Eddy current inspection instrument with noise shaping filter

1. A noise reduction circuit for non-destructive testing (NDT) of a test object with a NDT probe, the circuit comprising:

a direct digital synthesizer (DDS);

a noise shaping filter;

a digital-to-analog converter (DAC);

an analog filter; and

amplifier;

wherein the (DDS) is configured to produce a digital waveform having a periodic excitation frequency and having a number of bits equal to the sum of a first number of bits of the DAC and a second number of bits of the DDS;

wherein the noise shaping filter is configured to produce a digital shaped output having a number of bits equal to the number of bits of the DAC according to the digital waveform and to shape a noise frequency distribution so that a first noise power corresponding to the excitation frequency is smaller than a second noise power corresponding to frequencies greater than the excitation frequency;

wherein the (DAC) is configured to sample the digital shaped output at a sampling frequency and to produce an analog shaped output having a quantization noise power distributed according to the noise frequency distribution;

wherein the analog filter is configured to produce an analog filtered output by removing from the analog shaped output signals having a frequency greater than a filter bandwidth; and

wherein the amplifier is configured to amplify the analog filtered output to produce an amplified analog filtered output.

2. The circuit of claim 1 wherein the NDT probe is an eddy current probe.

3. The circuit of claim 1 wherein the amplified analog filtered output is a drive signal for the NDT probe.

4. The circuit of claim 1 wherein the amplifier comprises a pre-amplifier and a power amplifier.

5. The circuit of claim 1 wherein the digital waveform is a sine waveform.

6. The circuit of claim 1 wherein the DAC is a 12-bit converter and the DDS is a 20-bit synthesizer.

7. The circuit of claim 1 wherein the sampling frequency is greater than twice the excitation frequency.

8. The circuit of claim 1 wherein the sampling frequency is greater than one hundred times the excitation frequency.

9. A method of reducing noise in a circuit for non-destructive testing (NDT) of a test object with a NDT probe, the method comprising the steps of:

producing a digital waveform having a periodic excitation frequency and having a number of bits equal to the sum of a first number of bits of a digital-to-analog converter (DAC) and a second number of bits of a direct digital synthesizer (DDS);

shaping a noise frequency distribution of the digital waveform as a digital shaped output having a number of bits equal to the number of bits of the DAC according to the digital waveform and to shape a noise frequency distribution so that a first noise power corresponding to the excitation frequency is smaller than a second noise power corresponding to frequencies greater than the excitation frequency;

converting the digital shaped output at a sampling frequency to an analog shaped output having a quantization noise power distributed according to the noise frequency distribution;

filtering the analog shaped output to produce an analog filtered output by removing from the analog shaped output signals having a frequency greater than a filter bandwidth; and

amplifying the analog filtered output to produce an amplified analog filtered output.

10. The method of claim 9 wherein the NDT probe is an eddy current probe.

11. The method of claim 9 wherein the amplified analog filtered output is a drive signal for the NDT probe.

12. The method of claim 9 wherein amplifying the analog filtered output to produce an amplified analog filtered output comprises pre-amplification and amplification.

13. The method of claim 9 wherein the digital output waveform is a sine waveform.

14. The method of claim 9 wherein the DAC is a 12-bit converter and the DDS is a 20-bit synthesizer.

15. The method of claim 9 wherein the sampling frequency is greater twice the excitation frequency.

16. The method of claim 9 wherein the sampling frequency is greater than one hundred times the excitation frequency.