EMC Symposium: An on-site EMI tester

On August 18 at the EMC Symposium, Thursten Schrader of Physikalisch-Technische Bundesanstalt (PTB), the national standards lab of Germany, presented “On Site EMC Testing and Interference Prevention.” Schrader discussed a weighing system for large trucks, which has electronics. Operators noticed unusual results when suing a cell phone. “EMC test is only performed on the components, but not on the installation.”

Engineers made measurements testing the response of a weigh-bridge around transmitters of 4.5 MHz at 0.5 W. The check indicators as the moved a transmitter closer to the display. With no radio, they measured 0.1 V/m. With the transmitter at 4-m distance, they measured a 20 kg change and a change of up to 60 kg at 1 meter.

Engineers designed an on-site EMC test system. Transmitters in the area ranged from 27 MHz (Citizens Band) up to 5.8 GHz for WLAN. Other frequencies used corresponded to popular transmit frequencies such as 918 MHz for GSM. Modulation for test is AM at 80%. The design goal was to produce 20 V/m at 1-m distance. The tester had to be portable. They used an anechoic chamber used for calibration. The test system is based on a laptop PC.

Next, the engineers had to design an interference prevention system. They designed a 6-channel circuit breaker that responds to field strength to avoid measurement errors in the presence of strong electromagnetic fields. They tested the breaker in an anechoic chamber. The multisensor inputs covers the wide range of frequencies. The circuit breaker ensures against anyone trying to use an intentional transmitter as a way to produce a false reading.