Due to an aging population and increasingly sedentary lifestyles around the world, cardiovascular disease and diabetes are affecting a larger number of people. Because of the asymptomatic or intermittent properties of these diseases, long-term continuous and ubiquitous real-time health monitoring—without limiting the patient’s activities—are critical to their detection and treatment. Such continuous and automated monitoring demands ultra-low-power wireless connectivity among devices placed in, on, and around the human body. At the Korea Advanced Institute of Science and Technology (KAIST), an energy-efficient, crystal-less, double-frequency-shift-keying (double-FSK) transceiver for wireless-body-area-network (WBAN) sensor nodes has been designed by Joonsung Bae, Kiseok Song, Hyungwoo Lee, Hyunwoo Cho, and Hoi-Jun Yoo.

In a WBAN, which consists of a hub node (base station) and several sensor nodes, body-worn sensor nodes periodically send physiological or multimedia data to a central hub node. According to the IEEE 802.15.6 standard, energy efficiency is a top priority for WBAN sensor nodes. The lifetime of sensor nodes is expected to extend to five years for implants and one week for a wearable device using limited energy resources, such as coin batteries. Among other design challenges is the need for a low-power transceiver with fast turn-on time with tight duty-cycle control between sleep and active modes.

To lower the cost of their device and its energy consumption, the researchers replaced the crystal oscillator with an injection-locking digitally controlled oscillator (DCO). By choosing a calibration method that uses an injection-locking detector, they found that the frequency drift of that DCO could be calibrated within 100-kHz accuracy over a 100° temperature variation. To satisfy WBAN requirements like quality of service (QoS), they adopted the double-FSK modulation scheme with a divider-based transmitter by a power-efficient switching modulator. Upon its fabrication, the resulting crystal-less, double-FSK, WBAN-compatible sensor-node transceiver consumed 1 and 2 mW in calibrating and transmitting modes, respectively, at a data rate to 10 Mb/s. It provided an 80-MHz reference source with 100-kHz accuracy by auto-calibrated digitally controlled oscillator. The transceiver, which was fabricated in 0.18-μm CMOS technology, measured 2.5 x 5.0 mm. See “A Low-Energy Crystal-Less Double-FSK Sensor Node Transceiver for Wireless Body-Area Network,” IEEE Journal Of Solid-State Circuits, Nov. 2012, p. 2678.