Method and system for producing a magnetic field signal usable for locating an underground object

A method for generating a magnetic filed signal useable for locating an underground object comprising:

• generating a reference signal having a reference signal frequency substantially equal to the integer multiple of 300 Hz;
• receiving an information signal;
• encoding the information signal to produce an encoded information signal;
• modulating the reference signal with the encoded information signal at a predetermined bit rate to produce the drive signal, the drive signal including the carrier component and a lower information sideband and an upper information sideband, each of the lower and upper information sideband including respective sideband energy, the encoding and bit rate causing
• (i.) a substantial portion of the lower sideband energy to be contained between the carrier component frequency and a frequency spaced 50 Hz below the carrier component frequency, and
• (ii.) a substantial portion of the upper sideband energy to be contained between the carrier component frequency and a frequency spaced 50 Hz above the carrier component frequency;
• and
• driving a transponder with the drive signal to generate the magnetic filed signal, the magnetic field signal having a magnetic field carrier component equal to the carrier component frequency.

A system for generating a magnetic filed signal usable for locating an underground object comprising:

• a subsystem to produce a drive signal including a carrier component frequency substantially equal to an integer multiple of 300 Hz;
• a transponder to generate the magnetic field signal when driven by the drive signal, the carrier component equal to the carrier component frequency;
• wherein the subsystem comprises:
• a reference signal generator to produce a reference signal having a reference signal frequency substantially equal to the integer multiple of 300 Hz;
• at least one sensor to produce an information signal;
• an encoder to encode the information signal to produce an encoded information signal;
• a modulator to modulate the reference signal with the encoded information signal at a predetermined bit rate to produce the drive signal, the drive signal including the carrier component and at least one information sideband including sideband energy, the encoder and bit rate causing a substantial portion of the sideband energy to be contained between the carrier component frequency and a frequency spaced 50 Hz from the carrier component frequency;
• and
• a transponder to generate the magnetic filed signal when driven by the drive signal, the magnetic filed signal having a magnetic filed carrier component equal to the carrier component frequency.