When measuring small magnetic fields as they appear in medical or biological applications, both, very small and very large signal amplitudes are observed at the same time. Small amplitudes stem, e.g., from human sources such as the human heart (magnetocardiography) or brain (magnetoencephalography). Low frequency signals (0.5 to 40 Hz) can be measured here with peak amplitudes of about 100 pT (heart) or even less than 1 pT (brain). Superposed to these small signals are usually large signal components that stem either from artificial sources, such as excitation signals utilized for modulation techniques, or from natural sources such as the magnetic field of the earth. Creating digital signals that can be used for detailed (medical) analyses is an interesting challenge for both, material scientists and engineers. Methods for improving the signal quality (mainly in terms of signal-to-noise ratio) can be grouped into analog and digital approaches, indicating whether they are performed prior or after the analog-to-digital (AD) conversion.
After the AD conversion so-called reference sensors can be used for recording signals that show a strong correlation with the disturbing but not with the desired signal component. Again, adaptive filtering techniques can be used to enhance the signal quality. However, such approaches are only successful if the analog amplifier followed by the AD converter are not saturated. Furthermore, often magnetoelectic sensors can be read out in a multitude of modes. This allows for adaptive combination of the individual signals, leading to improved robustness and better signal-to-noise ratio. In addition, several sensors can be combined and postprocessing such as digital noise suppression can be applied finally.
As a consequence, more “ingredients” than just the sensor are required for an entire sensor system. This leads to very interesting multidisciplinary research approaches. From sensors to sensor systems: it’s a rocky road.
Short CV of Gerhard Schmidt:
Gerhard Schmidt received the Dipl.Ing. degree in 1996 and the Dr.Ing. degree in 2001, both from Darmstadt, University of Technology, Germany. After his Ph.D., he worked in the research groups of the acoustic signal processing departments at Harman/Becker Automotive Systems and at SVOX, both in Ulm, Germany. Parallel to his time at SVOX he was a part-time professor at Darmstadt, University of Technology. Since 2010 he has been a full professor at Kiel University, Germany. His main research interests include adaptive methods for audio, SONAR, and medical signal processing.
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