Eddy Current NDT instrument - basics

Hello all,

I am trying to make a simple circuit to show the basics of Eddy current testing principle.
I do have experience in firmware programming of various circuits mostly on LPC ARM Cortex M0/M3 MCUs so that part is not a problem - to setup the system to acquire analog data, send it to PC over USB, etc.

Problem is in how to design a circuit to measure on the impedance plane.

My plan is to use a signal generator to generate sine wave of the specific frequency which is being fed to a coil.

Usually for Eddy current testing people use Maxwell-Wien bridge where one coil is test coil (in contact with the target object under test) and another is sitting on the defect free area. Before test, bridge is balanced.

Then I would need to measure voltage changes (magnitude) coming out of the bridge as well as phase difference in the signal from the original signal generator signal.

Another setup which is one of the first and basic for Eddy current testing is to have a resistor and coil in the series fed by sine wave where voltage is being measured on the coil for magnitude and phase changes.

My question would be in how to measure magnitude of voltage changes from various frequencies that can be fed from the signal generator (50khz, 100khz, 300khz, 600khz)?

For phase change (0-360 degrees) I was thinking feeding signal generator to zero cross detector and coil signal also to zero cross detector and measure time difference between two zero crossings. But is there a simpler way to get voltage out and use AD converter for mangitude and phase changes which I can sample from MCU by max speed?

Measuring phase as zero crossing delay is simple but not accurate, affected by noise and circuit non-linearities. Accurate vectorial voltage measurements are usually referring to quadrature demodulators, review the design of high performance LCR meters.

FvM said:

Measuring phase as zero crossing delay is simple but not accurate, affected by noise and circuit non-linearities. Accurate vectorial voltage measurements are usually referring to quadrature demodulators, review the design of high performance LCR meters.

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Thanks for the input. Where could I find LCR meter designs on the Internet as a reference to look at?