TekBox DC-LISN-Combo1 Line Impedance Stabilization Network Combo

The TBOH01 5µH LISN is a device required to setup conducted noise measurements of DC-powered devices. It is designed to be used for EMC pre-testing in the frequency range of 150kHz to 110 MHz according to the CISPR- 25 standard.

The LISN is inserted into the supply line of the DUT (Device Under Test). Conducted noise which is present at the supply terminals of the DUT can be measured at the BNC connector using a spectrum analyzer or a measurement receiver. The source (supply) terminal and the DUT terminal are decoupled by a 5µH inductor.

Application

A Line Impedance Stabilization Network basically is a coupling device to measure conducted noise present on supply lines with a measurement receiver or spectrum analyzer. It establishes a defined impedance setup for the measurement, independent of the impedance of the power source. Hence the name Line Impedance Stabilization Network.

A LISN has three terminals – both ends of a feed-through path for DC and a 50 coaxial terminal to connect a measurement receiver or spectrum analyzer.

basic functionality of a LISN

Conducted noise from the Device Under Test (DUT) is coupled from the supply line to a 50 Ohm BNC terminal. Coupling loss from the DUT connector to the BNC connector is less than 1 dB from 3 MHz to 100 MHz. On the other hand, the LISN isolates the DUT and BNC terminals from conducted  noise coming from the source side.

The DUT can be any DC powered electronic device. The Source can be any DC power supply up to 200V max.

Though the LISN is normally used to measure conducted noise from a DC powered device, it could also be used to inject RF to the supply lines of the DUT. This setup requires an additional 50 ? feed-through terminal at the BNC connector. Furthermore the LISN could be used to measure the noise of a source, for example a switched mode power supply or a DC/DC converter output. In this case, the power supply has to be connected to the DUT terminals and a load has to be connected to the Source terminals.

The test configurations shown in the following chapters are just examples and not strictly according to standards. They should rather serve as an easy means to carry out EMC pre-testing in the lab, before going to a certified test house.

Setup dimensions, Limits for spurious levels, bandwidth and detector settings for the measurement receiver or spectrum analyzer have to be derived from the applied standards.

Standard conformant measurements would make use of two LISNs, if the vehicle power return line of the DUT is longer than 200 mm. One in the positive and another one in the negative supply line and then alternatively measure the noise. For a precompliance measurement, measuring the positive supply line gives a good indication on the performance of the device. The measurement can then be repeated with the LISN inserted into the negative supply line to check if the noise levels are any different.

Professional conducted noise measurements are done in shielded chambers, as any RF smog picked up by the wires from LISN to DUT or by the DUT itself, will be present at the BNC terminal. Consequently, a measurement with the DUT powered off should be done upfront in order to distinguish between real conducted noise generated by the DUT and RF smog from other sources (background noise).