A High-Frequency and High Spatial Resolution Probe Design for EMI Prediction

In this paper, a simple and accurate equivalent circuit of the magnetic probe is proposed. Based on the equivalent circuit, a high-frequency probe for near-field measurements is proposed, manufactured, and tested. To increase its working frequency, its parasitic inductance and capacitance are reduced by using a small loop aperture and a tapered transition. The working frequency of the probe is up to 30 GHz. At the same time, sidewall metallization and bottom-vias are used in the probe, which result in a good performance of electric field suppression. The transmission section of the probe is optimized at high frequencies by using via fence and coax-thru-hole vias. The magnetic field probe is built on a four-layered printed circuit board with Rogers substrate. The loop aperture size of the probe is 250 μm × 250 μm, so it has a high spatial resolution and is helpful to measure the detailed magnetic field from a small device. The probe is calibrated by a standard coplanar waveguide with a backside ground (CPWG). Finally, a near-field scanning system with the fabricated probe is set up to test the magnetic field above an unknown CPWG and microstrip line. From the near-field scanning results, it can be concluded that the probe can accurately measure the magnetic field over a wide frequency band.