How a LISN Provides 50-Ohm Impedance for EMC Testing

This article explains how a Line Impedance Stabilization Network (LISN) establishes a consistent 50-ohm radio frequency (RF) impedance during conducted emissions testing of power electronics. By isolating the equipment under test (EUT) from the unpredictable impedance of the AC or DC power grid, the LISN ensures repeatable and accurate electromagnetic compatibility (EMC) measurements. This overview covers the internal components of a LISN—specifically the inductors, capacitors, and termination resistors—and details how they work together to pass power while routing high-frequency noise to a 50-ohm measurement receiver.

The Problem with Power Grid Impedance

When testing power electronics for conducted emissions, engineers measure the unwanted RF noise the device pushes back into the power lines. However, the impedance of a standard wall outlet or power supply varies significantly depending on frequency, wiring geometry, and other connected loads. Because the amplitude of measured noise depends directly on the impedance of the power source, testing the same device in two different laboratories would yield completely different results. A LISN solves this by placing a predictable, standardized impedance between the power source and the EUT.

Core Components of a LISN

A standard LISN achieves a 50-ohm RF impedance using a specific combination of passive components. While values vary depending on the testing standard (such as CISPR 16 or MIL-STD-461), the fundamental architecture relies on three main elements:

  • Series Inductors (Chokes): Typically ranging from 5 µH to 50 µH, these are placed in series with the power line.
  • Shunt Capacitors (Coupling and Filtering): Placed on both the power source side and the EUT side of the inductor.
  • 50-Ohm Measurement Port: An RF connector (usually BNC or Type-N) connected to the EUT side capacitor, often paired with a built-in 1 kΩ bleed resistor for safety.

Step-by-Step: How the 50-Ohm Impedance is Created

The LISN creates a 50-ohm environment exclusively for high-frequency signals while remaining virtually invisible to the DC or 50/60 Hz AC power required to run the EUT.

1. Blocking the Grid Impedance with an Inductor

The series inductor allows DC or low-frequency AC power to pass through to the EUT with minimal voltage drop. However, an inductor's impedance increases with frequency (\(Z = 2\pi fL\)). At the RF frequencies of interest (typically 150 kHz to 30 MHz), the inductor presents a very high impedance. This effectively blocks the unpredictable impedance of the external power grid from affecting the measurement side of the circuit.

2. Filtering Incoming Grid Noise

On the power source side of the inductor, a large capacitor (typically 1 µF) is connected to the chassis ground. This capacitor acts as a low-impedance path to ground for any RF noise already present on the building's power lines, preventing external noise from contaminating the test results.

3. Coupling the EUT Noise to the 50-Ohm Receiver

On the EUT side of the inductor, a coupling capacitor (commonly 0.1 µF) is placed in series with the RF measurement port. At low power frequencies (50/60 Hz), this capacitor has a very high impedance, blocking dangerous mains voltage from entering the sensitive test equipment.

At RF frequencies, the impedance of this coupling capacitor drops to near zero (\(Z = 1 / 2\pi fC\)). It acts as a short circuit for the high-frequency noise generated by the power electronics.

4. Establishing the 50-Ohm Termination

The RF measurement port of the LISN connects directly to an EMI receiver or spectrum analyzer via a 50-ohm coaxial cable. Because the receiver itself has a built-in 50-ohm input impedance, the RF noise passing through the coupling capacitor sees a direct 50-ohm load to ground.

The Complete RF Equivalent Circuit

At test frequencies (e.g., above 150 kHz), the LISN's internal components behave in a specific way to lock in the 50-ohm value:

  1. The coupling capacitor becomes a short circuit.
  2. The series inductor becomes an open circuit (blocking the power source).
  3. The EMI receiver provides a 50-ohm resistor to ground.

Therefore, when the power electronics under test generate high-frequency switching noise, that noise looks out into the power cord, is blocked by the inductor, passes effortlessly through the coupling capacitor, and terminates directly into the 50-ohm input of the spectrum analyzer.

Note that at the lower end of the frequency range, the inductor is not a perfect open circuit, and the capacitor is not a perfect short. Consequently, the impedance actually follows an impedance curve (for example, rising from a few ohms at low frequencies and stabilizing at 50 ohms around 150 kHz to 30 MHz). Despite this low-frequency curve, the network guarantees that the impedance is strictly defined and identical across all compliant test setups worldwide.