Troubleshooting EMI with Close-Field Magnetic Probes

Close-field magnetic probes act as miniature loop antennas, coupling to the magnetic fields generated by high-frequency currents on a target PCB. High-frequency current passing through a trace creates an associated magnetic field proportional to the current. By placing the probe loop close to the trace, a voltage is induced across the probe terminals, which can be measured with a spectrum analyzer, EMI receiver, or oscilloscope. Passive probes, such as the 11940A and 11941A, require no external power supply. They rely on a dual-loop sensor design to provide high rejection of electric fields. This balanced dual-loop configuration ensures that common-mode E-field noise is canceled out, permitting highly selective measurements of only the magnetic field component. This is critical for accurate troubleshooting because strong electric fields can otherwise mask the magnetic field emissions under investigation.

When conducting an EMI sweep, the operator moves the H-field probe systematically across the PCB surface while monitoring the frequency spectrum. The orientation of the probe is important: magnetic field coupling is maximum when the magnetic flux passes directly through the probe loop. For a microstrip trace, this occurs when the loop is held parallel to the trace. If the probe is rotated 90 degrees, the coupling drops to a minimum. This directional property allows designers to trace current paths, locate ground plane splits, identify noisy integrated circuits, and find poorly shielded cables. Low-frequency troubleshooting from 9 kHz to 30 MHz is best performed with a probe optimized for that band, like the 11941A, while high-frequency troubleshooting from 30 MHz to 1 GHz is handled by a probe like the 11940A. Both probes are combined in the 11945A close-field probe set to cover the full spectrum of interest.

While many near-field sweeps are purely qualitative (looking for relative 'hot spots' on a board), quantitative measurements are sometimes necessary to estimate whether a board will pass compliance. Precision probes are individually calibrated with their antenna factors plotted directly on the probe handle or body. This allows engineers to convert the voltage displayed on a spectrum analyzer into absolute magnetic field strength (in microteslas or amperes per meter) with a typical accuracy of ±2 dB. These measurements must be conducted within safe operating limits, such as keeping continuous input RF power below 0.5 W and avoiding environments where electric fields exceed safe thresholds, like 1000 V/m for the 11940A.