Design Engineer’s Guide to Setting Up Conducted Emissions Testing in Your Own Lab - Part 1

For design engineers developing electronic products, passing EMC tests is one of the most important—and often most challenging—steps toward achieving CE, UKCA or FCC marking. While complying with electrical safety or machinery standards is usually straightforward once the principles are understood, EMC compliance tends to be far less predictable.

EMC test failures are common, and in many teams, they are almost expected. However, this does not need to be the case. Repeat visits to an EMC test lab not only increase costs, but also consume engineering time and delay product launch.

A strong understanding of EMC design principles is an essential first step, but without testing, there is no way to be fully confident that a product will meet regulatory requirements. Although the EMC Directive uses the term "assessment" rather than "testing"—implying that testing is not strictly mandatory—practical testing remains the best way to ensure compliance. This differs from FCC regulations, where formal testing is often required.

Why In-House Testing Was Once Out of Reach

Historically, most design teams lacked the resources to perform EMC testing in-house. The required equipment, such as EMI receivers, LISNs, and shielded rooms, was expensive. Many engineers also lacked EMC test experience, and the calibration process often seemed obscure or inaccessible.

Today, this situation has changed. With the availability of more affordable equipment and an abundance of technical guidance online, it is now possible to perform accurate conducted emissions testing in a standard laboratory environment, particularly for mains-powered commercial products. While radiated emissions and immunity testing still require specialized facilities such as anechoic chambers, conducted emissions and immunity can be handled in-house with appropriate engineering knowledge and a proper setup.

Test Setup Overview

This article explains how to establish a conducted emissions test setup using a bench-top configuration. The goal is to enable engineers to validate product designs before sending them to an accredited lab. The focus here is on commercial mains-powered products. Similar methods apply to industrial products, while military and defence applications require different setups that will be covered in a separate article.

Figure 1 Conducted emission test set up in our own lab

We follow the CISPR 32 recommended setup for tabletop devices, and set up the test, as shown in Figure. Key points include:

1. The use of both horizontal and vertical test ground planes.

2. A table height of exactly 80 cm. The table must be non-conductive. Polyform is ideal; wood is acceptable.

3. The Device Under Test (DUT) must be positioned 40 cm from the vertical ground plane. The mains cable must be exactly 1 metre in length, with 80 cm hanging freely in the air. Any excess cable should be bundled and supported by a non-conductive material.

We recommend use galvanized steel sheets for the ground planes due to their affordability and good conductivity. However, this approach presents a few challenges:

1. It typically requires multiple large sheets to construct both planes.

2. The sheet edges are sharp, so appropriate safety measures such as gloves are needed.

3. Electrical continuity between overlapping sheets must be ensured using fastening points.

Aluminium is another option, but its anodized surface can interfere with conductivity between sheets, making it less ideal.

To resolve these issues, we used a roll-up ground plane from Tekbox. The model we used measures 2.4 metres by 1.5 metres and includes eyelets for vertical mounting. Using two of these panels allowed us to quickly assemble a reliable ground plane system.

LISN and Power Considerations

For mains-powered devices, a 50 microhenry LISN (Line Impedance Stabilization Network) is required. The LISN serves several critical purposes in conducted emissions testing:

1. It provides a defined impedance—typically 50 ohms—within the relevant frequency range for EMC conducted emissions testing, which is from 9 kHz to 30 MHz in this case. This standardization ensures that the test conditions are consistent and comparable.

2. It offers noise decoupling between the DUT and the rest of the mains supply network. This helps ensure that the measured noise originates from the DUT itself and not from other appliances operating in the same building.

3. It functions as a voltage measurement device. When connected to a spectrum analyser (with 50 ohm input impedance), the results can be read directly in dBμV, which is the standard unit for conducted emissions.

The LISN must be bonded to the horizontal test ground plane to establish a consistent EMC measurement reference ground. It must also be earthed to ensure operator safety. This can be seen in Figure 2.

Figure 2 Recommended grounding of LISN, the green and yellow earthing wire ensures safety, the bottom plate of the LISN is directly connected to the test ground for RF measurement

In addition, an isolation transformer is recommended when powering the LISN from the building's mains supply. The LISN tends to draw a significant leakage current, which can trip the RCD (Residual Current Device) in typical building installations.

Measurement Equipment

The output of the LISN is connected to an EMI receiver or spectrum analyser. For this demonstration, we used the Tekbox TBMR-110M EMI receiver. Alternative spectrum analysers from Rigol or Siglent are also viable, but to ensure accurate and repeatable results, they should be used in conjunction with dedicated EMC software such as EMCview. Otherwise, measurement inaccuracies may occur due to the limitations of basic sweep modes in lower-cost analysers.

A transient limiter must be placed at the RF input of the analyser or receiver to protect against spurious low-frequency transients on the mains supply. Without this protection, the receiver input can be damaged—often without the user realising—leading to inaccurate results. A transient limiter from Tekbox is shown in Figure 3.

Figure 3 An EMI receiver is used for the demonstration. A transient limiter is visibly connected to the RF input

DUT and Reference Results

For the test, we used a switched-mode power supply (SMPS) evaluation board from Monolithic Power Systems (MPS). This board was chosen because the manufacturer provides conducted emissions test data, allowing us to cross-validate our results.

Using the Tekbox EMI receiver, we compared our measurements to those published by the manufacturer. The profiles matched closely, with slightly higher emission levels recorded in our setup. This is expected, given that filter tolerances in mass-produced units can vary by ±10 percent. The outcome confirms that our in-house setup delivers results comparable to those from an accredited lab.

Figure 4 Comparison between the manufacturer’s datasheet results and our own conducted emissions measurements

Video Demonstration

A full video demonstration of the test setup and results is available here