Patent ID: 7310584

Claim:
A detector for detecting an oscillating magnetic signal produced by a sonde, the detector comprising: a plurality of antennas for detecting electromagnetic signals with a plurality of frequencies comprising the magnetic signal produced by the sonde, each antenna having an analogue output; an analogue to digital converter for sampling the analogue output of the antennas to produce a digital output; and digital processing means, for processing the digital output from the analogue to digital converter to isolate the magnetic signal produced by the sonde, wherein the digital processing means comprises a phase feedback loop to allow the digital processing means to follow variations in the frequency of oscillation of the magnetic signal produced by the sonde, wherein the digital processing means comprises a low-pass filter to isolate the magnetic signal produced by the sonde, wherein the bandwidth of the low-pass filter is less than 0.5 Hz at −140 dB, the detector comprising a frequency spectrum analyzer having frequency bins and an output, wherein the frequency bins of the frequency spectrum analyzer are substantially aligned with a fundamental frequency and a plurality of the harmonic frequencies of the magnetic signal produced by the sonde.