RF integration: Full SoC, or just a SiP?

The radio-frequency circuitry in mobile and portable devices remains some of the most resistant to integration. RF devices are very picky about who their neighbors are, and they sometimes find surprising ways to add interference to the signals they are responsible for handling. In the push for smaller footprints, the RF circuitry needs to shrink. The question is, what should be integrated, how, and how much?

There are competing strategies here: Integrate the power amplifier (PA) with the transceiver in a system-on-chip (SoC), leave it to stand alone, or combine it with the passive and control circuitry that is off-chip in a system-in-package (SiP). Which one is the winner? For applications requiring low output power and short ranges (think Bluetooth), the answer is SoC all the way. For applications requiring long range and high output power (think 802.11n WLAN connections), the best bet is to go with a SiP for the RF front-end circuitry (the PA, maybe the low-noise amplifier, matching circuitry, passives, switches, etc.). The standalone PA may be left out in the cold.

At Ralink Technology Corp. (Cupertino, Calif.), the R&D team is supportive of integrating the PA. Recognizing that integration is a great way to reduce cost, form factor and component count, Keng Fong, Ralink's RF engineering director, remarked, "The ultimate goal of integration is to integrate all blocks, which include digital circuits, RF transceivers and PAs. Since the digital circuits use CMOS technology, it becomes the only choice for the highest level of integration. On the other hand, designers need to consider the trade-off among integration, time-to-market, performance, device yield and ruggedness. A compromise approach is to use BiCMOS technology to integrate PAs with the transceivers, and to use CMOS technology for digital circuits in a separate die."

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Fong sees promise in the integrated PA SoC approach: "The [integrated PA] trend started with applications that have low performance and output power requirements, such as Bluetooth, and moved toward applications that require medium output power, such as wireless LAN," said Fong.

SiGe Semiconductor (Ottawa) sees another trend: requests for more functionality in a PA SiP. SiGe specializes in front-end modules (FEMs), which include the PA and associated circuitry.

When the PA is not integrated with the transceiver, you can "optimize the process technology for the functionality you are trying to achieve," said Stefan Fulga, director of strategic marketing at SiGe. "There are more requirements in Wi-Fi MIMO and WiMax, and it is beneficial to [be able] to optimize for given functions."

Fulga added that the flexibility of a PA SiP is valuable. "The FEM is fairly customized to a given application. It is sort of a best of both worlds--integration plus customization," he said.

Integrating the PA with the transceiver has its advantages. However, these devices tend to have lower output power and power-added efficiency, which can be a huge trade-off for mobile wireless devices. Until someone comes up with an exotic new process technology, the smart money is on PA SiPs.