This section first discusses the LISN set-up for mains-related tests, such as those outlined in CISPR 11, CISPR 14, and CISPR 22. A table-top Device Under Test (DUT) is positioned on a non-conductive support, often a wooden table, located 80 cm above the test ground plane to which the LISN is connected. Subsequently, we examine the conducted emission set-up for DC-powered electric control units (ECUs) as defined in CISPR 25. In this scenario, two DC LISNs are employed, and the DUT is placed on a 5 cm tall insulation support above the ground plane, with a short cable length connecting it to the LISN.

Setting Up a Mains Powered Conducted Emission Test using a LISN

A quick test set-up can be easily achieved as shown in the picture. Following the relevant standard, the DUT is placed on a wooden table with a height of 80 cm above the test ground plane. A 16A-rated LISN is directly bonded to the test ground plane, which is also safety grounded to the earthing point. The LISN must be connected to an isolation transformer to function correctly; otherwise, the leakage current drawn from the capacitors in the LISN could trip the circuit breaker in the building. For mains-powered conducted emission tests, ensuring high voltage safety is the top priority. As mentioned in the previous section, we also need to check if the mains harmonics spurious is high to decide whether we should use a transient protector or not. Some LISNs, like the one shown in the picture, have a built-in transient protector. When using a transient protector, there will be some insertion loss, which need to be compensated in the final test results. A video demonstration can also be found here.

Setting Up a DC Powered Conducted Emission Test for Automotive Applications

In the following demonstration set-up, we follow the CISPR 25 standard for EMC testing. Given that the specifics of an EMC test set-up often hinge on parasitic elements (like stray capacitance, which affects the common mode current return path), the key elements in this arrangement are:

•LISNs ground connection to the test ground plane: I typically use a galvanized steel plate, readily available at the local tool shop. The LISNs need to be bonded firmly to the plate.

•Wiring connection between the device under test (DUT) and the LISNs: The length should be around 20 cm, in accordance with the standard.

•Insulation support: Its height needs to be 5 cm above the test ground plane, as this dictates the parasitic capacitance between the DUT and the ground.

Minimizing the Ambient Noise

To perform the conducted emission test, we require an electromagnetically quiet environment. However, this cannot be easily achieved in a typical working environment. In the R&D workspace, EM interference originates from:

•Noise generated by the benchtop power supply;

•Noise generated by nearby equipment, which radiates out and couples with the cables of the benchtop power supply and the wiring connection between the DUT and the LISNs; or

•Local radio transmitter signals coupled to the wiring.

It is generally considered good practice to install a DC filter between the benchtop power supply and the LISNs. Alternatively, depending on the noise characteristics of the supply, it is often possible to mitigate the noise by applying multiple-turn ferrite cores. During my assistance with clients in setting up tests in their offices, I observed that placing a two-turn ferrite core on the mains input cable to the power supply also significantly aids in noise suppression. This is particularly effective because switched-mode power supplies generate common-mode noise, so addressing noise from both ends of the power supply is advantageous. An even better solution is to use a linear power supply, although they are often heavy, bulky, and have limited power capability.

Suppressing signals measured in the FM band due to local radio transmitters is often impossible without an EMC tent or chamber. However, it is worth noting that the characteristics of the radio signal spectrum in this band are distinctive and can be readily identified.

This video link that demonstrates how to minimize ambient noise.

The Devil Often Lies in the Details

Since, in this case, the test is often performed by electronics design engineers rather than skilled EMC engineers, unnoticed mistakes are often made. One of the most commonly seen mistakes is that the test engineer forgets to terminate the LISNs using the 50ohm termination. This can lead to measurement errors of up to a few dBs.

Another topic I would like to discuss is the 1 µF input capacitor to the LISNs. In certain commercially available LISNs, there’s a switch designed for toggling the 1µF input capacitor. This capacitor proves useful in conducted emission tests but must be switched off when conducting any form of transient test. Failing to do so might result in the capacitor inadvertently shorting the transient.

It’s worth noting that the LISNs featured in this article do not come equipped with the 1µF input capacitor. The manufacturer recommends that users install it themselves for the correct set-up.

Utilizing a Reference Noise Source

It’s always advantageous to employ a reference noise source when assessing noise levels with a spectrum analyser. This enables you to check the set-up’s integrity. In accredited test labs, it’s common practice to verify set-ups with a reference signal source. I came across a Texas Instruments small evaluation board TPS54361EVM-555 with conducted emission test results, and I’ve adopted it as my personal reference source.

A video demonstration can be found here.