Conducted emission testing and LISN basics

Conducted emission testing measures the high-frequency noise a product sends back into the mains through its power cord. For ordinary mains ports the usual range is 150 kHz–30 MHz. Other product families (lighting, vehicle components, etc.) may use a different range and LISN.

Why a LISN is needed

• Fixed impedance: mains impedance differs from place to place and makes results vary. A LISN presents a defined impedance over the test range (the CISPR 16-1-2 V-network: 50 Ω ∥ (50 µH + 5 Ω), within tolerance).
• Mains noise isolation: it strongly attenuates noise coming from the mains so it does not mix into the result.
• Measurement port: it delivers the noise voltage between each conductor (L, N) and reference ground to a 50 Ω receiver input.

Units and conversion

Mains-port conducted emission limits are in dBµV, per detector (quasi-peak, average). Add the LISN correction factor (voltage division factor, including the built-in attenuator) and the cable loss to the receiver reading before comparing. A spectrum analyzer dBm value becomes dBµV with +107 at 50 Ω.

Cautions for pre-compliance

• Distance to the reference ground plane and mains cable length are specified in the standard. Reproduce the same layout so values agree.
• Terminate unused LISN measurement ports with 50 Ω.
• Run the product in the mode that maximizes emissions (maximum load is not always the worst case) and measure both L and N.
• If the result exceeds the limit, first decide whether the problem is common mode or differential mode; it speeds up countermeasures.

Field notes

• In our experience cable details often affected the result. Check whether the cable shield is grounded at both ends or only one, whether the cable is twisted, and its length.
• Also check the LISN grounding. The LISN base is normally in contact with the ground plane; if units are stacked in layers the ground connection can become weak and influence the result.