Line locator with a metal detector

1. A line locator system, the system comprising:

a digital signal processor in a housing;

a line locator portion mounted in the housing and coupled to the digital signal processor; and

an active metal detector mounted in the housing and coupled to the digital signal processor, the active metal detector comprising:

a first receive antenna mounted in the housing;

a second receive antenna mounted in the housing parallel with the first receive antenna and a distance from the first receive antenna along a direction perpendicular to a plane of the first receive antenna; and

a transmitter antenna mounted between the first receive antenna and the second receive antenna and oriented to provide a magnetic field in a first direction through the first receive antenna and in an opposite direction through the second receive antenna, the magnetic field being of equal strength at the first receive antenna and the second receive antenna;

wherein the digital signal processor is configured to:

receive a signal from the active metal detector;

determine an in-phase signal I and a quadrature signal Q of the received signal;

determine an amplitude A and a phase P of the received signal from the in-phase signal I and the quadrature signal Q;

determine changes in the amplitude A and changes in the phase P; and

generate outputs that indicate proximity to a metal object based on the changes in the amplitude A and the phase P.

2. The system of claim 1 , wherein the first receive antenna and the second receive antenna are electrically coupled and further coupled to a receiver, the receiver including a differential amplifier coupled to provide a signal related to a difference in the magnetic field detected by the first receive antenna and the magnetic field detected by the second receive antenna to the digital signal processor.

3. The system of claim 2 , wherein the magnetic field provided by the transmitter antenna is distorted with a tuner such that an output of the receiver is biased.

4. The system of claim 3 , wherein the tuner includes one or more metallic plates mounted proximate to the transmitter antenna.

5. The system of claim 1 , wherein the transmitter antenna is coupled to a transmitter, the transmitter receiving a sinusoidal signal from the digital signal processor and providing a driving current to the transmitter antenna.

6. The system of claim 5 , wherein the transmitter includes:

a zero crossing detector that receives the sinusoidal signal and produces a first square wave with a same phase as the sinusoidal signal;

a divide-by-2 that produces a second square wave having a frequency that is half the frequency of the first square wave and a third square wave that is complementary to the second square wave; and

a switching bridge driven by the first square wave and the third square wave, the switching bridge coupled to the transmitter antenna.

7. The system of claim 1 , wherein the digital signal processor is further configured to:

generate a sinusoidal wave having frequency 2f that is output to the active metal detector, wherein the received signal has a frequency f.

8. The system of claim 7 , wherein generating outputs includes providing display signals to a display coupled to the digital signal processor.

9. The system of claim 7 , wherein generating outputs includes providing audio signals to an audio output coupled to the digital signal processor.

10. A method of detecting a metal object with a line locator system, comprising:

generating a sinusoidal signal with a digital signal processor;

generating a transmitted magnetic field based on the sinusoidal signal with a transmitter antenna, the transmitter antenna being mounted in a housing;

generating a first signal by detecting the transmitted magnetic field in a first direction with a first antenna, the first antenna being mounted in the housing;

generating a second signal by detecting the transmitted magnetic field in a second direction with a second antenna, the second antenna being mounted in the housing parallel with the first antenna and a distance from the first antenna along a direction perpendicular to a plane of the first antenna;

generating a detection signal based on a difference between the first signal and the second signal;

determining, using the digital signal processor, an in-phase signal I and a quadrature signal Q of the detection signal;

determining, using the digital signal processor, an amplitude A and a phase P of the detection signal from the in-phase signal I and the quadrature signal Q;

determining, using the digital signal processor, changes in the amplitude A and changes in the phase P;

determining, using the digital signal processor, proximity to the metal object based on the changes in the amplitude A and the phase P; and

displaying an indication of proximity to the metal object;

wherein the transmitter antenna is mounted in the housing between the first antenna and the second antenna and oriented to provide the transmitted magnetic field in the first direction through the first antenna and in the second direction through the second antenna, the transmitted magnetic field being of equal strength at the first antenna and the second antenna and the first direction being opposite in direction from the second direction.

11. The method of claim 10 , wherein generating the transmitted magnetic field includes:

receiving the sinusoidal signal from the digital signal processor;

generating a first square wave based on zero-crossings of the sinusoidal signal;

providing a second square wave and a third square wave that is complementary to the second square wave; and

controlling a switching bridge coupled across the transmitter antenna with the second square wave and the third square wave.

12. The method of claim 10 , further including tuning to bias the detection signal.

13. The method of claim 12 , wherein tuning includes moving one or more metallic plates within the transmitted magnetic field generated by the transmitter antenna.

14. An apparatus comprising:

a metal detector coupled to a digital signal processor, the metal detector comprising a housing including:

a first antenna mounted in the housing and configured to produce a first signal in response to detecting a magnetic field in a first direction;

a second antenna mounted in the housing parallel with the first antenna and a distance from the first antenna along a direction perpendicular to a plane of the first antenna, the second antenna configured to produce a second signal in response to detecting a magnetic field in a second direction, the second direction being opposite in direction to the first direction; and

a third antenna configured to produce the magnetic field and mounted between the first antenna and the second antenna, the third antenna being oriented to provide the magnetic field in the first direction through the first antenna and the second direction through the second antenna, the magnetic field being of equal strength at the first antenna and the second antenna; and

a receiver including a differential amplifier configured to receive the first signal and the second signal and output an output signal to the digital signal processor;

wherein the receiver includes:

a zero crossing detector that receives a sinusoidal signal and produces a first square wave with a same phase as the sinusoidal signal;

a divide-by-2 that produces a second square wave having a frequency that is half the frequency of the first square wave and a third square wave that is complementary to the second square wave; and

a switching bridge driven by the second square wave and the third square wave, the switching bridge coupled to the third antenna;

wherein the switching bridge includes:

a first switch and a second switch coupled to a first node, the first node being coupled to a first side of the third antenna;

a third switch and a fourth switch coupled to a second node, the second node being coupled to a second side of the third antenna, the first side being an opposite side of the third antenna from the second side;

wherein the first switch and fourth switch are coupled to receive the third square wave and the second switch and third switch are coupled to receive the second square wave such that current driven through the third antenna is driven in alternate directions.

15. The apparatus of claim 14 , wherein the switching bridge is configured to alternately switch voltage across the third antenna to produce an alternating current.

16. The apparatus of claim 14 , wherein the magnetic field provided by the third antenna is distorted with a tuner such that an output of the receiver is biased.

17. The apparatus of claim 16 , wherein the tuner includes one or more metallic plates mounted proximate to the third antenna.

18. The apparatus of claim 14 , wherein the digital signal processor is configured to:

generate a sinusoidal wave having frequency 2f that is output to the metal detector;

receive a signal from the metal detector having a frequency f;

determine an in-phase signal I and a quadrature signal Q of the signal;

determine an amplitude A and a phase P of the signal from the in-phase signal I and the quadrature signal Q;

determine changes in the amplitude A and changes in the phase P; and

generate outputs that indicate proximity to a metal object.