As Bluetooth spreads into applications requiring significant levels of integration, it begins to present a problem for SoC designers. A Bluetooth transceiver is a relatively sophisticated 2.4 GHz spread-spectrum radio: not the sort of thing you license at RTL and hand off to a subcontractor to synthesize and integrate. In fact, there are only a few complete Bluetooth radio cores on the market. Most of the IP out there is in the form of building-blocks, intended for design teams who know their way around the inside of a digital radio.
This situation makes an announcement by IP developer Orca Systems this week quite intriguing. The company has released a complete, turnkey Bluetooth radio—both the RF transceiver and the modem—supporting the original, EDR, and low-energy modes. Supporting all three modes in a compact core necessarily means reusing most of the blocks in the signal paths. That necessity in turn means power-management must exploit every opportunity for clock-gating afforded by the standard, or your low-energy mode will consume about the same power as your original mode. Even so, the design may not be as energy-efficient as a low-energy-only core. But for an SoC that must work in multiple modes, one does what one can.
The block is tiny: 2.7 square mm in Fujitsu’s 90nm CMOS including both transceiver and modem. Better yet, unlike most RF CMOS designs that are nearly process-specific, it should scale well into finer geometries. Orca delivers the core as a single hard macro, by the way, including both transceiver and modem.
All this is possible, according to Orca vice president for business development Joe Thome, because of the digital radio technology that makes up Orca’s crown jewels. The company’s foundation is a digital open-loop frequency synthesizer, capable of sufficient spectral purity for a Bluetooth transmitter, but with very low frequency-shift latency because of the open-loop architecture. Basically, it doesn’t have to search and lock the way a phase-locked loop does—it simply generates whatever frequency it is programmed to generate.
Orca designers directly modulate the synthesizer at the Bluetooth 1600 Hz hop rate, providing a fully digital signal source. Going a step further, Orca has developed a digital power amplifier to drive the antenna. This is not unprecedented: Class-D amplifiers at 2.4 GHz have appeared in the literature, notably from UC Berkeley at the 2008 Custom Integrated Circuits Conference. But it is hard to find any reference to a similar commercial product.
The designers did not stop there. Thome said Orca has developed a mostly-digital receive chain as well. The design starts with a conventional analog low-noise amplifier (LNA). But then instead of analog filtering and mixing, the Orca design directly samples the RF, turning the continuous-time radio signal into a very-high-frequency sampled-data stream. The core does the filtering and mixing functions in the sampled-data domain, apparently somehow using digital circuitry to do so, and then passes the output to an ADC for use by the baseband processor. Orca, liking their acronyms as well as the next man, calls this technology DSP-RF, and the core itself the DRBT.
So what does that mean for the SoC designer? There are several advantages, Thome said. The first is the size of the core. Without large matched RF transistors and passive components, the design is small. Next comes scalability. Since only the LNA is a true analog device, the radio area should scale almost like a purely digital block. Thome said Orca has produced the block in 130, 90, and 65nm geometries so far, and it has scaled as predicted.
A further benefit, Thome said, is that the DSP-RF architecture uses many fewer inductors then would a conventional design. All this digital talk suggests a standard digital process. But in fact a white paper from Fujitsu, which appears to describe the development of the DRBT sample chip, says the process the team used was Fujitsu’s 90nm low-power RF CMOS, with a special RF process design kit.The DRBT digital Bluetooth core is available now, as are the aforementioned prototype IC and an evaluation board.