A 700-μ A 405-MHz all-digital fractional-N frequency-locked loop for ISM band applications

Several wireless biomedical transceivers, including medical implants communication systems (MICSs), require ultra-low-power low-complexity frequency synthesizers. This paper presents an all-digital frequency-locked loop (ADFLL)-based frequency synthesizer with a built-in frequency-shift keying modulator for MICS and industrialscientificmedical band applications. Unlike all-digital phase-locked loops that rely on a power-hungry time to digital converter, the proposed ADFLL employs a high-resolution single-bit Σ Δ frequency discriminator in the feedback path and a noise-cancelling Σ Δ phase-accumulator-based frequency controller in the reference path, achieving fractional resolution with low power consumption. The loop compensation is implemented digitally using an infinite impulse response filter followed by a digital-intensive current-steering DAC driving a ring-oscillator-based voltage-controlled oscillator. The ADFLL achieves 9.5-Hz frequency resolution, spanning the ISM 400410-MHz band. A worst case near-integer spur of -62 dBc and a phase noise of -83 dBc/Hz at 300-kHz offset are measured. The ADFLL is fabricated on a 0.18-μm CMOS process, occupying a 0.14-mm² die area, with a quiescent current consumption of 700 μ A.