NF: Cellular phones have gotten much “smarter.” For all this power and convenience, RF/microwave frequencies (such as those used in WCDMA and LTE systems) must be kept separated within compact handsets. What type of filtering technologies does this require?

TD: As fourth-generation (4G)/Long Term Evolution (LTE) networks deploy, the number of RF bands within each device is increasing significantly. At the same time, the global demand for more spectrum is leading governments around the world to re-farm existing spectrum and allocate new bandwidth for wireless services within a crowded landscape. Robust filter technology is critical to mitigate potential interference issues. While surface-acoustic-wave (SAW) filter technology is widely used for band frequencies up to about 1.9 GHz, higher frequencies are better served by advanced bulk-acoustic-wave (BAW) and temperature-compensated-SAW (TC-SAW) technologies.

NF: As smartphones increasingly support WiFi, what demands are placed on filtering technologies?

TD: LTE bands are commonly located next to the unlicensed, international, industrial, scientific, and medical (ISM) bands between 2.4 and 2.5 GHz. These bands are used worldwide for WiFi and Bluetooth signals—often with very narrow guard bands. As WiFi becomes ubiquitous and new wireless spectrum is allocated for commercial applications, interference issues multiply. To address this, design engineers are turning to high-performance LTE/WiFi coexistence filters. They have steep skirts that roll off quickly for frequency rejection coupled with low insertion loss.

NF: Because of the multiple frequency bands being processed within such small circuits, do you see increased opportunities for combination filters or duplexers in emerging smartphone designs?

TD: TriQuint organizes filters in various combinations. These range from duplexer banks that consolidate several filters into a single module [thus reducing printed-circuit-board (PCB) space] to two-in-one duplexers that permit the use of two-receiver operation simultaneously and independently. For wideband TD-LTE filter applications, it’s conceivable that two or more bands could be accommodated in one device, which would eliminate the need for a separate filter.

NF: TriQuint offers extensive lines of BAW and SAW filter products. How do your filter technologies differ in their capabilities and how do they match up to different frequency bands within a smartphone?

TD: SAW filters are well suited for frequencies through 1.9 GHz, such as standard GSM, CDMA, and third-generation (3G) bands—with the exception of the US-PCS band (Band 2). Some new 3G and 4G WCDMA duplexers and filters are best served by TC-SAW, which reduces temperature drift for more challenging specifications. For example, TriQuint uses TC-SAW to support Band 13, Band 20, and Band 26 duplexers.

BAW is ideal for many of the new LTE bands above 1.9 GHz, delivering superior performance with lower insertion loss, steeper slopes, and excellent rejection. BAW excels in applications where the uplink and downlink separation is minimal and when attenuation is required in tightly packed adjacent bands. TriQuint’s BAW advantages are instrumental in serving the following bands: Band 25, Band 3, Band 7, Band 38, Band 40, and Band 41 LTE filters.

NF: For these emerging smartphone applications, which architectures make the most sense for designers?

TD: Designers select the best architecture to meet a specific set of requirements. For a low-band-count phone with more board space, some designers may like the flexibility of a discrete approach. For a high-end smartphone, they often opt for an integrated RF front end that allows them to squeeze in more bands and offer feature-rich content. LTE filters are more likely to be discrete because they’re relatively new. Designers simply add LTE “satellite” components to existing layouts to offer regional 4G variants.

NF: Because TriQuint offers many other components for a communications device, including amplifiers and frequency converters, is the company looking to increasingly combine component functions (such as amplifiers, attenuators, and filters) into a multifunction “super-component?”

TD: Yes, we’ve been selling hundreds of millions of multifunction units in different combinations. Increasing RF complexity is driving the trend toward integrated solutions. One approach for multi-band devices is an integrated module that combines power amplifiers with duplexers (PADs) in single-, dual-, and multi-band configurations. This optimizes performance along the transmit path while reducing the amount of PCB space required. TriQuint has powered the world’s top smartphones with over a half billion PADs. Our integration-enabling technologies like CuFlip™ shrink size, improve performance, and reduce cost. We’re also adding wafer level packaging (WLP), which offers significant advances in miniaturization and reduced height compared to traditional chip-scale packages.

Another integrated approach for RF design is the multi-mode, multi-band power amplifier (MMPA), which gives OEMs more PCB room for richer feature sets while minimizing engineering time and resources. Strategy Analytics predicts the market for MMPAs will reach more than $700 million in 2016. TriQuint’s MMPAs are found in some of the world’s most sought-after smartphones and on leading chipset suppliers’ reference designs.

NF: What are the most challenging performance requirements for filters used in LTE handsets? What tradeoffs are involved in achieving the required performance levels?

TD: Deciding which filter technology is right for a particular band is usually a balancing act between performance, size, and cost. Some LTE bands require higher-performance filters with lower insertion loss for improved signal reception and longer battery life. BAW filters deliver the lowest loss, which helps compensate for the higher losses associated with combining multiple bands in a smartphone. Featuring steep filter skirts and superior out-of-band rejection, BAW filters also provide higher isolation for better receive sensitivity, higher attenuation to cope with increased band coexistence, and higher linearity to handle LTE modulation.

NF: What percentage of your filters is used in tablets?

TD: Many of our smartphone customers offer tablets, but they don’t report to us the breakdown between phone and tablet models. Although tablet shipments are increasing steadily, they represent a much smaller percentage of the overall mobile-device market. Tablets contain WiFi but, so far, only a portion includes the cellular option.

NF: How have you had to adapt or modify TriQuint’s filters to satisfy tablet designs?

TD: Our customers’ tablet requirements are very similar to those for handsets in terms of height, performance, size, and cost. Most often, they simply reuse our smartphone parts for tablet devices.  

NF: For customers who have a particularly difficult requirement, will you share some of your in-house filter models so that they can perform software simulations—at the circuit or system level—to better understand the impact of your filters on their designs prior to fabrication?

TD: We collaborate closely with customers as they develop their next-generation products. As trusted partners, we provide technical information, such as S-parameters and other data, to help them implement their software simulations and design our products into their systems.

NF: Does TriQuint provide testing services to help these cellular device customers evaluate the performance of new filters? If so, are those services available at circuit and system levels?

TD: We provide customers with excellent applications support—including on-site support and recommendations for layout options and optimizing matching solutions—to help them meet their performance targets. Our field application engineers provide all of the data required to use different filters, as well as detailed application notes for each device. 

NF: How do customers work with you to define these tests?

TD: We collaborate very closely to compare bench set-ups and often provide detailed specifications that they incorporate into their test programs and calibration routines.