Eddy current tester front end

I have experience with analog and RF design, but none with eddy current testers. I am on a new project which requires a front end for an eddy current crack tester similar to the MIZ-23 and MIZ-27. I have found some technical information and patents, but I want to make sure I don't miss anything. I would be very grateful for any help or pointers.

The transmit coil sends a CW sinusoid into the conductive test material, and a receive coil picks up the EM wave created by the resulting eddy currents. The transmit coil requires a varying amplitude and frequency, but I'm not sure if it is normally driven constant-voltage or constant-current.

The usual receiver seems to be a quadrature direct-conversion circuit, mixing the receive signal with I and Q versions of the transmit carrier. The probe designs I've seen use orthogonal tx and rx coils so the rx picks up as little of the tx as possible, but there will still be a carrier plus close-in sidebands on the rx signal. Can anyone tell me about what to expect for the rx signal? Like signal amplitudes, bandwidth, and how close to the carrier useful rx signals will be? Maybe some particular specs to shoot for, like phase noise and quadrature accuracy in the I and Q references? If rx phase noise is important, tx phase noise should also be. Anything else to watch for?

Many thanks! Gerrit

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Gerrit Barrere
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