The Use of LISN’s When Powering a Unit Under Test


When bringing a unit in for Electromagnetic Compatibility (EMC) testing, one important thing to remember is how the Unit Under Test (UUT) or Equipment Under Test (EUT) is going to be powered. Whether using a power supply to provide it a DC voltage, wall power (60/50 Hz) or airplane power (400 Hz), almost all standards require the power to go through a Line Impedance Stabilization Network (LISN – pronounced lisən).

Image provided by ETS-Lindgren

What is a LISN? 
A Line Impedance Stabilization Network (LISN) is a device called out in most standards to create an impedance match between the power supply and the UUT. The LISN acts as a low-pass filter allowing the power to come through, but rejecting the high frequency noise of the power supply. The LISN also has an RF port for measuring the conducted RF noise on the power lines.

Why is impedance matching important?
If there is a mismatch in impedance along the power line you can get reflective power (as power reaches your unit some of the power is reflected back to the power supply). This can cause a ripple on the voltage line and cause the current to either increase or decrease because of it. It may also lower the voltage at the input of the UUT, so if you set the power supply to 28 VDC as an example you might only see 22 VDC at the UUT. Because of these changes each scan could give a different result. Impedance matching ensures repeatable measurements.

What does a Professional Testing client need to provide?
Normally when powering your unit at Professional Testing, the only thing that the customer will need to provide is the power line from the output of the LISN to the UUT. The power supply is connected to power filters attached to the sides of the EMC testing chambers. This is to ensure that the power coming in to UUT is clean with no external noise on the lines. Power is then connected to the LISN and from there to the UUT.

How long must the power line be?
Depending on what standard you are testing to, there may be a requirement for how long the power line has to be. The required length of the power line can be found in the standard you are testing to, or you can contact Professional Testing and we will be happy to provide the answer for you. The standard will also call out which type of LISN will be required for testing such as 5µH or 50µH.

What connectors are needed?
LISNs can have different ways to connect to them. If you’re unsure what to put on your power leads to connect to the LISN, feel free to contact Professional Testing.

Jonathan-PTI-2563 web 400x500About Jonathan Morris
Born and raised in San Antonio, Texas, Jonathan Morris is a graduate of the University of Texas at San Antonio where he got his B.S. in Electrical Engineering. Jonathan moved to Austin to take a position as Calibration Engineer at Professional Testing in January of 2011 and soon transferred from Calibration to EMC Test Engineer. Jonathan is currently the Lead EMC Engineer for the Avionics/Military/Automotive division of Professional Testing.

About Professional Testing
Professional Testing (EMI), Inc. is a NVLAP accredited full service test laboratory providing EMC, Product Safety, Reliability and Hazardous Location testing and consulting services. Our team of experts can assist you at almost any stage of your product development cycle: from design assistance to beyond the final product test report.

Photo credit and thanks for the LISN image goes to ETS-Lindgren

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