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Employ ISN design techniques to create RF filters

22 Sep 2015  | Bob Hammond

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The demand for wireless capacity to connect mobile devices and the emerging Internet of Things (IoT) to data networks is growing astronomically. Analysts forecast that there will be 11.5 billion mobile-connected devices by 20191, and that by 2019, mobile data traffic will reach 25 exabytes—or 25 million gigabytes—per month, a ten-fold increase from 2014 levels.2

Manufacturers of connected devices are already struggling to keep up with the proliferation of spectrum bands needed to carry this data. While available space on the circuit board continues to shrink, the need to provide users with access to these bands continues to increase.

As a result, connected devices require more RF components, adding to costs and power demands. This is forcing mobile phone manufacturers to add new models just to keep up with bandwidth needs, at a time when they would actually prefer to be reducingthe number of different models and SKUs.

The proliferation of frequency bands and the desire to reduce the number of phone SKUs is putting enormous pressure on RF component and module manufacturers to reduce size and cost, while at the same time improving performance. Going forward, carrier aggregation and more complex Multiple-In-Multiple-Out (MIMO) designs, enabling the emerging LTE-Advanced and 5G networks, will require dramatically smaller and cheaper RF filters.

Infinite Synthesized Networks (ISN) is a new filter design technique that can be used to develop a state-of-the-art Band 3 duplexer using low-cost SAW fabrication, while achieving performance results that equal or exceed the performance of the very best avail┬Čable, but more costly, BAW Band 3 duplexer.

RFFE complexity
The mobile RF front-end is becoming increasingly complex. With more spectrum auctions slated for the future, leading to the adoption of LTE-Advanced with key features of carrier aggregation (CA) and multiple-in-multiple-out (MIMO) for increased data throughput, there appears to be no slowdown in this increasing complexity—or in the need to simplify the RF front-end (RFFE) architecture.

The current RFFE architecture comprises the following components (figure 1):

 • Antenna tuner
 • Antenna switch
 • Duplexers and filters
 • Power amplifier switch
 • Multi-mode, multi-band power amplifier (MMMB PA)
 • Low noise amplifiers
 • Matching networks

Figure 1: RF Front-end architecture for the Low-Band frequencies (700-900MHz), illustrating the complexity of the front-end.

Power amplifiers, which in the past were the high cost component of the RFFE, have now advanced to a point where a single power amplifier can cover multiple technology modes, such as CDMA, LTE, W-CDMA and multiple frequencies/bands—hence the terms Multi-Mode/Multi-Band (MMMB) PAs. Although filters and duplexers are relatively low-cost items, a filter is required for every RF path. Thus the most significant cost for the RFFE moving forward is the filters.

Infinite Synthesized Networks
Traditional acoustic wave filter design uses a ladder structure and empirical models (linked to a particular fab manufacturer). ISN are design tools that bring together for the first time the following elements:

 • Modern filter theory
 • Finite element modelling (both electro-magnetic and acoustic)
 • New optimisation algorithms
 • Design for multiple temperatures: The ISN framework allows optimisation over multiple temperatures, with optimum performance on the higher temperatures expected in the RF module.
 • Designs optimised for high power performance: LTE operates at higher power than CDMA, requiring designs that can withstand high power, at elevated temperatures, for extended periods of time.

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