Collin Arocho
9 June 2020

As the digitalized world is flooded with new electronic devices and components, electromagnetic interference (EMI) is becoming a much bigger problem. As part of the public-private collaboration between Axign and the University of Twente, researchers are discovering how to reduce this EMI from audio amplifiers to keep your car running smoothly.

As the world pushes further into the digital era, the growing use of electronics continues to pave the way. Take, for instance, your car, where an increasing number of modern-day features, like eye detection, parking assist, various cameras and radar systems, are more and more common. But as consumer demands and expectations are always on the rise, these features can sometimes intrude on many of the technologies that have long been utilized, forcing the industry experts to continually find innovative solutions to overcome new challenges.

One such obstacle is the interference caused by audio amplifiers in automobiles. As cars start to implement more radio frequency (RF) technology for safety, communications and entertainment, designers are stuck in a balancing act to include all the latest technology, while also preventing the bleed-over of RF signals, which can restrict or corrupt other radio systems. Looking to find new and inventive methods to mitigate this interference, Enschede’s Axign turned to the University of Twente to establish a public-private collaboration on a project called Intelligent Class D Control (ICDC). Chipmaker NXP, Teledyne Dalsa and Bruco also joined to help guide the project. The goal: to create a cost-efficient method to amplify sound, while adhering to strict emissions standards of the automotive industry.

Electronic switches

“Traditional amplifiers use an analog, non-switching design, meaning the power transistors in the output stage are continuously conducting current. This type of amplifier has worked great for many decades, but the power efficiency is really low,” describes Ronan van der Zee, assistant professor of integrated circuit design at the University of Twente (UT). “Switching amplifiers have a much better power efficiency, but they come with disadvantages.” “That’s how the project came to be,” adds Daniel Schinkel, technology advisor at Axign. “We wanted to make switching power converters better, in the sense of lower emissions, which is known to cause big issues for amplifiers.”

Credit: University of Twente/ICD group/Arnoud Rop

Different from analog, these modern switched-mode audio amplifiers utilize transistors that act as electronic switches to push the sound through a speaker – resulting in amplified sound. The issue that both Van der Zee and Schinkel refer to is that in commonly used class D amplifiers, as the signal passes from the sound source to the speaker, it often causes interference with the other radios in the car, ie distortion or even failure of systems like the car’s Bluetooth connection, among many others. “With a class D amplifier, it’s high frequencies that make efficient amplification possible. But with this also comes electromagnetic radiation that can interfere with the car’s other devices,” explains Chris Lokin, PhD student in the UT’s Integrated Circuit Design group. “In my research, I worked to develop a new technique that works as an add-on for these amplifiers and is designed to significantly lower the EMI emissions.”


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Digital domain

Under the guidance of Axign and assistant professor Van der Zee, Lokin worked, as part of his PhD, to design a filter that could sift out the excessive EMI content at the output of the amplifier – the part that feeds into the speaker. “It’s not an actual component that Chris developed, it’s electronic. We’re an integrated circuit design group, so we aim to get everything on a chip,” depicts Van der Zee. “The filter is synthesized in the digital domain and it behaves similarly to a physical component, but it does it all on-chip.”

“Our filter-response model was made specifically for Axign’s chip. We were able to design a filter that can fit inside their current product,” adds Lokin. “Then, by adding one extra component on the outside, we could filter out the high-frequency components from the signal that are known to radiate and cause interference with other devices.”

How exactly will this help society? What is it that consumers will notice?

“Well, first and foremost probably, people will notice that their devices simply work. By reducing the EMI emitted from amplifiers and power converters, it will be less likely to corrupt other nearby components,” says Lokin.

According to Schinkel, another benefit would be for those living in rural areas, with more limited access to radio communications. “It’s well known that the EMI of the high-frequency pulses of class D amplifiers has a direct effect, for instance, on the AM radio band,” he elaborates. “While that’s no longer widely used in more urbanized countries, like the Netherlands, in many places around the world, like in rural America, people still rely on AM. These emissions also have some effects on the FM band, but that’s something we’ll look deeper into during the next phase of the project.”

Credit: University of Twente/ICD group/Arnoud Rop

A factor of 100

Though the newly designed chip add-on hasn’t been through an official testing and measuring environment, which requires a very costly anechoic (echo-free) chamber that adheres to strict guidelines, the ICDC project has yielded strong results. Saving on both cost and time, EMI performance was measured in the lab and translated to real-life automotive testing parameters through simulation models developed by NXP. While the chip hasn’t quite hit its goal to reach the threshold rate of emissions set by regulators, it’s getting very close. To date, the ICDC team has managed to reduce EMI emissions by more than a factor of 100 – from way over the emissions allowance, to just over regulations. Perhaps more notably, however, is that this feat was achieved by using inexpensive, and sometimes suboptimal, components.

“The crux of this project is to see if we could solve this fundamental problem,” depicts Van der Zee. “So far, we’ve been able to do this on a budget and with a timeline that focuses on Chris being able to complete his PhD. Our results are very promising and we believe we can likely achieve even better reductions by further integrating the components on a single chip. We could also utilize more-expensive components, but the whole idea of the project is to be as relevant to industry as possible, which means making a tradeoff between performance and cost.”