Design and Calibration of an Ungrounded Double-Loop Active Differential Magnetic Probe

In this paper, an ungrounded double-loop active differential magnetic field probe is proposed. The proposed probe includes two low noise amplifiers and an ungrounded double-loop detection structure, which can ensure a high magnetic field response. The detection structure also includes a shielded ground plate for suppressing electric field coupling and improving the common-mode rejection ratio (CMRR). Meanwhile, a rotate asymmetric calibration method (RACM) is used to correct the probing system’s asymmetry, improve the CMRR, and broaden the measurement frequency range. Therefore, the probing system combines several advanced design ideas, including adding two amplifiers, designing an ungrounded double-loop detection structure with shielded ground plate, using the asymmetric calibration method, and reserving differential output. The calibration data show that the proposed probe has a high magnetic field response factor of -19.8 dB at 2 GHz and a suppressed electric field response factor below -90 dB at 2 GHz. After using the RACM, a CMRR of more than 31.8 dB can be obtained when the frequency is less than 9 GHz. When the frequency is up to 14 GHz, the CMRR is better than 16 dB. The validation results of measuring three stand-wave points prove that the proposed probe and probing system can realize radio frequency magnetic field detection along a trace of up to 14 GHz.