  Efficiency and multisimultaneous-frequency (MSF) output capability are two
major criteria characterizing the performance of a power amplifier in the
application of multifrequency eddy current testing (MECT). Switch-mode power
amplifiers are known to have a very high efficiency, yet they have rarely been
adopted in the instrumental development of MECT. In addition, switch-mode power
amplifiers themselves are lacking in the research literature for MSF
capability. In this article, a Class D power amplifier is designed so as to
address the two issues. An MSF selective harmonic elimination pulsewidth
modulation method is proposed to generate alternating magnetic fields, which
are rich in selected harmonics. A field-programmable-gate-array-based
experimental system has been developed to verify the design. Results show that
the proposed methodology is capable of generating high MSF currents in the
transmitting coil with a low distortion of signal.
