In this article, we introduce a fractional-N all-digital phase-locked loop (ADPLL) architecture based on a single LC-tank, featuring an ultra-wide tuning range (TR) and optimized for ultra-low area in 10-nm FinFET CMOS. Underpinned by excellent switches in the FinFET technology, a high turn-on/off capacitance ratio of LC-tank switched capacitors, in addition to an adjustable magnetic coupling technique, yields almost an octave TR from 10.8 to 19.3GHz. A new method to compensate for the tracking-bank resolution can maintain its quantization noise level over this wide TR. A new scheme is adopted to overcome the metastability resolution problem in a fractional-N ADPLL operation. A low-complexity TDC gain estimator reduces the digital core area by progressive averaging and time-division multiplexing. Among the published fractional-N PLLs with an area smaller than 0.1mm2, this work achieves an rms jitter of 725fs in an internal fractional-N mode of ADPLL's phase detector (2.7-4.825GHz) yielding the best overall jitter figure-of-merit (FOM) of -232dB. This topology features small area (0.034mm2), wide TR (56.5%) and good supply noise rejection (1.8%/V), resulting in FOMs with normalized TR (FOMT) of -247dB, and normalized TR and area (FOMTA) of -262dB.

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The current paradigm of frequency synthesis for short-range wireless transceivers, such as BLE, is to use a crystal oscillator (XO) in the tens-of-MHz range as a frequency reference (FREF) to phase lock an RF oscillator [1-4]. This ensures a sufficiently wide PLL bandwidth of tens to hundreds of kHz to quickly acquire a new channel and to suppress lower-frequency phase noise (PN) of the RF oscillator. The latter requirement can be alleviated by substantially lowering the flicker PN of a digitally controlled oscillator (DCO) thus allowing to freeze its tuning word updates during receive (RX) packets and further directly FM-modulating the DCO during transmit (TX) packets [1]. However, an all-digital PLL (ADPLL) is still needed just to quickly settle the DCO to each new channel.

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In this paper, we introduce an all-digital phase-locked loop (ADPLL) for Bluetooth low energy (BLE) that eliminates the need for a crystal oscillator (XO) other than a 32.768-kHz real-time clock (RTC) already present in wireless systems. Specifically, we propose to replace the conventional channel settling with a band settling that would be carried out only once per global device power up. The ADPLL locks to the center of the Bluetooth band (2440 MHz) upon system power-up and jointly performs an instantaneous channel hopping and Gaussian frequency shift keying (GFSK) modulation in a two-point manner to overcome the narrow PLL bandwidth (BW) due to the 32.768-kHz reference. Extensive calibrations linearize the effective cubic digitally controlled oscillator (DCO) transfer function to achieve a precise frequency range of hopping and modulation. Realized in 16-nm FinFET, it consumes <1 mW at ≤0.45 V, while achieving best-in-class performance and <100-ns hopping time.

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Energy harvesting (EH) is a topic of intensive research promising battery-free operation of massive networks of wireless IoT devices. To simultaneously satisfy the EH and IoT, ultra-low-power (ULP) consumption with ultra-low-voltage (ULV) supply are required. Some, e.g., photovoltaic, EH output voltage (<0.3V) is below the threshold voltage (V_t) of even the most recent FinFET CMOS transistors, which makes it extremely challenging for circuit designers. Thus, new special circuit techniques are needed to make the IoT devices work properly with supplies below V_t.

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A tiny LC-tank-based ADPLL in 10nm FinFET CMOS achieves an area comparable to that of inverter-based ring-oscillator PLLs. A DCO occupying 0.016mm2 uses a controllable multi-turn magnetic coupling transformer to extend its tuning range to 10.8-19.3GHz (56.5%). A diversity of fine-tune capacitor banks limits the max/min step-size ratio to 2.3×. A new metastability-resolution scheme allows to use the frequency reference (FREF) clock directly instead of a retimed FREF (CKR) of conventional ADPLLs. A low-complexity estimator calculates inverse of the TDC. The fractional phase jitter (725fs) reaches sub-ps for the first time among PLLs of <;0.1mm2. Frequency pushing is 1.8%/V, which is at least 50× better than in traditional ring-type PLLs.@en