Resonant’s RF filters designed to improve 5G data rates, bandwidths

Article By : Maurizio Di Paolo Emilio

Resonant's new XBAR resonator technology has been optimized to improve 5G data rates and bandwidths.

The radio frequency (RF) filters embedded in smartphones differentiate the spectrum bands and control data flow to avoid collision and an irregular and slow user experience. Simply put, RF filters allow wireless devices to accept the data signals they need to work correctly, while filtering out unwanted signals. Resonant has created designs for filters that can improve 5G data rates and bandwidths.

In an interview with EE Times, George Holmes, chairman and CEO at Resonant, and Mike Eddy, the company’s VP corporate development, explained how their new XBAR resonator technology has been optimized to create filters for 5G and WiFi networks. “We are working with several RF module and filter providers for 4G, and 5G filters, and recently achieved an important milestone, that will lead to mass production of critical 5G RF filters,” said Holmes.

The proliferation of 4G LTE networks, the deployment of new 5G networks, and the pervasive nature of Wi-Fi are leading to a dramatic increase in the number of RF bands that smartphones and other mobile devices must support. RF filters are not new, and our smartphones won’t work without them. The first smartphones had less than 10 filters because they didn’t have many RF signals. Today — with Wi-Fi, Bluetooth, GPS, and 2G, 3G, 4G, and now 5G — 100+ filters are trying to keep signals coming into the phone from colliding with each other.

5G networks are not quite ready for prime time, however. The challenge being that each 5G band must be isolated using filters to avoid interference that drains battery life, reduces data rates, and causes dropped calls. Today, filter technologies are just not capable of delivering the performance that these new networks promise.

“We are focused on 5G right now because the requirements have changed dramatically from 4G filters. If you look at an iPhone 11, there are almost 100 acoustic wave filters; in an iPhone 11 each frequency band that needs to be processed within the phone needs a filter. As we looked at 5G, much higher frequency, much wider bandwidth, and much more complex, it became clear the market needed a different kind of acoustic wave building block for these filters. So, we developed technology to address this new market for 5G and Wi-Fi bands at five gigahertz and six gigahertz, and ultra-wideband (UWB) at six to eight gigahertz,” said Eddy.

Figure 1: RF Filter Market. RF Front end (FE) growing from 15B – 26B. RF Filter market is growth from 9B to 15B (Source: Resonant). Click the image above to enlarge.

RF filters 
In addition to mobile handsets, the Internet of things (IoT) is a rapidly growing set of connected devices, and they also use RF filters to communicate. At its simplest level, RF filters allow “good” signals to be able to make their way into the PCB tracks and others to be rejected to avoid interference. “Without the help of RF filters, you wouldn’t be able to stream a video on your phone, or even call or text,” Eddy said.

RF filters will also allow surgical robots, working in medical settings to be revolutionized by communicating remotely with the operations center, eliminating any failures in speed or performance that could prove catastrophic. RF filters manage the entire smart home to be controlled, from turning on the lights to starting the vacuum cleaner, and they are being incorporated into modern electric vehicles, aiding autonomous driving technology.

Early cell phones used ceramic monoblock filters with very low insertion loss, but essentially these phones needed relatively few filters compared to today’s phones. Ceramic one-piece filters are now limited in modern phones due to their large size and high cost. Modern cell phone RF architectures and the explosion of smartphone use have been made possible by the development of acoustic wave resonators based on the piezoelectric effect.

Cellular radios are operating in multiple frequency bands using multiple transmit and receive chains, each with its own set of amplifiers, switches, and filters. Each signal chain relies on a series of filters to eliminate unwanted interference. Almost all of these filters are piezoelectric devices, fabricated using photolithographic processes to create a surface acoustic wave (SAW), a bulk acoustic wave (BAW), or an acoustic resonator (AR) structure. SAWs and their temperature-controlled counterparts have been favored due to their low cost. However, their high signal losses at higher frequencies pose a serious problem, as it is difficult to operate much above 2.5 GHz.

“We realized that these types of acoustic filters had difficulty with high frequencies and bandwidths. So, we came up with a structure we call XBAR, where we’re using metal fingers on top of a thin film of single-crystal, lithium niobate, to set up a bulk acoustic wave within that piezoelectric.  It’s a very different structure, it kind of looks like a surface acoustic wave structure, but it’s actually a bulk acoustic wave. And it’s perfectly optimized for high frequency, wide bandwidth and high power. Sorry, that’s a long-winded explanation,” said Eddy.

Figure 2: These acoustic wave filter technologies are manufactured using photolithographic processes (Source: EDN).

To design a filter, multiple resonators must be coupled together to form a passband. The first parameter to consider is the bandwidth related to the acoustic wave resonator’s key parameter, namely the coupling coefficient. Other parameters include the frequencies of operation, losses, and power levels. Low loss maximizes signal efficiency which extends battery life. All of these parameters are a function of the material, design and manufacturing process (Figures 1 and 2).

RF Filter for 5G
XBAR resonators consist of a single crystal, piezoelectric layer, with a metal interdigital transducer (IDT) on the top surface. The metal traces excite a bulk acoustic wave within the piezoelectric layer, the primary frequency and coupling characteristics being determined by the physical dimensions and properties of the piezoelectric.

XBAR devices propagate the signal through the bulk of the piezoelectric material, rather than along its surface, thus offering low insertion losses at high frequency and wide bandwidth, suitable for 5G. “XBAR is Resonant’s BAW resonator structure developed using our design software platform, ISN. It is manufactured using standard SAW processes, with higher native operating frequency (3-7 GHz) and 4x wider operating bandwidth, up to 24%,” said Holmes.

He added, “We use mathematical models to rapidly design and simulate filters, specifically against the targeted foundry’s capabilities, which yields fewer turns through the fab. These simulations are based on thousands of variations that are contemplated against the target specifications yielding better results. As a result, few engineers are needed to do the same work with fewer turns, making this process more cost-efficient.”

5G cell phones depend on high bandwidth to have high data rates, and therefore much larger portions of the spectrum are needed. So, 5G has new spectrum allocations that are much larger and at higher frequencies than 4G, requiring hundreds of megahertz of spectrum and frequencies above 3 GHz – instead of the tens of megahertz of spectrum around 2 GHz.

Resonant highlighted as XBAR fits well with 5G and, as users grow, interference problems will occur and RF filters will play their essential role in optimizing 5G transmission. “Filters developed using XBAR have the features needed to maximize efficiency, including bandwidth up to 1,200MHz, support for frequencies higher than 3GHz, and low loss,” said Eddy. In addition, low-cost manufacturing techniques, leveraging existing processes will be an important aspect for all RF filter manufacturers as 5G becomes more prevalent.

This article was originally published on EE Times.

Maurizio Di Paolo Emilio holds a Ph.D. in Physics and is a telecommunication engineer and journalist. He has worked on various international projects in the field of gravitational wave research. He collaborates with research institutions to design data acquisition and control systems for space applications. He is the author of several books published by Springer, as well as numerous scientific and technical publications on electronics design.

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