Beat Interferences in Fundamental Mode Orthogonal Fluxgates

Fundamental mode orthogonal fluxgates (FM-OFGs) can be made very thin so that one can assemble them in an array to image magnetic field distributions. The dc-biased ac excitation current is fed to the U-shaped wire core of the sensor head; therefore, one can enclose the area by the excitation of the current path, which is made small. Due to the high sensitivity of FM-OFG, tiny but substantial (several hundreds pT) sinusoidal oscillations are visible in the output when two FM-OFGs are placed close to one another. The frequencies of such oscillations are found to be exactly the difference of two ac excitation frequencies independently operated upon the sensor head and higher harmonics of the difference frequency. The interference can be described as a beat of the excitation magnetic fields. We will show that interferences are mainly caused by the inductive coupling of the excitation magnetic field and that, although interferences decay quickly in intensity with increased distance between the sensor heads, they remain at the level of several hundreds pT at 5.8 cm apart. A solution to the problem is to use the same excitation frequency for all the sensor heads, which will suppress beat interferences where they are converted to tiny dc components.