In the paper, the principle is described of measuring magnetic fields with very low levels of magnetic induction, of the order of unities of T. The method described in the paper aims to ascertain the presence of a small amount of ferromagnetic material on a small area. It can, however, be also applied when examining the homogeneity of weak magnetic fields. The most important part of the device proposed here is the detection element, which evaluates the changes in magnetic properties of the surrounding environment, and interacts with the given ferromagnetic material. An integral part of the device is the demodulator, which processes the detected signal, from which only the part carrying useful information is selected. The useful information is deformed by convolution distortion and thus the signal must be reconstructed in an appropriate way. The reconstruction is associated with the decoder, which facilitates obtaining additional information in cases when the results of reconstruction are not unambiguous. A detailed analysis of the functions of detectors working on different principles has shown that our requirements are best satisfied by a simple ferromagnetic probe with a core of small diameter. The detecting part is designed in the form of a pen complemented with an oscillator with a resonance frequency of 300 kHz. The demodulator is implemented in the form of two PLL phase detectors, which are followed by a third-order low-pass filter with a cut-off frequency of 1 kHz. The circuitry terminates in a high-pass filter with a cut-off frequency of 0.34 Hz, which removes the dc component of the signal. The proposed meter will be used to establish the homogeneity of a weak magnetic field in biomedical applications.
