A typical EMI emission test is performed using broadband EMC antennas such as biconical, log periodics & combilogs and horn antennas. These antennas are usually placed at 1, 3 or 10 meter distance as required by the test specification. These are considered far field measurements and the emission limits are given for the specific distance by the specification. During EMI compliance measurements, the emissions levels from products are compared with these limits. If the product exceeds these limits it is considered failing. These tests are typically conducted in an open area test site (OATS) or inside an anechoic chamber.
If the product is failing, because of the higher emissions must be investigated and fixed to make the product compliant. To find the EMI emission source may require making measurement closer to the circuit. This is not possible using typical EMC antennas.
Near field magnetic (H-field) and electric (E-field) probes, aka sniffer probes let you "sniff" around the circuits, cables, and enclosures to find the source. H-field probes use a conductive loop to detect magnetic fields produced by clock signals, serial data streams, control signals, and switching power supplies. You can often uncover the source of EMI emissions with near field probes.
Near field probes are only sensitive to sources at close proximity. They are generally immune to background noise or hand position. Once the emission source has been located, the appropriate fix can be implemented to reduce emissions. Since the near field probe measurements do not provide any indication of compliance, the measurement must be retaken with an EMC antenna at the specified distance.
Near Field Probe measurement do not take into account any other contributors to higher radiated emissions levels at the specified distance. In addition, Near Field Probes can be a valuable tool for troubleshooting EMI problems at the factory. They do not require any special calibrated test facility.
Once you've made your baseline measurements at an EMI test lab to determine failing frequencies, you can take the product back for troubleshooting at the factory. You can experiment with design modifications and repeat near field measurements to reduce the emission levels. Once the emissions levels successfully reduced, the product can be taken back to the EMI test lab for additional compliance measurements. This process will reduce the cost of the overall product compliance process.Features: