Conducted emissions are regulated by the FCC over the frequency range 450 kHz to 30 MHz, and the CISPR 22 conducted emissions limits extend from 150 kHz to 30 MHz. When testing a device for compliance with the FCC and CISPR 22 regulatory limits, a line impedance stabilization network (LISN) must be inserted between the ac power cord of the device under test and the commercial power outlet. Due to the difference in regulated frequency ranges between the FCC and CISPR 22 regulations, the LISNs for the two have similar layouts but the component values are different. A spectrum analyzer is connected to the LISN and measures the conducted emissions from the product under test.
The purpose of conducted emissions testing is to measure noise currents that exit the product under test’s ac power cord and make sure these currents are within the regulated limits. FCC and CISPR 22 regulations require that measured data be correlatable between measurement facilities. Since the currents exiting the device under test are dependent on the load on the ac power cord, and this load is the impedance seen by the device looking into the ac power outlet, which varies considerably over the measurement frequency range from outlet to outlet and from building to building, it is not sufficient to measure the noise currents on the power cord with a current probe. Instead, the product under test is connected to a LISN, which stabilizes the impedance seen by the product looking out the ac power cord. This is the first of the two objectives of the LISN. The second objective of the LISN is to block external noise that exists on the power system net from entering the product’s ac power cord. Any noise currents from the power system net that were to enter the product’ ac power cord would add to the conducted emissions from the device. Since we are only interested in the conducted emissions that signal from the device under test, it is important that the LISN prevent noise from the power system net from entering the product’s ac power cord. The LISN must satisfy both of these objectives over the entire conducted emissions frequency range (450 kHz-30 MHz for FCC regulations and 150 kHz-30 MHz for CISPR 22 regulations), yet still allow the 60 Hz power to reach the device under test.