How to locate the source of radiated noise from an FTU (Flexible Transmission Unit) in a power system?

Near-field scanning and spectrum analysis were used to locate the radiated noise source. Tools: Near-field probe kit, spectrum analyzer. Steps:

1. Measure the radiated spectrum in an anechoic room or open field to identify the frequency points exceeding the standard;

2. Scan the PCB with a near-field probe to find strong radiating areas at the same frequency points;

3. Focus on checking the clock circuit, switching power supply, and high-speed digital lines.

Rectification after location: Add shielding to the clock circuit; optimize the layout of the switching power supply; add grounding to the high-speed lines.

Through location and rectification, the radiated emissions in the 30MHz-1GHz frequency band are more than 6dB below the limit, meeting the requirements of GB/T 9254 Class B.