Application Note, AN-7
Measuring a Line Impedance Stabilization Network (LISN)

Conducted emissions is a measure of the amount of Electromagnetic Interference (EMI) that is conducted from a unit under test (EUT) to the outside world via the power cables. A Line Impedance Stabilization Network (LISN) is used for conducted emissions testing, and is inserted between the power mains and the EUT.

The LISN is a passive network that serves to isolate the test system with a reference impedance, and provide a measurement point to monitor the conducted emissions. The measurement bandwidth is in the 9Khz to 30Mhz region. A spectrum analyzer or RF noise meter is used to monitor the conducted emissions of the EUT via an RF (BNC) connector to a (test receiver) port on the LISN.

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This application note demonstrates how the SA-100 Vector Network Analyzer can be used to measure and verify the operation of an LISN device for:

- magnitude and phase of the impedance   at the EUT terminal with respect to   ground with the receiver terminated in 50   ohms.

- insertion loss between each EUT terminal   and corresponding receiver port,

- isolation between each mains terminal   and corresponding receiver port,

Figure 1 illustrates a circuit model for a single channel LISN. Most LISN devices have two identical channels; one for power and another for return.

The input (Power Source) is connected to the AC main. Capacitor, C2 and inductor, L1 form a filter to help reject high frequency noise from the main power