How to locate the source of radiated noise in an electrical force measurement switch (PMS)?

When PMS radiated emission (RE) tests exceed limits, quickly and accurately locating the internal noise source is the first step in rectification. Locating tools include near-field probes, spectrum analyzers, and receivers. First, in an anechoic chamber or open area, use a receiver to determine the specific frequency point exceeding the limit. Then, with the PMS powered on, use near-field probes (magnetic and electrical probes) to scan the PCB, cables, and gaps at close range: magnetic probes are sensitive to current loops and can be used to locate the power loop of a switching power supply and clock traces; electrical probes are sensitive to high-voltage points and can be used to locate switching nodes, crystals, etc.

Connect the probes to a spectrum analyzer and observe the signal strength at the exceeding frequency point; the location with the highest strength is the main noise source. Common noise sources include: poorly filtered switching power supplies, high-speed digital clocks, long traces or cables acting as antennas, and leakage through gaps in shielded enclosures. After locating the source, targeted measures can be taken, such as adding local shielding to the noise source, connecting ferrite beads in series on the clock line, and strengthening cable shielding grounding. Systematic location can significantly improve rectification efficiency.