The electromagnetic spectrum is one of the modern world’s most valuable resources, yet only a minuscule fraction, primarily radio waves representing less than one billionth of one percent, is suitable for wireless communication. The Federal Communications Commission (FCC) meticulously regulates the available bands in that part of the spectrum. Recently, they opened up the Frequency Range 3 (FR3) band, which includes frequencies from about 7 GHz to 24 GHz, for commercial use.
According to Troy Olsson, Associate Professor in Electrical and Systems Engineering (ESE) at Penn Engineering, the new tool could “enable the next generation of wireless communications.”
To date, wireless communications have predominantly used lower-frequency bands. “Right now we work from 600 MHz to 6 GHz,” explained Olsson, who is also the senior author of a new paper in Nature Communications that describes the filter. “That’s 5G, 4G, 3G.”
Wireless devices require different filters for various frequencies, leading to the need for numerous filters to cover all bands, which takes up considerable space. A typical smartphone contains over 100 filters to prevent signals from different bands from interfering with each other.
“The FR3 band is most likely to roll out for 6G or Next G, and right now the performance of small-filter and low-loss switch technologies in those bands is highly limited. Having a filter that could be tunable across those bands means not having to put in another 100+ filters in your phone with many different switches. A filter like the one we created is the most viable path to using the FR3 band,” Olsson said.
Obstacles in Higher-Frequency Band Adoption
Using higher-frequency bands presents a challenge because many frequencies are already reserved for satellites.
Olsson mentions that Elon Musk’s Starlink operates within those bands, and he points out that the military has occupied many lower bands, showing no intention of giving up radar frequencies or their satellite communications within those ranges.
As a result, Olsson’s lab collaborated with Mark Allen, Alfred Fitler Moore Professor in ESE, and Firooz Aflatouni, Associate Professor in ESE, to design the adjustable filter, which allows engineers to selectively filter different frequencies, eliminating the need for multiple separate filters.
“Being tunable is going to be really important because at these higher frequencies you may not always have a dedicated block of spectrum just for commercial use,” Olsson explains.
The Unique Role of the Yttrium Iron Garnet (YIG) Material
The filter’s adjustability comes from a special material called “yttrium iron garnet” (YIG), which is a mix of yttrium, a rare earth metal, along with iron and oxygen. As Olsson further points out, the YIG material is unique because it creates a magnetic spin wave – a type of wave formed in magnetic materials when electrons spin together in a coordinated way.
When exposed to a magnetic field, the magnetic spin wave generated by YIG changes its frequency. This enables the YIG filter to achieve continuous frequency tuning across an extremely broad frequency band through adjustments in the magnetic field.
“By adjusting the magnetic field, the YIG filter achieves continuous frequency tuning across an extremely broad frequency band,” said Xingyu Du, a doctoral student in Olsson’s lab and the first author of the paper.
Along with its tunability, the new filter is remarkably small, approximately the size of a quarter. This is a significant departure from previous generations of YIG filters, which were comparable in size to large packs of index cards.
Discovered in the 1950s, YIG has been used in filters for decades. However, by integrating a novel circuit with extremely thin YIG films micromachined in the Singh Center for Nanotechnology, the new filter achieves significantly reduced power consumption and size. “Our filter is 10 times smaller than current commercial YIG filters,” Du said.
Next, Olsson and Du will showcase the new filter at the 2024 Institute of Electrical and Electronics Engineers (IEEE) Microwave Theory and Techniques Society (MTT-S) International Microwave Symposium in Washington, D.C., next month.
