LISN-mate for a dual channel DC LISN

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LISN-mate for a dual channel DC LISN


This design is for a “LISN mate”, suitable for a dual channel LISN. In particular, it was designed for the dual DC LISN published in Elektor (part 1 in the Sept./Oct. 2021 issue, part 2 in the Nov./Dec. 2021 issue), and which is available from the “elektorstore”. as a kit.

A LISN lets you test the conducted emissions in power cables. If the emissions exceed the limit, and you need to adjust the design to reduce the RF noise, the first thing you will want to know is, whether the noise is “common mode” or “differential mode”. A LISN on its own does not tell you that, because it only measures the total RF noise. A LISN-mate separates common mode noise from differential mode, and feeds each into a different output.

When you want a quantitative value for the common mode noise and/or the differential mode noise, a LISN-mate is actually somewhat involved. However, for the qualitative answer of whether emissions are primarily either common more or differential mode, all it takes is a transformer (suitable for the frequency range).

The design is in KiCAD. Gerber files for the PCB are provided as well. The components are:

  Number     Part number     Manufacturer     Description  
  1     ADT1-6T     Mini-Circuits     RF Transformer 50 Ohm, 30 kHz - 125 MHz  
  4     132289     Amphenol     SMA receptable, edge-mount  
  1     4607     Adafruit     copper foil, 0.1 mm thick  

The case is 3D printed; an STL file is provided. The design is in OpenSCAD (and the design source file is provided as well). Since the case is in plastic, and therefore not shielded, I added a shield from copper foil over the entire top of the PCB. It is soldered onto the SMA connectors. (The bottom of the PCB is an uninterrupted ground plane over the entire surface.)

Using the LISN-mate

The LISN-mate sits between the LISN and the spectrum analyzer. On one side of the LISN-mate are two SMA connectors that are marked “IN” (on the PCB). These must be connected to the outputs of the LISN. Both channels of the LISN must always be connected to the inputs of the LISN-mate —and hence, you need a dual-channel LISN.

The other side of the LISN-mate has two SMA connectors marked “CM” and “DM” (for “common mode” and “differential mode”). These are the outputs of the LISN-mate. If you have a dual channel spectrum analyzer, you can connect both outputs to the spectrum analyzer at once (which will give you two graphs in sync). When you have a single channel spectrum analyzer, you can only connect either the “common mode” or the “differential mode” output to the spectrum analyzer. The other output of the LISN-mate must then be terminated with a 50Ω load.

In brief, no input and no output of the LISN-mate should ever be left floating.

The flaw in this simple design of this LISN-mate is that the impedances of the “CM” and “DM” outputs deviate from the standardized values. As a result, the absolute levels of “common mode” and “differential mode” noise (what you'll read from a spectrum analyzer), are not normative. On the other hand, the FCC and CE directives specify only a limit on the total conducted RF noise —there is no norm for common mode versus differential mode separately. So, in my opinion, this is somewhat of a moot point: you will have to perform the pre-compliance test without LISN-mate anyway.

The point of the LISN-mate is that if you need to reduce conducted emissions, this requires different actions for common mode versus differential mode. And therefore, you will want to know whether the (total) RF noise that you measured is primarily common mode or primarily differential mode.


The LISN-mate is distributed under the CC BY-NC-SA 4.0 license (Attribution-NonCommercial-ShareAlike 4.0 International). All files are available from the GitHub project (see the link below).

For the 3D printed case, this project uses the Chamfered primitives for OpenSCAD v1.2 by TimeWaster, which is published under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 licence.

  GitHub project  
  3D printed case on