A ${\hbox{700-}}\mu{\hbox {A}}$ 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 industrial-scientific-medical 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 ΣΔ frequency discriminator in the feedback path and a noise-cancelling ΣΔ 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 400-410-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-mm
²
die area, with a quiescent current consumption of 700 μA.