amplifier 12 with input 20, output 22, feedback path 16 from the output to the input, and a narrow band filter 14 in the feedback path. The bandpass or bandstop filter is tuned or tunable to either wanted signals or interference signals. The sensor and the filter are arranged so as to alter the relative amplitudes of the wanted signals and the interference signals in order to increase the relative amplitude of the wanted signals and reduce the relative amplitude of the interference signals. The reduction in dynamic range achieved permits the use of 16 bit rather than 24 bit A/D converters with sensitive sensors such as an induction magnetometer with search coil (122, figure 15) or electric potential sensors. The filter may comprise a plurality of filters. The filter may have variable quality factor Q, may comprise a twin T network, and/or be tunable using switched capacitors (figures 7,8). A delay may be included in the feedback path to give a comb filter characteristic.