The smartphones that permeate today’s world wouldn’t be possible without the RF filters found inside of them. And with 5G rapidly approaching, the need for high-frequency filter solutions is only going to intensify. One company that’s making a significant impact within the mobile-communications filter space is Resonant. The firm recently unveiled its new technology, which it believes holds great promise for future RF filters for 5G mobile devices.
A Look at the Company and the Filter Market
Before getting into the announcement, it’s worthwhile to first examine Resonant along with the filter market itself. Founded in May 2012, the firm now has over 60 employees with more than 60 designs under contract from 10 customers. The company’s business model is based on licensing rather than manufacturing.
“We’re a licensing company,” says Mike Eddy, vice president of marketing at Resonant. “We don’t make the filters, multiplexers, etc. We create the designs for our customers and then we license those designs on a per-unit royalty basis. Also, we announced the addition of filter IP library products to our offerings. Library products are designed and developed by Resonant against one of its foundry partner’s processes, tested against the latest industry and phone board requirements, and then made available to license.”
The market for RF filters targeted at mobile communications has grown significantly over the last several years. And with 5G coming, that demand is expected to rise. “The highest-growth element in an RF front end (RFFE) is the filter,” explains Eddy. “The reason why is you need a filter for every frequency band the phone is required to operate at. Not only do you need filters for every band, but you need more filters as you add more antennas to get higher data rates. A power amplifier (PA) can operate at multiple frequencies, so you can have one PA for multiple frequency bands. That’s not the case with filters.”
Of course, filters play a critical role in smartphone RFFE architectures—a role that will only be magnified by 5G requirements. Resonant has developed a technology, known as infinite synthesized networks (ISN), that’s made a significant impact in the RF filter realm (see figure). “We have developed a software platform called ISN, which we use to design complex filters for our customers that go into a mobile handset,” says Eddy. “ISN represents a very different approach compared to the way filters are designed in most cases. What ISN does is bring fundamentals to the design space of a filter.
Resonant’s ISN technology, based on circuit models and physical models, is used to develop filters for mobile handsets.
“The fundamentals we care about are physical dimensions and material properties. Given those fundamentals, we can precisely simulate the performance of our designs using our finite-element analysis. That’s why our customers work with us—what we design is going to be absolutely replicated in the actual measured performance of the filter.”
ISN technology also allows the company to investigate potential new solutions. Eddy adds, “The other part of ISN is that it enables us to start to look at different kinds of structures so that we can come up with structures that are much more applicable to new applications. We’ve been using ISN to look for new structures that will be applicable to 5G filters.
“5G filter requirements involve much higher frequencies because that’s where you have some bandwidth available for use in the mobile market. Higher frequencies mean lower propagation. To get around that, you need higher power. So, you need filters that can handle much higher power.”
Requirements for 5G filters don’t stop there. Eddy continues, “With 5G, the bandwidths will be much larger than the bandwidths currently used by cellular networks at lower frequencies. At lower frequencies, typical bandwidths are approximately 60 to 80 MHz. Now, you’re talking about bandwidths in the 500 to 800 MHz range—and even up to 1 GHz. That means the coupling coefficient of resonating structures must be significantly larger than what’s currently available. Also, you want much higher quality factors (Qs). Q generally drops as a function of frequency, so you want to increase the Q at high frequencies in order to make useful filters that have low loss. Finally, you want to make sure that any kind of spurious modes are eliminated.”
Everything discussed so far leads to Resonant’s recent announcement of its new resonator technology, known as XBAR. “We’ve spent time looking at different kinds of structures using ISN that would meet all the new requirements,” explains Eddy. “We’ve developed a structure called XBAR, which satisfies the requirements for 5G at these high frequencies. XBAR-based designs will first be offered through our library products program.”
Resonant speaks very highly of XBAR, boasting that it’s a “revolutionary acoustic resonator.” So, what’s special about it? Eddy says, “With XBAR, we now have a resonating structure that has the strongest acoustic-wave coupling of any resonating structure on the market right now. It offers three to four times more bandwidth than the 4G resonators used in the mobile space right now. This technology gives us the ability to design filters for very wide radio channels—the bandwidths we’re talking about are in the hundreds of megahertz. We’re also looking at going up to 1 GHz.
“XBAR also offers performance at very high frequencies—much higher than other acoustic-wave filter technologies. It goes beyond 3 to 4 GHz into the millimeter-wave (mmWave) range, i.e., 30 to 300 GHz. The other aspect of this resonating structure is that it can handle very high power. Our ISN toolset can not only model performance with regard to loss and rejection, but it can also model power—which is essential for these very high frequencies.”
Eddy sums it up with these words: “Mobile filters are critical for smartphones now and they’ll be even more critical for 5G. Our ISN design platform has allowed us to invent new structures that can be applied to the new requirements for 5G. Our XBAR simulations currently show that we can achieve high Q along with resonances at very high frequencies with high coupling coefficients to enable large bandwidths. In addition, the simulations show that XBAR can handle high power. We currently have test structures going through fabs that show that our simulations match the measured performance of these resonators. The agreement has been excellent so far.”
Stay tuned for more on Resonant’s XBAR technology.