RF filters allow for different spectrum bands and improve the user experience by avoiding collision and controlling data flow in smartphones. Resonance filters are designed to optimize 5G data rates and bandwidth. In an interview with EE Times, George Holmes, Chairman and CEO of Resonant, and Mike Eddy, the company’s Vice President of Corporate Development, explained how the new XBAR resonator technology has been optimized to create filters for 5G and WiFi networks. “We are working with several providers of RF filter and modules for 4G and 5G filters, and recently we have achieved a significant milestone that will lead to mass production of the important 5G RF filters,” Holmes said. The proliferation of 4G LTE networks, the deployment of new 5G networks, and the pervasive nature of Wi-Fi are greatly increasing the number of radio frequency bands that smartphones and other mobile devices must support. RF filters are not new and our smartphones will not work without them. The first smartphones had fewer than 10 filters because they did not contain many radio frequency signals. Today – with Wi-Fi, Bluetooth, GPS, 2G, 3G, 4G, and now 5G – more than 100 filters try to prevent incoming signals into the phone from hitting each other. However, 5G networks are not completely ready for prime time. The challenge is that every 5G band must be isolated with filters to avoid interference that drains battery life, lowers data rates, and causes dropped calls. Today, filtering technologies are unable to deliver the performance these new networks promise. “We are focusing on 5G at the moment because requirements have changed dramatically from 4G filters. If you look at iPhone 11, you will find nearly 100 audio wave filters; on iPhone 11, every frequency band that needs to be addressed inside the phone needs a filter. To 5G, a much higher frequency, a much wider bandwidth, and a lot more complex, it became clear that the market needed a different kind of sound wave building blocks for these filters. Therefore, we developed a technology to deal with this new market for 5G bands and Wi-Fi quickly. Five gigahertz and six gigahertz and Ultra Wideband (UWB) from six to eight gigahertz, “Eddy said. RF Filters In addition to cell phones, the Internet of Things (IoT) is a rapidly growing group of connected devices, and they also use RF filters for communication. At its most basic level, RF filters for “good” signals allow the ability to make its way into the pathways of PCBs 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. Figure 1: The RF Filter Market. The front RF interface (FE) grows from 15B – 26B. The RF filter market is growing from 9B to 15B (Source: Resonance). Click on the image above to enlarge it. Radio frequency filters will also allow to revolutionize surgical robots operating in medical settings by communicating remotely with the operations center, eliminating any failure in speed or performance that could be catastrophic. RF filters drive the entire smart home to be controlled, from turning on the lights to starting the vacuum cleaner, and are being integrated into modern electric cars, aiding in autonomous driving technology. Older cell phones used monoblock ceramic filters with very low input loss, but essentially these phones needed relatively few filters compared to today’s phones. One-piece ceramic filters are now limited in modern phones due to their large size and high cost. Modern cell phone radiofrequency architectures and the explosion in the use of smartphones are made possible thanks to the development of sound wave resonators based on the piezoelectric effect. Cellular radios operate 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, manufactured using optical lithography processes to create a surface sound wave (SAW), sound wave large (BAW), or acoustic resonator structure (AR). SAWs and their temperature controlled counterparts were preferred due to their low cost. However, its high signal loss at high frequencies is a serious problem, as it is difficult to operate above 2.5 GHz. We realized that these types of audio filters had difficulty at high frequencies and bandwidth. So, we came up with a structure we call XBAR, where we use metal fingers over a thin layer of a single crystal, lithium niobate, to create a sound wave assembled within this piezoelectric. It’s a completely different structure, it kind of looks like a surface sound wave structure, but it’s actually a huge sound wave. It is fully optimized for high frequencies, wide bandwidth, and high power. “Sorry, that’s a long-term explanation,” Eddy said. . To design a filter, several resonators must be paired together to form a passage band. The first parameter to consider is the bandwidth associated with the main parameter of the acoustic wave resonator, that is, the coupling coefficient. Other parameters include operating frequencies, losses, and power levels. Low loss maximizes signal efficiency which results in extended battery life. All these parameters are a function of the material, design, and manufacturing process (Figs 1 and 2). Figure 2: These acoustic wave filter technologies are manufactured using optical lithography processes (source: EDN). The RF filter for XBAR 5G resonators consists of a single crystal layer piezoelectric layer, with an interdigital transducer (IDT) on the upper surface. Metal traces excite a massive sound wave within the piezoelectric layer, the fundamental frequency and coupling properties are determined by the physical dimensions and piezoelectric properties. XBAR devices spread the signal across the bulk of the piezoelectric material, rather than along its surface, providing low input losses at the high frequency and wide bandwidth, suitable for 5G. “XBAR is Resonant’s own BAW resonator chassis developed using our design software platform, ISN. It is manufactured using standard SAW processes, with a higher native operating frequency (3-7 GHz) and a 4 times wider operating bandwidth, up to 24%” Holmes said. “We use mathematical models to design and simulate filters quickly, specifically against the capabilities of the target forge, resulting in fewer turns through the factory. This simulation is based on thousands of visualized variations against the target specifications leading to better results. As a result,” he added. Fewer engineers are needed to do the same work with fewer roles, which makes this process more cost effective. ” 5G cell phones rely on high bandwidth to obtain high data rates, so much larger portions of the spectrum are needed. Therefore, 5G has much larger new spectrum allocations and is at frequencies above 4G, and requires hundreds of megahertz of spectrum and frequencies above 3 GHz – instead of tens of megahertz of spectrum around 2 GHz. Resonance was highlighted because XBAR matches well with 5G, and as users grow, interference problems will occur and RF filters will play their primary role in improving 5G transmission. Eddy said: “The filters developed with XBAR have the features to maximize efficiency, including bandwidth up to 1200MHz, support for frequencies above 3GHz, and lower loss. In addition, manufacturing technologies are low-cost, Utilizing existing operations will be an important aspect of all RF filter manufacturers as 5G networks become more prevalent.
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