Signal select in underground line location

1. A transmitter, comprising:

at least one direct digital synthesizer, the direct digital synthesizer producing a composite output signal in response to an input square wave signal, the composite output signal having a first frequency component for a first period followed by a second frequency component for a second period; and

a feedback loop providing a feedback signal coupled with the at least one direct digital synthesizer that adjusts the first and second frequency components of the composite output signal;

wherein the at least one direct digital synthesizer includes a first digital synthesizer that produces an output signal from the input square wave signal and a control phase signal and a second digital synthesizer that provides a sine wave output for generation of the input square wave signal and signals for generating a current phase signal from the feedback signal.

2. The transmitter of claim 1 , wherein the current phase signal is mixed with a dispersion offset signal and integrated to form the control phase signal.

3. The transmitter of claim 1 , further including an amplifier that couples the output signal to a load conductor.

4. The transmitter of claim 1 , wherein an operator can request a fixed current at a defined carrier frequency through a user interface coupled to the feedback loop.

5. The transmitter of claim 1 , wherein the first frequency component is f0*50/51 and the second frequency component is f0*50/49, wherein f0 is a carrier frequency.

6. The transmitter of claim 5 , wherein the composite output signal has 8 cycles at each of the first frequency component and the second frequency component.

7. A transmitter, comprising:

a signal generation module that generates a composite output signal, the composite output signal having a first frequency component for a first period followed by a second frequency component for a second period, the signal generation module including:

a first direct digital synthesizer that provides the composite output signal, the composite output signal being based on a control phase signal and an FSK control signal;

a second direct digital synthesizer that provides a quadrature signal, the quadrature signal being based on the control phase signal and the FSK control signal; and

a third direct digital synthesizer that provides an in-phase signal, the in-phase signal being based on the control phase signal and the FSK control signal;

an output stage, the output stage coupling the composite output signal to a load conductor;

a measurement circuitry that receives a signal from the output stage and phase signals from the signal generation module and providing a feedback signal; and

a microprocessor coupled to the signal generation module and the measurement circuitry, the microprocessor controlling the signal generation module in response to the feedback signal.

8. The transmitter of claim 7 , wherein the output stage includes:

a pulse-wave modulation generator that receives the composite output signal from the signal generation module;

a digital filter that receives a modulated output signal from the pulse-wave modulation generator; and

an output circuit that couples a filtered output signal from the digital filter to the load conductor, the output circuit providing a signal to the measurement circuitry.

9. The transmitter of claim 8 , wherein the measurement circuitry includes:

a measurement amplifier coupled to receive the signal from the output circuit;

a quadrature detector coupled to receive an amplifier signal from the measurement amplifier; and

a measurement signal that provides the feedback signal to the microprocessor based on a quadrature detector signal from the quadrature detector.

10. A method, comprising:

providing, by a first direct digital synthesizer, a composite output signal based on a control phase signal and an input square wave, the composite output signal including a first frequency component for a first period followed by a second frequency component for a second period;

providing, by a second direct digital synthesizer, a sine wave output for generation of the input square wave and for generation of a current phase signal from a feedback signal; and

providing, by a feedback loop coupled to the first direct digital synthesizer and the second direct digital synthesizer, a feedback signal that adjusts the first frequency component and second frequency component of the composite output signal.

11. The method of claim 10 , further comprising mixing the current phase signal with a dispersion offset signal and integrating to form the control phase signal.

12. The method of claim 10 , further comprising amplifying the composite output signal and providing it to a load conductor.

13. The method of claim 10 , wherein providing the feedback signal by the feedback loop comprises:

providing, by the first direct digital synthesizer, the composite output signal to an output stage, the output stage providing a first intermediary feedback signal to a measurement circuitry;

providing, by the measurement circuitry, a second intermediary feedback signal to a microprocessor; and

generating, by the microprocessor, the feedback signal in response to the second intermediary feedback signal.

14. The method of claim 10 , further comprising:

providing the composite output signal to a pulse-wave modulation generator;

modulating the composite output signal, by the pulse-wave modulation generator, and providing a modulated output signal to a digital filter; and

filtering, by the digital filter, the modulated output signal, and providing a filtered output signal.