EMI and RFI testers monitor the presence of unwanted electromagnetic interference (EMI) and radio frequency interference (RFI). EMI and RFI is electromagnetic radiation emitted by electrical circuits which, as a by-product of normal operation, carry quickly-changing signals that cause interference or noise in other circuits. EMI and RFI interrupts, obstructs, and limits the effective performance of these other circuits. EMI and RFI can be induced intentionally, as in electronic warfare, or unintentionally, as a result of spurious emissions and responses. EMI and RFI testers identify interference that could potentially cause a failure, which might interfere with medical equipment or halt a manufacturing process.

EMI and RFI testers use several approaches to test for interference. Typically, an EMI and RFI tester uses a line impedance stabilization network (LISN) that is connected in continuity with the power lines to the equipment. These passive LISN networks establish a consistent impedance to allow repeatability of test results. Conducted emissions are measured by using an RF connection to a port on the LISN. Radiated emissions are measured using an antenna or current-detecting probe. An EMI tester has four operational methods: horizontal high band, horizontal low band, vertical high band and vertical low band. EMI testing generally has a low band range of 30 MHz to 200 MHz and a high band range of 200 MHz to 1 GHz. Conducted RFI is often found in the low frequency range of several kHz to 30 MHz. Radiated RFI is most often found in the frequency range from 10 GHz to 30 MHz. RFI testing uses a frequency range of 30 MHz to 3000 MHz. If the equipment being tested has the potential for radiation above 3000 MHz, the frequency range of the RFI tester can be extended for particular measurements. In this way, EMI and RFI testers can be used to keep in, keep out, or reduce interference levels.

EMI and RFI testers are used in military and medical equipment. Typical sources of conducted interference include switching power supplies, AC motors, and microprocessors. Almost any electrical and electronic device has the potential to generate conducted and radiated interference. The 89/336/EEC electromagnetic compatibility (EMC) directive mandates that all electronic equipment comply with the applicable specifications for EMI and RFI testers. Regulatory agencies also define limits for the maximum leakage current, operating temperature rise, and mechanical design. EMI and RFI tester suppliers also adhere to standards from the U.S. Federal Communications Commission (FCC), the International Electrotechnical Commission (IEC), and the Society of Automotive Engineers (SAE).