We present the results of combining a hardware implementation of an analog comb filter with an ultralow noise electromagnetic sensor. The comb filter is designed to attenuate mains related interference, at either 50 or 60 Hz, and related harmonics. The sensor chosen for this work is an induction magnetometer, but the method is applicable to any low noise high dynamic range sensor. The resultant system, in this case, uses only a single coil, not a gradiometric configuration, thus providing a magnetometer capable of sensing field as opposed to field gradient. This combination of filter and sensor allows additional gain to be added and the full sensitivity of the system to be achieved, previously only realized in an electromagnetically screened room. At the same time, the high dynamic range, low noise performance, and original bandwidth of the sensor are maintained. The technique is illustrated by using the system in an urban environment to observe Schumann resonance phenomena. This approach to acquiring small signals in a noisy environment is compared with conventional analog filter and digital signal processing techniques. (c) 2007 American Institute of Physics.
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