Bluetooth wireless connectivity has been acknowledged for its versatility in linking everything from cell phones and hand-held devices to office and automotive electronics. But until recently, the cost of implementing this wireless standard couldn't compete with simple hard-wired cable.

Enter the design engineers at Cambridge Silicon Radio (CSR), led by James Collier, technical director and a co-founder of the company. Their approach was to integrate the Bluetooth wireless functions onto a single CMOS (complementary metal-oxide silicon) chip. This is the most popular material used in microchip fabrication, and thus has the lowest basic cost in materials and manufacturing.

Chip shot. But once engineers selected that approach, the greatest challenge became designing in the degree of circuit integration needed to implement the Bluetooth standard. Having a majority of functions on one chip allows faster signal transfer, smaller footprint, lower parts count and cost, and easier fabrication. And the fewer electrical leads and connections that come with fewer chips usually means there are fewer "wires" to act as EMI and RFI inducing antennas, particularly at high clock speeds such as the Bluetooth 2.45-GHz reference frequency.

But in fact, the latter is not the case with the Bluetooth wireless radio on a chip. The reason: The noisy digital circuits have the potential to interfere with sensitive radio frequency (RF) analog circuits. In the case of competitive designs using multiple chips, the sensitive RF components are on more exotic materials, which makes them easier to design but more costly. Consequently, the digital portions are on separate silicon chips.

Collier's team was able to solve the problem using modern design tools "coupled with some 100-plus man-years of experience in RF on CMOS design," he notes. By this concentrated effort on designing complex RF circuitry into CMOS, the result is an economical, smaller, single chip that is easier to design into products, Collier highlights. Packed into the single chip, called BlueCore, are the complete RF radio circuit, baseband DSP, microprocessor, and on-board memory (Bluetooth software stack), along with support for a USB connection and piconets.

Engineers also included a built-in self test capability. In warm environments, the radio frequency components may drift. The CSR chip checks itself and compensates for temperature-induced changes.

Modern design tools Collier cites include ECAD, which helped reduce the number of physical prototypes, and thus cut time and cost. Finite element analysis was used to analyze the 3D structures etched into the silicon, such as the conductive spirals that form inductors for the radio circuit. Finally, the team also came up with a hardware development kit, named Casira, to support customers in creating products incorporating Bluetooth.