IEEE JOURNAL OF SOLID-STATE CIRCUITS,VOL. 46, NO. 5, MAY 2011 1049 A Sub-100 W MICS/ISM Band Transmitter Based on Injection-Locking and Frequency Multiplication Jagdish Pandey, Student Member, IEEE, and Brian P. Otis, Senior Member, IEEE Abstract—For fully autonomous implantable or body-worn devices running on harvested energy, the peak and average power dissipation of the radio transmitter must be minimized. Additionally, link symmetry must be maintained for peer-to-peer network applications. We propose a highly integrated 90 W 400 MHz MICS band transmitter with an output power of 20 W, leading to a 22% global efficiency—the highest reported to date for low-power MICS band systems. We introduce a new transmitter architecture based on cascaded multi-phase injec- tion locking and frequency multiplication to enable low power operation and high global efficiency. Our architecture eliminates slow phase/delay-locked loops for frequency synthesis and uses injection locking to achieve a settling time 250 ns permitting very aggressive duty cycling of the transmitter to conserve energy. At a data-rate of 200 kbps, the transmitter achieves an energy efficiency of 450 pJ/bit. Our 400 MHz local oscillator topology demonstrates a figure-of-merit of 204 dB while locked to a stable crystal reference. The transmitter occupies 0.04 mm of active die area in 130 nm CMOS and is fully integrated except for the crystal and the matching network. Index Terms—Injection-locking, frequency multiplication, ring oscillator, ultra-low power, medical implants, transmitter, efficiency. I. INTRODUCTION I N 1999, the FCC established the Medical Implant Com- munications Service (MICS) band to address the need for ubiquitous body area networks (BAN) comprising body-worn or implanted devices that continually sense vital body param- eters such as heart-rate, blood pressure, glucose, acceleration, and even brain signals [1]. Battery replacement for these de- vices may not be feasible or desirable, placing severe constraints on the power dissipation of the radio transceiver that tends to consume the bulk of total power. Other interesting applications enabled by ultra-low-power radios are long-range, long-dura- tion untethered animal tracking systems such as small bird flight recorders that require payloads less than one gram [2], [3]. High power consumption in the radio will lead to reduced battery life and an increased system weight typically dominated by the bat- tery weight (Fig. 1). Neuroscience research/brain machine inter- Manuscript received September 02, 2010; revised January 16, 2011; accepted January 21, 2011. Date of publication April 05, 2011; date of current version April 22, 2011. This paper was approved by Guest Editor Yuhua Cheng. J. Pandey is with Qualcomm Inc., San Diego, CA 92121 USA (e-mail: jnpandey@uw.edu). B. P. Otis is with the Department of Electrical Engineering, University of Washington, Seattle, WA 98195-3770 USA. Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JSSC.2011.2118030 faces for disorders such as epilepsy, animal-worn wireless sen- sors for social networking, animal psychology research [4], [5], and smart buildings with intelligent regulation of energy con- sumption are yet another class of emerging applications facing a common challenge: the need for a near complete energy au- tonomy [6]. The typical power harvested from common surroundings is on the order of 100 W/cm [6], [7]. Since the transmitter tends to dominate the total power budget, its power consump- tion needs to be below 100 W for always-on connectivity. However, the state-of-the-art ultra-low-power transmitters dissipate significantly higher power [8]–[10]. Both the peak and average power dissipation of the node must be minimized as the harvested energy sources/small batteries exhibit large source impedances. Therefore, high-data rate systems that exhibit very low energy per bit are typically not viable due to their high peak powers [11], [12]. There are two additional constraints that ultra-low-power ra- dios must satisfy. 1) For autonomous sensors in a peer-to-peer network, the radio link should be symmetric in terms of energy consumption. That is, the power/complexity burden cannot be shifted from transmitter to receiver and vice versa. Energy per transceived bit is a more meaningful evaluation for such sys- tems. 2) Ultra-low-power radio systems typically employ very aggressive duty cycling to conserve power. In such cases, the finite locking time of carrier generation loop places the upper limit on the duty cycling, and the start-up energy overhead be- gins to dominate the energy used in communication, signifi- cantly increasing energy per bit [13]. Fig. 2 shows the measured time domain power consumption of a commercial off-the-shelf transceiver wherein the phase-locked loop (PLL) lock time ex- ceeds the communication time period when the packet size be- comes relatively small. Additionally, FCC regulations in the MICS band currently specify a listen-before-talk (LBT) pro- tocol to avoid channel collision. However, as the number of de- vices operating in this band increases, the time spent in finding a clear channel may significantly increase, leading to higher drain on the battery. In the wake of this, FCC and other regula- tory agencies are considering low-power low-duty cycle (LP- LDC) protocol for interference mitigation wherein extremely agile duty cycling (0.1%) at very low output power (1 W) can be used without significant carrier-to-interference degradation from other such transmitter operating even in the same channel [14]. In such scenarios, the LO settling time should be made ex- tremely small. Finally, an important characteristic of a portable transmitter is a high global efficiency: the ratio of power delivered to the antenna to the total transmitter power consumption. In systems 0018-9200/$26.00 © 2011 IEEE