Nuts and Volts - June 2017

Near Field Tools for Debugging

Far field tests can accurately tell whether a product passes or fails as a whole, but cannot pinpoint the source of a problem. Using only the far field test, you can’t isolate problems down to specific components or locations — like too much RF energy “leaking” from an opening in a metal enclosure or a cable radiating too much RF energy. The near field test is a good way to locate such emission sources and is typically performed using a spectrum analyzer and near field probe.

Near field probes are electromagnetic pickups used to capture either the electric (E) or magnetic (H) field at the area of interest.

Manufacturers provide kits of probes that offer the best compromise between size, sensitivity, and frequency range, and you may need all the sizes in your toolkit to solve your problem.

Selection between an H-field or E-field probe may be driven by the location of a signal in your design, or by the nature of its source (voltage or current).

For example, the presence of a metal shield may suppress the E-field, making it necessary to use an H-field probe for the application. Near field probes must be used to pick up the signal near the device under test.

Summary

Most hobbyists don’t need to worry much about FCC rules and compliance testing. That is, until the hobby produces an interesting design with market potential.

Even for small-scale manufacturing, you risk running afoul of FCC rules around electromagnetic emissions without the stamp of approval from an FCC approved test facility.

Pre-compliance testing using low cost USB based spectrum analyzers can give you the confidence that your design will meet FCC requirements and you’ll sail through compliance testing the first time out. NV