RF filters—which isolate radio signals from the different spectrum bands that phones use to receive and transmit information—are going through a makeover while adapting 4G structures for higher-frequency 5G bands. The growing filter content in 5G designs leads to significant improvements that target higher bandwidth, higher operating frequencies, and higher output power than the incumbent polycrystalline bulk acoustic wave (BAW) technology.
Qualcomm, for instance, is bolstering its thin-film technology for surface acoustic wave (SAW) filters operating in 600 MHz to 2.7 GHz frequencies. The company claims that its ultraSAW filters have achieved 1 dB improvement in insertion loss compared to competing BAW filters. The low insertion loss leads to longer battery life in mobile devices.
Figure 1 Qualcomm is integrating ultraSAW filters in RF front-end (RFFE) modules for 5G designs. Source: Qualcomm
However, while Qualcomm is trying to bolster the SAW filter technology, traditionally used for lower-frequency bands with a good level of interference and co-existence blocking, others are reengineering the existing BAW filters for much higher frequency bands.
BAW filter reengineering
Take the case of Akoustis that received the order to develop a BAW filter for 5G mobile handset in May 2019. According to founder and CEO Jeff Shealy, Akoustis delivered the 5G mobile handset filter within two months. Besides delivering BAW filters for mobile handsets in n79 band, a 5G NR spectrum operating at 4.7 GHz, the Charlotte, North Carolina-based supplier of BAW filters has also delivered n77 band filters for small-cell 5G infrastructure designs. Akoustis also provides BAW filters for Wi-Fi and defense applications.
Another BAW supplier in the news is Resonant, which has currently entered into a multi-year partnership with Murata, the world’s largest manufacturer of RF filters. The deal will provide Murata rights to Resonant’s XBAR filter technology, and in return, Murata will make a strategic investment of $7 million in the Goleta, California-based supplier of RF filters and modules.
XBAR, Resonant’s brand new BAW filter technology, is targeted at the high-bandwidth and high-frequency requirements of 5G mobile applications. Resonant claims to have developed filters for 5G New Radio (NR) devices operating in the 3 GHz to 6 GHz frequency range and millimeter-wave (mmWave) systems operating at 28 GHz and higher frequencies.
Figure 2 The XBAR filter was demonstrated at the Mobile World Congress (MWC) in Barcelona. Source: Resonant
The percentage bandwidth that XBAR technology can deliver is more than 10% compared to 3% to 5% for 4G devices. Likewise, it can handle the high-power RF signal needed to accommodate the signal attenuation at these frequencies. Finally, with approximately 1 dB of passband insertion loss from the center of the passband, XBAR filters can facilitate longer battery life.
RF filters are a critical building block in 5G designs for their role in accommodating a higher number of bands and support for higher bandwidth applications such as HD video. Their ability to allow 5G bands to coexist with Wi-Fi spectrums is also be an important consideration for mobile designers.
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