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This paper describes a new method of measuring magnetoelastic sensor resonance frequency. In traditional bell-ring interrogating method, the sensor response decays rapidly if placed in viscous liquid and its resonant frequency is hard to be measured. The new method using a persistent excitation makes the sensor keep vibrating during measuring. To eliminate the interference of excitation signal to output signal, we designed a figure-8 antisymmetric structure dictation coil. The paper describes the working principle and gives out the main function circuit. This new method obtained a good result in a-amylase concentration measurement.