What is a Line impedance stabilization network (LISN)?

There are two types of Radio Frequency (RF) emission - Radiated Emission (propagates through the air), and Conducted Emission (propagates along the wire). Conducted RF Emissions are the noise currents generated by the electrical and electronic equipment that propagate through the power cord or power line to other components/systems or power grid. These unwanted emissions affect the other connected electrical/electronic equipment performance.

Fig. 1 LISN in a conducted emission measurement test

Hence, the EMC standards such as CISPR 16-1-2 & MIL-STD-461 have set conducted emission (i.e., disturbance voltages) limits for equipment over different frequency ranges for avoiding interference with other equipment under normal operation. Usually, the conducted emission measurements are carried out over the frequency range of 150 kHz to 30 MHz. In order to measure the conducted emission from a EUT (Equipment-under-test), a special device called LISN (line impedance stabilization network) is used.

A Line Impedance Stabilisation Network (LISN) or artificial main network (AMN) or a V network is an ancillary device used in conducted emission measurement EMC testing. The LISN is basically a low-pass filter used to measure conducted emissions (generated by the EUT) on power lines as per international EMC standards set by the FCC, CISPR/EN/ETSI, and other worldwide standards bodies. 

LISNs are ideal for use in both AC (single & three phase) and DC conducted emission measurement tests and usually support the power line frequency range from DC to 400 Hz. 

Fig. 2 LISN for DC conducted emission test

Functions of a LISN:

Draws power from the ordinary wall power plug and supplies it to the equipment under test (EUT).

Provides stable line impedance (50 Ω) to the EUT during conducted emissions testing, which is independent of the external power line impedance. The stable line impedance greatly improves the test result repeatability & accuracy at different test locations (i.e., different power plugs). 

Acts as a low pass filter. Hence, it blocks RF signal (noise) from entering the EUT via its AC power cord. It also decouples the measuring instruments (e.g., spectrum analyzer/ EMI receiver) from RF signal (noise) from the power line.   

Provides a measurement port with output impedance (50 Ω) for connecting measuring instruments. This port couples the RF noise generated by the EUT to the spectrum analyzer or EMI receiver for measurement or recording.    

In addition to the above functions, a LISN may also offer the following additional functions:

Artificial hand: This function simulates the capacitive effect of the human body on hand-held devices (e.g., electric drill)

High pass filter: A high pass filter in the measurement path helps to suppress the low-frequency signals (e.g., from the switched-mode power supply)

Transient limiter: Protects the measuring instrument from high voltage spikes.

Pre-amplifier: Recovers the insertion loss of the transient limiter.

Remote: Allows remote or automated operation.

Specifications  of LISN:

Frequency: Represents the operating frequency of the LISN. Usually, the operating frequency range is from 150 kHz to 30 MHz. As per EMC standard, it is the frequency range of conducted emission tests. 

Line impedance: Represents the stable line impedance provided by the LISN. It is usually 50 ohms.

Maximum continuous current: Represents the maximum continuous current that can flow through the LISN. It is expressed in Ampere (A).

EMC standards: Represents the ESD test standards to which the LISN complies.

AC voltage: Represents the maximum RMS AC voltage that supports by the LIAN (line to line for 3 phase or line to ground for single-phase). It is expressed in volts (V).

DC voltage: Represents the maximum DC voltage that supports by the LIAN. It is expressed in Volts (V).

Power line frequency: Represents the power line frequency that supports by the LISN. Usually, the LISN supports the frequency range DC to 400 Hz. It is expressed in Hertz (Hz).

Insertion loss: It is the loss due to the insertion of LISN in the measurement circuit. It is expressed in decibels (dB).   

Inductance: Represents the value of inductance present in the LISN network. It is expressed in microhenries (μH).  

Operating temperature range: Represents the operating temperature of the LISN for safe operation. It is represented in degrees Celsius.

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