Detection of weak magnetic fields propagated through a ferrous steel boundary using a super narrowband digital filter

An instrumentation system is described that is capable of launching high frequency magnetic fields through a large mild steel plate 2 mm in thickness, and detecting them on the face opposite to the transmitter with remarkable signal to noise ratios. Results for signal frequencies ranging between 4.5 kHz and 13 kHz are reported. The skin depth at 9 kHz, for the steel used, is approximately 137 μm. The detection of the minute fields arriving at the receiving coil is made possible by the use of digitally synthesized input signals, low-noise amplification, and in particular the use of a real time digital signal processing system that isolates the signal of interest using a super-narrowband IIR filter and very high levels of distortion-free gain. Although traditional methods of weak signal detection, such as lock-in amplification, may also be applied in this context, the digital approach discussed here is both more cost effective and flexible, allowing the simultaneous detection of multiple frequencies.