Ultrasound Circuit: Topological Device Could Cut IoT Interference

The proliferation of IoT technology has made chatterboxes out of everyday hardware and new gadgets too, but it comes with a downside: the more devices sharing the airwaves the more trouble they have communicating. The nearly 30 billion connected devices expected by 2030 will be operating using different wireless standards while sharing the same frequency bands, potentially interfering with one another. To overcome this, researchers in Japan say they have developed a way to shrink the devices that filter out interfering signals. Instead of many individual filters, the technology would combine them onto single chips.

For smartphones to work with different communications standards and in different countries, they need dozens of filters to keep out unwanted signals. But these filters can be expensive and collectively take up a relatively large amount of real estate in the phone. With increasingly crowded electromagnetic spectrum , engineers will have to cram even more filters into phones and other gadgets, meaning further miniaturization will be necessary. Researchers at Japanese telecom NTT and Okayama University say they’ve developed technology that could shrink all those filters down to a single device they describe as an ultrasonic circuit that can steer signals without unintentionally scattering them.

The ultrasonic circuit incorporates filters that are similar to surface acoustic wave (SAW) filters used in smartphones. SAW filters convert an electronic RF signal into a mechanical wave on the surface of a substrate and back again, filtering out particular frequencies in the process. Because the mechanical wave is thousands of times shorter than the RF wave that creates it, SAW filters can be compact.

Today’s filters screen out unwanted RF signals by converting them to ultrasonic signals and back again. New research could lead to a way to integrate many such filters onto a single chip.NTT Corporation

“In the future IoT society, communication bandwidth and methods will increase, so we will need hundreds of ultrasonic filters in smartphones, but we cannot allocate a large area to them,” because the battery, display, processor and other components need room too, says Daiki Hatanaka a senior research scientist in the Nanomechanics Research Group at NTT. “Our technology allows us to confine ultrasound in a very narrow channel on a micrometer scale then guide the signal as we want. Based on this ultrasonic circuit, we can integrate many filters on just one chip.”

Valley Pseudospin-dependent Transport

Guiding ultrasonic waves along a path that changes direction can cause backscattering, degrading the signal quality. To counter this, Hatanaka and colleagues tapped Okayama University’s research into acoustic topological structures. Topology is mathematics concerned with how different shapes can be thought of as equivalent if they satisfy certain conditions—the classic example is a donut and a coffee mug being equivalent because they each have just one hole. But as highlighted by the 2016 Nobel Prize in Physics, it’s also used to explore exotic states of matter including superconductivity.

In their experiments, the researchers in Japan fashioned a waveguide made up of arrays of periodic holes with three-fold rotational symmetry. Where two arrays with holes that were rotated 10 degrees apart from each other met, a topological property called valley pseudospin arises. At this edge, tiny ultrasonic vortexes “pseudospin” in opposite directions, generating a unique ultrasonic wave known as valley pseudospin-dependent transport. This propagates a 0.5 GHz signal in only one direction even if there is a sharp bend in the waveguide, according to NTT. So the signal can’t suffer backscattering.

“The direction of the polarization of the valley states of ultrasound automatically forces it to propagate in only one direction, and backscattering is prohibited,” says Hatanaka. “

NTT says the gigahertz topological circuit is the first of its kind. The research team is now trying to fabricate a waveguide that connects 5 to 10 filters on a single chip. The initial chip will be about 1 square centimeter, but the researchers hope to shrink it to a few hundred square micrometers. In the second stage of research, they will try to dynamically control the ultrasound, amplify the signal, convert its frequency, and integrate these functions into one system.

The company will consider plans for commercialization as the research proceeds over the next two years. If the research becomes a commercial product the impact on future smartphones and IoT systems could be important, says Hatanaka. He estimates that future high-end smartphones could be equipped with up to around 20 ultrasonic circuits.

“We could use the space saved for a better user experience, so by using ultrasonic filters or other analog signal components we can improve the display or battery or other important components for the user experience,” he says.