A two‐turn loop active magnetic field probe design for high sensitivity near‐field measurement

Abstract

This paper develops an active magnetic near field probe (H-field probe) by using a fourlayer printed circuit board (PCB) technique. Two-turn detection structure and a low noise amplifier are used to improve probe's frequency response. The two-turn detection structure can maximize the use of PCB stack resources, and a 14.5 dB gain amplifier can increase signal output capability. The probe can then be used for electromagnetic compatibility (EMC) test from 150 kHz to 3 GHz. Compared with traditional active shielded-loop H-field probe (TAHP) with loop area of 500 𝜇m 2 , the frequency response of the proposed probe can improve 20 dB at 0.5 GHz. Different from the method of comparing the sensitivity of the probe only through the frequency response, this paper measures and analyzes the real sensitivity of the probing device composed of the proposed probe. Under the condition of the same receiver parameter setting, the sensitivity at 0.5 GHz is increased by 11.7 dB compared with commercial passive probing device and 20.2 dB at 0.5 GHz compared with the probing device with TAHP device. It reaches −32.4 dB µA. In addition, the proposed probe has acceptable spatial resolution of 1.28 mm at 1 GHz, which is more suitable for printed circuit board level EMI analysis.

Design and characterisation of a dual probe with double‐loop structure for simultaneous near‐field measurement

“…Sensitivity is an important parameter of the near-field probe. It is used to estimate the minimum electromagnetic component that the near-field probe can measure [10,34]. The sensitivity of the near-field probe is affected by the noise floor of the receiver and the structure of the near-field probe.…”

“…The equations of equivalent current source and equivalent voltage source are as follows, $$${U}_{m}=N\mathrm{j}\omega \int B\mathrm{d}S,$$$ where N is the number of the loop, B is the magnetic flux density through the loop and S is the area of the detection loop. The output voltage of magnetic field detected by the dual probe is [34], $$${U}_{om}=\frac{{\mathrm{Z}}_{\mathrm{i}\mathrm{n}}}{\mathrm{j}\omega {L}_{s}/2+{\mathrm{Z}}_{\mathrm{i}\mathrm{n}}}{U}_{m},$$$ where Z in is the internal impedance of the instrument and L s is the parasitic inductance in the probe. It can be seen from the formula that the frequency response of the probe is higher than that of the probe with single‐loop structure due to the structural characteristics of two‐turn loop.…”

“…When the resolution bandwidth (RBW) is constant, the measurement noise floors of different passive probes are the same. And the noise floor of the active probe is increased by the low noise amplifier [34]. In view of this, the characterisation method of the sensitivity of the dual probe is proposed as follows.…”

“…The equations of equivalent current source and equivalent voltage source are as follows, where N is the number of the loop, B is the magnetic flux density through the loop and S is the area of the detection loop. The output voltage of magnetic field detected by the dual probe is [34],…”

“…Half of the distance from the maximum to 6 dB decrease is regarded as the spatial resolution of the probe, which is often used for comparison of different probes [32,33]. The third is sensitivity, if the measurement data of the near-field probe is 6 dB larger than the floor noise, the measurement result is considered to be reliable, which is used to characterise the minimum field component that the near-field probe can detect [10,32,34]. The fourth is isolation of probe, the isolation is used to evaluate the interaction between the output of the electric field component and the magnetic field component of the dual probe.…”