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Institute for Measurement Technology
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Bachelor Theses.

Investigation of Moving Magnetic Structures by Means of a Faraday Magnetometer Using the Example of Audio Tapes

Jonas Peter Lindenberger

©Jonas Peter Lindenberger

Nowadays, information is often encoded using magnetic structures. Banknotes for instance depend on magnetic patterns to avoid falsification. In the production process the validity of these patterns needs to be guaranteed. Therefore, it is necessary to develop real time measurement procedures.

 

Figure 1: Magnetization of an audio tape of a compact cassette. Figure 1: Magnetization of an audio tape of a compact cassette.

In this thesis dynamic measurements of magnetic structures are discussed. An existing setup of a Faraday Rotation Magnetometer is used in order to analyze the magnetic properties of audio tapes. A cassette player was converted for moving the tape in front of the sensor of the measurement setup.

 

The characteristic of the measuring process is calculated as a function of the measuring parameters. Subsequently, the parameters are chosen in accordance to a desired system response. The acquired data are then processed in order to create a continuous digital audio track with reduced noise. The most important steps of data processing are explained and illustrated with measurement data. Measurements are carried out using two different magneto-optical sensors - both based on the Faraday effect. The results are compared and the differences as well as advantages and disadvantages are outlined. Moreover, the hysteretic behavior of one of the sensors is elaborated in more detail and its impact on the resulting audio signals is discussed. The audio signals are compared to the output of the built-in digitizer of the cassette player both in the time and the frequency domain. Finally, the setup's potential with respect to high tape speeds yet maintaining significant frequency information is debated.

Figure 2: Spectral comparison of a resulting signal sequence (crystal type A) to the reference signal (digitizer). Figure 2: Spectral comparison of a resulting signal sequence (crystal type A) to the reference signal (digitizer).

Keywords: Faraday magnetometer, magnetic structures, magnetic tape, audio signals

July 25, 2021