According to researchers at the Interuniversity Microelectronics Centre (IMEC), the number of “things” connected to the Internet in 2008 was more than the entire population of the Earth! And by 2020, it projects billions of devices connected to the Internet. Eventually, IMEC researchers say, the number of short-range radios will outnumber smartphones and tablets.
Such wireless innovations applied in smart buildings, smart metering, personal healthcare with portable diagnostics, and monitoring that can be worn, will challenge designers to conserve energy and extend battery life without compromising performance. All of this, plus complying with standards for interoperability where applicable.
To help get us to this world of ubiquitous wireless, IMEC used this year's ISSCC gathering in San Francisco to describe a CMOS-based Bluetooth Low Power/ZigBee 802.15.6 combo radio architecture for low-power, personal/body-area networks (PAN/BAN) that is worth examining for its design choices. This is one of the first development ideas that the IMEC research team has conceived toward their goal of a more efficient set of ULP (ultra-low power) radio ICs.
In general, widespread, use-it-everywhere wireless will be enabled by circuit integration. IMEC cites the following statistics that highlight how these wireless devices (and the technology behind them) are sneaking into our lives. Mixed signal: 10 papers published at ISSCC conference 2013, seven patents (filed but not granted in the last three years) Radio and DSP patents: 12 papers published at ISSCC conference 2013, 20 patents (filed but not granted in the last three years). These are sure signs of coming innovations to foster RF, processor, and mixed-signal integration, coupled with the drive for lower power technologies and architectures.
To help put all this in perspective and explore possible routes to meeting these on-going design challenges, I had a discussion recently with Harmke deGroot, Program Director, Ultra-low Power Circuits at the Holst Centre/IMEC, (Eindhoven, The Netherlands). Ms. deGroot, one of the ISSCC paper's co-authors, and her team, are working with various IC suppliers, like Renesas, to enhance ULP for short range communications while extending battery life in portable systems like those in PAN/BAN.
IMEC researchers predict that power and data will be simultaneously sharing a single antenna at long distances (They have already achieved 15-20m in 2012 with their Rectenna design). Others are also working on these power/data transmission modes. MIT is working on a mat that sits in your garage with an integrated coil over which you drive your electric vehicle to begin charging.
The PAN/BAN solutions were a subject of a 2013 ISSCC paper that attracted my interest because they were proposing a 2.4GHz radio that would be compliant with three wireless standards for PAN/BAN, with the caveat that it also improve energy efficiency of most existing radio designs. Figure 1 shows the block diagram for such a radio.
IMEC will use a CMOS process for radio integration with the transceiver discussed in this ISSCC paper being based on TSMC's 90nm technology. Figure 2 gives a good view of how all this will be squeezed onto the IC.
Normal analog integration is challenging enough, but to integrate RF VCOs (voltage controlled oscillators), PLLs (phase locked loops), ADCs, LNAs (low noise amplifiers), etc. — that's a real challenge. Crosstalk at these high frequencies can be problematic. EMI/RFI can plague a design if not done properly. The die itself should have good practices to minimize radiated energy as well as to de-sensitize the circuitry from radiated RF signals interfering with the IC function.
If possible, it's always best to make an IC whose die does not have to be encased in an EMI shield. Besides eliminating the cost of the shield, the die can more easily dissipate heat energy into the air.
The designers wanted to achieve an energy-efficient radio architecture, so a suitable local oscillator (LO) frequency plan was selected. Some other efficiency enhancement techniques were employed in the critical RF circuits (such as a push-pull mixer and a digitally-assisted power amplifier).
More clever techniques were employed by the designers, like the receiver (RX), which uses a sliding-IF architecture to reduce the power consumption by avoiding the need for a high frequency quadrature LO.
In addition, to save power and eliminate the lossy balun, the researchers used a single-ended LNA that also functions as an image rejection filter, and chose a push-pull mixer structure which improves both the transconductance efficiency and noise performance.
The delicate balance of a creative design that preserves performance needed while conserving power is an endeavor that has always involved trade-offs. Size can be one of those trade-off specs, yet the designers were able to keep the die at a core area of under 4mm2 .
IMEC's design achieved the best RX sensitivity versus energy efficiency performance of current state-of-the-art ULP 2.4GHz RXs while achieving a 3 to 10X improvement in energy efficiency.
- 1. “A 1.9nJ/bit 2.4GHz Multistandard (Bluetooth Low Energy/Zigbee/IEEE802.15.6) Transceiver for Personal/Body Area Networks”, Yao-Hong Liu, Xiongchuan Huang, Maja Vidojkovic, Ao Ba, Pieter Harpe, Guido Dolmans, Harmke de Groot, 2013