LISN
Line Impedance Stabilization Network
Introduction
Line impedance Stabilization network is a device that enables you to power your device while listening to the emissions that the power supply is sending.
In conducted emission measurements, LISN are inserted into the supply lines of the equipment under test and provide an RF output to measure conducted emissions.
In many other test set-ups, LISN are inserted into the EUT(Equipment Under Test) supply lines just for the purpose of creating a defined source (supply) impedance.
As the output impedance of the power supply is hardly ever known and also varies from one source to other, the EMC testing requires a well defined and known setup that is created by placing a LISN in between the source and EUT so that source impedance is no longer a concern and also we can modify the LISN impedance according to test requirements.
Why do we need LISN?
It is perfectly possible to measure the conducted emissions produced by the EUT without using the LISN, but the problem is that the conducted emissions should be universally the same and should not change from one lab to another.
If the physical and electrical test parameters are kept the same throughout the testing cycles of an EUT still the results will not remain same, the reason for that is change in power source, once the source changes the impedance changes and in turn the EMC test results change.
To avoid this confusion we use LISN to make the source impedance known and universally constant.
A LISN can be connected to any power supply and provide the same voltage and current capability as the power supply at the EUT terminals. However, the source impedance of the LISN is specified by EMC standards and enables repeatable conducted emission measurements in any laboratory.
How is LISN implemented?
Below is the impedance curve of a 5uH inductor(image in introduction):
As clear from the frequency impedance cure the impedance of a simple 5uH LISN is 50ohm above 5MHz and decreases almost linearly below 5MHz mark and close to zero for DC.
The LISN impedance must follow the specification independently of the source terminals shorted or open or whatever impedance connected to it. The impedance of the connected power supply does not have any influence on the impedance of the LISN within its specified frequency range.
This is just an example of a LISN. There are several specified variants, but the underlying principle is the same for all.
LISN Variants:
There are three types of LISN when categorized in terms of topology:
V LISN
Delta LISN
T-ISN
The V-LISN measures the disturbance voltage between one supply line and ground (e.g. positive supply line and ground) or two lines and ground (e.g. phase/neutral and ground). The disturbance voltage is the sum of differential mode and common mode disturbance voltage. The V-LISN is by far the most commonly used LISN.
The Delta LISN provides separate outputs for common mode disturbance voltage and differential mode voltage. A pair of LISN connected to a “LISN-Mate” will do the same.
The T-ISN misses the “L” because it is not used for monitoring supply lines, but twisted pairs connected to telecommunication and data transmission lines. It measures common mode disturbance voltage only.
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