Plug a noisy switch-mode power supply directly into a spectrum analyzer's input and the reading you get depends almost entirely on the building's wiring impedance, which varies lab to lab and outlet to outlet — a useless basis for a compliance limit. The line impedance stabilization network (LISN) solves this by inserting a known, standardised impedance between the EUT and the mains supply.
What a LISN Actually Does
Provides a defined, repeatable impedance at the EUT's power terminals, typically 50Ω∥50μH+5Ω (the "V-network" per CISPR 16-1-2), making measurements comparable between any two labs using compliant LISNs.
Isolates the EUT from incoming mains noise by acting as a low-pass filter, blocking noise already on the mains supply from corrupting the measurement.
Couples the EUT's conducted noise out to the measuring receiver via a dedicated RF output port with defined impedance and frequency response.
Why Inductance Value Is the Key Selection Parameter
50μH LISNs are the standard for commercial and industrial testing under CISPR 16-1-2, reflecting the longer cable runs typical of a building's internal wiring.
5μH LISNs are used for automotive testing under CISPR 25, reflecting a vehicle's much shorter wiring harness. Using a 50μH LISN for automotive testing (or vice versa) produces an impedance profile that doesn't match the standard, making the result invalid regardless of measurement care.
MIL-STD-461 and DO-160 bring their own LISN requirements worth checking directly against the applicable standard.
Single-Phase, Three-Phase, and DC
Single-phase and DC equipment typically needs a pair of LISN networks; three-phase equipment needs three or four depending on configuration. A LISN purchased for single-phase testing won't cover three-phase equipment without additional units.
What Sits Around the LISN in a Complete Test System
Isolation transformer further decouples the setup from mains noise and provides a safety barrier.
EMI receiver with EMI-compliant detectors — quasi-peak, average, and peak detection with correct resolution bandwidths (commonly 9kHz for 150kHz–30MHz) per CISPR 16-1-1. A general-purpose spectrum analyzer without these functions won't produce compliant results. The LISUN EMI-9KC covers 9kHz–1GHz with CISPR 16-1-1 compliant detectors.
Coupling/decoupling networks (CDNs) for signal, control, or data lines beyond the power connection — a frequently missed gap for equipment with multiple port types.
A Practical Selection Checklist
| Question | Why it matters |
|---|---|
| Commercial, automotive, military, or aerospace scope? | Determines 50µH vs 5µH vs MIL-STD/DO-160 requirements |
| Single-phase, three-phase, or DC EUT? | Determines number of LISN networks needed |
| Maximum EUT current draw? | Undersizing causes thermal issues during extended tests |
| Compatible EMI receiver already in the lab? | A LISN's output is only as useful as the receiver measuring it |
For general commercial EMC capability, a 50μH LISN paired with a CISPR 16-1-1 compliant receiver covers the large majority of commercial standards. For labs extending into automotive work, a 5μH automotive LISN is near-mandatory — as covered in our automotive EMC guide, the wrong inductance value produces an invalid result, not just a less accurate one.