Under the Hood: CSR evolves along with Bluetooth

Cambridge Silicon Radio (CSR) has a reputation in the industry as a strong provider of Bluetooth solutions. The BlueCore 5 Media is one of the latest versions of the popular BlueCore series and integrates additional functionality over previous products. First and foremost, the BlueCore 5 is compatible with v2.1 of the Bluetooth standard. It operates at the 2.4-GHz bandwidth and is backward- compliant with the Bluetooth v2.0 + enhanced data rate (EDR) standard.

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Applications for the device are as varied as the Bluetooth products available in the market. Some of the more popular applications are headsets, wireless speakers, handsets and MP3 players. Connectivity into the BlueCore 5 supports all of the usual methods. These include USB, I²C and UARTs with dual-port bypass mode to 4 Mbits/second. External memory of up to 32 Mbits is supported to increase the capacity of the system. Since more consumer products are implementing multiple radio systems in their design, the BlueCore 5 is designed to coexist with 802.11.

Die photograph of Cambridge Silicon Radio's BlueCore 5 Media, with the logic circuitry—the largest consumer of space on the device—on top.

Over the years, CSR has evolved steadily in the Bluetooth market by continuing to support the latest version of the standard, improving its design process and including additional features. For example, the CSR BlueCore 2 device is compliant with Bluetooth standards v1.1 and v1.2 and is fabricated in a 0.18-µm RF CMOS process. The device utilizes four nonsymmetric, center-tapped, on-chip inductor structures for the low-noise amp, voltage-controlled oscillator and local-oscillator generator blocks. The inductors consist of multiple, parallel metal layers to reduce the series resistance and increase the Q-factor.

Like the BlueCore 2, the Blue- Core 3 complies with Bluetooth v1.1 and v1.2 and is fabricated in 0.18-µm RF CMOS. Its four on-chip inductors were redesigned from those used on the BlueCore 2. The LNA, VCO and LO generator blocks utilize symmetrically wound, tapered inductors with center taps. The LNA on the BlueCore 3 uses multiple parallel metal layers, as does the BlueCore 2, but the VCO and LO generator only use top metal in their inductors. In addition, the exclusion area around the inductors was significantly reduced in the BlueCore 3 design. The BlueCore 3 also uses metal-insulator-metal (MIM) capacitors.

The BlueCore 4, while still using a 0.18-µm RF CMOS process, is compliant with Bluetooth v2.0 with EDR.

SEM cross-section through the device's minimum observed transistor, measuring 110 nanometers.

Looking at another Bluetooth device on the market, Broadcom Corp.'s BCM2045 device is likewise compliant with Bluetooth v2.0 with EDR but is fabricated in a 0.13-µm CMOS process as opposed to 1.8-µm CMOS. Further, the BCM2045 uses nine symmetric, center-tapped, on-chip inductors, compared with the six in the BlueCore 5 Media.

The BlueCore 5 is divided into roughly two halves, with the RF circuitry on one end and the digital circuits making up the remainder of the device. The largest consumer of space on the device is the logic, followed by the embedded memory, the RF blocks and the I/O pad ring. The remaining sections around the identified functional blocks are unused, have other circuitry, or are used for routing.

The LNA design on the BlueCore 5 differs significantly from the BC2 and BC3 designs. The earlier designs use an LNA topology with a single inductor. The BC5 employs a more complex LNA, with two inductors, both having connections to the IO pad ring.

CSR has also announced the Blue- Core 6, which supports Bluetooth v2.1 with EDR and eSOC (extended synchronous connection-oriented) capability. The advantage over previous solutions is the improved voice-quality capabilities offered by the DSP.

All standalone Bluetooth devices, however, face tough competition from integrated solutions. For example, Marvell has announced its next wireless device, the 88W8688. Following on the heels of the wildly popular 88W8686 802.11 a/b/g wireless-LAN offering, the new device adds to the WLAN functionality with Bluetooth v2.0 + EDR. As more consumer products start offering multiple radio standards, expect to see more combined solutions enter the market.

Semiconductor Insights' Technical Marketing Manager Gregory A. Quirk