Reducing Energy Dissipation in ULP Systems: PLL-Free FBAR-Based Fast Startup Transmitters

The energy dissipated by conventional phase-locked-loop-based transmitters (TXs) during the long startup phase is a major bottleneck that reduces the energy autonomy of duty-cycled ultra-low-power systems. In order to tackle this problem, this paper presents a loop-free TX based on a film-bulk acoustic wave resonator (FBAR) that has a 5- μs startup time and requires just 3- μs for channel switching. The presented TX takes advantage of the high-frequency stability of the FBAR digitally controlled oscillator (DCO) to operate in open loop mode. In order to avoid the problem of the same frequency stability that also hinders addressing multiple channels, the DCO output is mixed with a divided down version (which gives the IF) of itself. By adjusting the division ratio suitably, frequency of the system can then be tuned. To further relax the tuning requirements on the FBAR, the divider for producing the IF is implemented as a phase-switching divider with a step size of 0.2. Integrated in a 65-nm technology node, the TX can address all the bands within the 2.36-2.5-GHz frequency range viz. Medical body area network, industrial, scientific, and medical, and low-power active medical implant bands. Moreover, this TX is able to reach up to 16-Mb/s peak data rate while consuming 9.2 mA at 1.2 V.