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Small RF filters handle multiple bands in single die

Posted: 27 Aug 2015 ?? ?Print Version ?Bookmark and Share

Keywords:radio frequency? RF filter? surface acoustic wave? smartphone? bulk acoustic wave?

Smartphones today need at least 60 RF filters to accommodate all the radio frequency (RF) bands, but this number may soon become lower as Resonant Inc. refocuses on the handset market. The company develops room temperature RF filters that are 25 per cent smaller in area and 50 per cent thinner. In addition, these devices can also handle two or three bands in the same die.

"Resonant is the world's first fabless RF filter maker," Bob Hammond, chief technical officer (CTO) of Resonant told EE Times. "Manufactured by leading RF foundries, our filters are not only smaller, thinner and pack more filters per die, but will also reduce the parts count and the bill of materials [BOM]."

Most RF filters are still manufactured by their designers using proven technologies that date back to the 1920s and address only one frequency per die. Resonant, through its experience making superconducting filters for base stations from which it has accumulated a 70-strong patent portfolio, claims to have developed multi-band techniques that allow two or three filters to share the same die, as well as creating tunable models that can change their frequency to suit the locale in which the smartphone is being used.

Resonant also claims superior specifications to the time-worn designs from the 1920s used by everybody else, including insertion loss of less than 1dB and high transmission power in excess of 33dBm. They also have reduced the size of filters, with duplexers measuring just 1.8mm x 1.4mm 0.38mm whereas incumbent duplexers performing the same function measure 2mm x 1.6mm x 0.9mm thick.

"We are not using microelectromechanical systems [MEMS], but some people compare us to MEMS because we have vibrating resonators in our surface acoustic wave [SAW] designs," Hammond told us.

Mid-bad SAW filters

Here are some of the mid-band surface acoustic wave filters used in the iPhone 6, allowing radio frequency (RF) signals to flow both to and from (duplex) the smartphone. (Source: Resonant)

Instead of going the venture capital route, the three-year-old company ran on founder seed funds and superconductor-filter sales during its start-up period creating the cash flow necessary to perform the research and development (R&D) for its latest foray into making less expensive, thinner smartphones for the consumer market.

Resonant made its initial public offering (IPO) in May of 2014, giving it the capitol to finish its R&D on its novel consumer RF filters for smartphones. Currently, they are in pilot production and being evaluated by a host of (unnamed) leading smartphone makers with the first design wins due to be announced early in 2016.

One high-volume customer is accepting shipments this year but will not have their smartphones in the marketplace until Christmas 2015. Also, a second high-volume customer has started the second stage of designing-in Resonant's RF filters, but it will not be allowed to announce the design win before the end of the year.

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