SAN FRANCISCO Chip designers reported on two separate projects to build combined 802.11b/Bluetooth RF chips at the 50th International Solid-State Circuits Conference on Monday (Feb. 10). While the two designs differed in detail, they shared a common strategy.
Hooman Darabi of Broadcom Corp. (El Segundo, Calif.) and Thomas Cho of Wireless Interface Technologies Inc. (San Diego) reported on their respective companies' combo chip efforts. Both designs included RF transmit and receive sections, clock generation and filtering. Both require an external power amplifier for 802.11b, and both require an external digital baseband chip with A/D converters.
Both chips also exploit the convenient fact that the center frequency for 802.11b is 2.4 GHz, the same as that for Bluetooth. So in principle a good deal of the RF stages of the two radios could be shared, but that was less true in practice. The two standards differ considerably in their bandwidth and dynamic characteristics, requiring significant differences in implementation.
Still, there is enough commonality for some economies. A single mixer is sufficient for up/down conversion for both standards. In Broadcom's case, a common low-noise amp and power amp are used. Beyond that point the two designs take different approaches.
Broadcom uses a switch to select between two sets of transmit/receive filters and amplifiers. Wireless Interface Technologies uses a similar approach on the transmit side, but on the receiver uses a programmable dual-mode complex filter and a dual-mode amplifier saturating for Bluetooth and programmable-gain linear for 802.11 instead of separate modules.
Both designs show promising economies by combining the two standards into a single architecture. But both also require some further investigation. In response to a question from the audience, Darabi said Broadcom is just beginning to investigate the possibility of a dual-mode baseband chip. Given the obvious application in mobile Internet and computing appliances, neither chip appeared to permit simultaneous 802.11b and Bluetooth operation. '"That is beyond our capability right now,'" Cho said.