ISN for high performance 5G RF filters

The ISN tool was initially used to design surface acoustic wave (SAW) filters. SAW filters traditionally supported lower-frequency bands with a good level of interference and co-existence blocking. However, the high accuracy of ISN’s design process resulted in better performing SAW filters. Since then, Resonant has been perfecting the use of the tool on higher-performing resonator technologies including temperature-compensated (TC-SAW) and, most recently, bulk acoustic wave (BAW) technologies.

BAW filters have traditionally been used at frequencies up to 2.7 GHz and in applications with difficult interference and coexistence challenges. Adapting these (4G) filter structures in order to filter the higher frequency, 5G bands is a significant challenge. In addition, the bandwidth requirements for 5G, to realize HD video to the phone, also is significantly wider than previous generations of wireless technology. The percentage bandwidth the filter can deliver is now more than 10% (compare to 3-5% for 4G). Another important element is handling the high power RF signal needed to accommodate the signal attenuation that occurs at these frequencies. The right technology for a 5G filter will support all three of these elements, and so far, no BAW technology, except XBAR, has been able to deliver all three.

In exploring the use of ISN for BAW, Resonant developed a brand-new BAW resonating structure called XBAR. In ISN simulations, XBAR technology can filter at much higher frequency bands, surpassing the performance of other BAW filters. XBAR technology is ideal for emerging 5G mobile devices that need higher bandwidth at higher frequency, yet XBAR filters can be manufactured using existing fabrication processes for fast production.

Figure 1: XBAR can deliver very large bandwidth, in this chart showing coupling of more than 500 MHz in the 5 GHz band.

ISN simulations have shown that XBAR technology could be potentially used to develop filters for 5G New Radio (NR) devices operating in 3 GHz to 6 GHz frequency range and also millimeter wave systems operating at 28 GHz and higher frequencies. Resonant has measured the performance of XBAR technology at up to 38 GHz.

First XBAR Filter

At MWC (Mobile World Congress) Barcelona 2019, Resonant demonstrated the performance of one of the first XBAR-based filters. The prototype XBAR filter demonstrated approximately 600 MHz of bandwidth in the 5 GHz frequency range which is approximately two times wider than any other announced filter bandwidth. Many next-generation 5G services will operate in this range. Production XBAR filters delivering this bandwidth will contribute to 5G by providing the next-generation services promised by the technology.

Figure 2: Prototype of XBAR filter at MWC Barcelona 2019.

In addition to bandwidth, the prototype XBAR device measured approximately 1 dB of passband insertion loss (from the center of the passband). In a mobile device, this low insertion loss will result in longer battery life.

For power handling, XBAR filters have demonstrated more than 30 dBm (1W) power handling performance at the band edge. Power handling is important because as frequencies increase, transmit distances are reduced and power is increased to the RFFE to achieve transmit distance.

Rejection of adjacent frequency bands is another critical performance characteristic of filters especially for certain 5G bands that are adjacent to Wi-Fi bands. The XBAR prototype filter has demonstrated Wi-Fi rejection at high frequencies to eliminate interference and allow for 5G and Wi-Fi co-existence in a phone or device. No other high performance integrated and low-cost solutions allowing for this co-existence have been announced.

XBAR holds the promise of delivering on a whole new range of “greenfield” filter designs that are brand new to the market for both 5G and Wi-Fi networks. The first planned device will support band n79, a 5G NR band operating at 4.7 GHz.

Since MWC, Resonant has continued to iterate its XBAR filter, leveraging the ISN platform.  ISN was used to simulate a prototype of a new XBAR architecture that provides for the simultaneous operation of n77, n79 and 5GHz Wi-Fi using a n77/n79 diplexer and 5GHz Wi-Fi filter. The only such public simulation of a Wi-Fi/5G co-existence solution. This architecture has been confirmed as a preferred solution by a leading wireless operator, and the smaller design footprint will be critical to all handset manufacturers looking to deliver 5G handsets without sacrificing device size and battery life.

Resonant’s ISN technology provides better performance for all RF filter technologies, and this has led to the development of XBAR. With ISN and XBAR, the promise of next-generation wireless technology bandwidth and resulting enhanced user experience can be fulfilled.