Even assuming perfect switches, it becomes increasingly difficult to maintain high performance for all bands as each new band is added, since compromises must be made in the shared circuitry.

Also, since every component in the chain has its specific fixed-frequency response, the band edge performance is typically suboptimal.

Single-chain solution

All of the above issues can be avoided if the RF front-end components are tunable. A single chain can then be optimized specifically for the channels currently in use.

The benefits of the single-chain approach are widely appreciated--as are the challenges remaining in the way of implementation.

Research into tunable front-end components has been progressing for a couple of decades, but only now are the requisite technologies maturing. The traditional stumbling blocks have been size, cost, repeatability, reliability and performance. Each of these has been addressed in part by previous work, but WiSpry is the first to bring to market a complete solution that is suitable for high-volume production at a price point that is compatible with the cell phone industry.

By pioneering the integration of high-Q MEMS capacitor elements into a mainstream RF CMOS process technology, WiSpry brings together the benefits of a high-volume, low-cost process with the advantages of high-performance RF MEMS technology. Individual capacitor elements are integrated on-chip as tiny parallel plate capacitors with a digitally variable air gap. Individual shunt or series elements are combined into capacitance cells and then into arrays that can contain any combination of individual cells, resulting in a digital capacitor that is well behaved and free of higher modes, with capacitance ratios (max/min) greater than 10 and a quality factor (Q-value) well over 200 at 1 GHz.

The manufacturing of this device benefits from the latest advancements in CMOS semiconductor process technology. WiSpry is using a fabless model that integrates the programmable digital capacitor technology monolithically on mainstream 8-inch RF CMOS wafers that can be produced in extremely high volumes, thereby eliminating the size and cost concerns traditionally associated with high-performance MEMS technology.

The process flow also includes wafer-level encapsulation so that finished wafers from the foundry can be utilized directly in conventional, automated back-end processing (such as bumping, thinning, dicing, packaging and test), making for a highly reliable end product that can be produced in a traditional RF semiconductor manufacturing flow.

No external circuitry

So how does the component work, and what must the designer need provide in order to use the technology?

There is no need for external circuitry, as the component truly works like a high-Q capacitor with integrated serial interface. All the support functions for the MEMS elements are integrated on the chip.

By loading a digital word that contains the desired setting of the digital capacitor elements over the serial bus, the internal logic and driver circuits immediately set the capacitor bank to the selected value.

This programming can be repeated at high rates to create dynamic RF functionality, which has a significant number of interesting applications.

As programmable chips are integrated into custom modules together with other high-Q integrated passive and active components and support circuitry, they form a platform that WiSpry will use to provide programmability to the complete RF front end.

The work starts t the antenna, with frequency agility functions and mismatch tuning, and will address the rest of the RF chain in a natural sequence.

Marten Seth (Marten.Seth@wispry.com) is director of product planning and business development at WiSpry, a fabless semiconductor company based in Irvine, Calif.

Arthur Morris (art.morris@wispry.com) is chief technology officer at WiSpry. The company is pioneering the use of RF MEMS in industry-standard CMOS technology and is currently sampling its first products to key customers.

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