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.
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.
Keywords: Faraday magnetometer, magnetic structures, magnetic tape, audio signals
July 25, 2021