Conducted Emissions is one of the EMC tests that any electronic device needs to go through. During the test, we check that the new device will not interfere with others through the cables. Cables conduct energy, so there is where the name of the test comes from.
Understanding the Conducted Emissions test is important for any Hardware company, designer or startup, entering the world of electronic design.
Why?
- These emissions can interfere with other devices, provoking integration issues. There is nothing more frustrating than passing all the tests in the laboratory and seeing the device failing when we connect it in the same network as other devices.
- It can fail compliance tests, and delay production timelines. It is not enough to pass functional tests, the amount of noise leaving the cables is limited by the standards.
Very hear often “Test early, test often”. However, it is hard to find trustable resources to learn where to start. Even if we want to test early, there are many incognita to unveil before making the first measurement:
- Which tools do we need
- What can make a test set-up fail
- Do I need an extra piece of Software?
- Is there any safety consideration that I need to keep in mind?
- And so forth
This article describes how to create an effective, budget-friendly test setup for conducted emissions testing. Whether you are a Hardware designer, a technical manager, CTO or a founder, knowing the fundamentals of this set up will save you time, frustration and ensure your devices meet industry standards.
Before we start
This article covers Direct Current (DC) conducted emissions. The set-up and procedure for AC conducted emissions is very similar. The main differences rely on the safety precautions and the need for an Isolation transformer.
Understanding Conducted Emissions
Conducted emissions are currents that unwantedly flow from a device into power lines or other connected systems through communication lines. These emissions can cause interference in other electronic devices, provoking a misfunction or a degradation of the performance. More details on conducted emissions can be found in Understanding Electromagnetic Compatibility Tests: Conducted EMC.
Frequency bands and Standards
Conducted emissions, in industry applications, are measured in the frequency range of 150 kHz to 30 MHz. In some cases, the measurement band can start in 9 kHz, raising the bar for passing the test. Standards such as CISPR 22 – Information technology equipment – Radio disturbance characteristics – Limits and methods of measurement or FCC Part 15 – Radio Frequency Devices define acceptable levels for emissions and provide testing guidelines. For more details on these standards and their importance, explore this beginner’s guide to EMC testing for emissions
Building an Affordable Test Setup
Creating a test bench for conducted emissions testing should not have to break the bank. For this test, you can build an accurate test set-up without needing to sacrifice the performance. The key to not overspend is a careful selection of tools and a precise building.
Essential Equipment
The basic material you will need is:
- Line Impedance Stabilization Network (LISN): Ensures consistent network impedance. It helps building repeatable tests.
- Spectrum analyzer: It measures voltages vs frequency.
- A conductive foil: to build a ground plane to give a return path to the Device Under Test (DUT).
- Copper tape: it provides a very low impedance connection from the different elements to the ground plane.
The price for these components can be from some hundreds to thousands. It is important to focus on what is good enough to perform pre-compliance tests. There are brands like Tekbox or Siglent that sell affordable solutions with an accessible price.
Setting Up the Test Environment
Choose a room with a low-noise room. The perfect place is a shielded area, but it is understandable that you do not have one. Try to use a room that is not close to highly noise tools like motors or drills.
- Over an non-conductive table, place the conductive foil. Make sure that it is well fixed.
- Place the Device Under Test (DUT) over a non-conductive foam of 5 cm height.
- Place the LISN between your power supply and the device under test (DUT)
- Connect the LISN and the power supply to the ground plane through a very low impedance connection (the copper tape).
- Use shielded cables to connect the LISN RF output to the Spectrum Analyzer (SA).
- Maintain a standardized layout, following precise instructions from the device manual.
Testing Procedures for Conducted Emissions
Now that you are equipped, let’s dive into the actual testing. Following a structured and repeatable approach ensures consistent results.
Pre-Test Preparations
Before you press start, ensure:
- The DUT is correctly grounded.
- All cables are securely connected
- External devices near the test area are turned off to minimize interference
Conducting the Test
- Power the DUT through the LISN
- Connect the LISN to the Spectrum Analyzer
- Measure the emissions with the Spectrum Analyzer
- Compare the emission values against regulatory limits
A note on protections
LISN have big capacitors, so when a fast voltage (a power supply) is applied on their inputs, a big current is generated. This current can kill the input of your Spectrum Analyzer, which is not fun at all. Two recommendations
-
- Switch the DUT AND connect the Spectrum Analyzer, following always this order.
- Add a transient limiter to the input of the Spectrum Analyzer, so you can be assured in case you forget to follow the step recommendation #1. A good transient limiter is the Tekbox TBFL1
Conclusion
- Building your own conducted emissions does not need to be crazily expensive. Today, there are many manufacturers selling affordable solutions. Following standardized practices, you can confidently test your Hardware before going to the compliance lab. It will save you testing hours, trips and frustration. On the top of that, your team and company will be upskilled in Radio Frequency measurement techniques and Electromagnetic Compatibility (EMC).