Investigation of the Near-Carrier Noise for Strain-Driven ME Laminates by Using Cross-Correlation Techniques

The near-carrier noise around the longitudinal mechanical resonance of a magnetoelectric laminated composite has been investigated. By simultaneously applying a high-frequency electric field across the piezoelectric phase, the sensor response to low-frequency magnetic signals can be shifted around the "carrier" frequency as side band modulation signals. This magnetoelectric response can appear either as an electric charge via piezoelectric-to-piezoelectric (PP) modulation effects or as a magnetic signal via piezoelectric-to-magnetic (PM) modulation effects. These two signals are detected either with a charge preamplifier or with a coil surrounding the sample and the low-frequency sensor response to the applied magnetic field can be recovered by using two independent synchronous detectors. We have designed an experimental setup to observe the direct (passive) low-frequency noise and the noise corresponding to the two above modulations. Noise cross-correlating measurements were also carried out to investigate the origin of the near-carrier noise. No noise coherence was found between the direct low-frequency noise and the noise resulting from either the PP or the PM modulations. However, a noise coherence factor of more than 50% has been found between the signals recovered from the two modulation techniques. A simple model has been used to explain this effect. The magnetoelectric sensor is considered as a nonlinear forced vibration system. Noise sources passing through such a system can be amplified and distributed around the carriers as side band noise where it hampers the equivalent magnetic noise performance. Electronic-thermal noise caused by dielectric dissipations in the piezoelectric phase can be considered as a noise source with a negligible contribution to the total noise floor. Mechanical-thermal low-frequency excess noise is found to be the only intrinsic noise source which is filtered by the nonlinear ME system and is present as an output near-carrier noise which dominates the noise level after the demodulation processes.