A 0.0043-mm2 0.3–1.2-V Frequency-Scalable Synthesized Fractional-N Digital PLL With a Speculative Dual-Referenced Interpolating TDC

This paper presents a synthesized fractional-N digital phase-locked loop (PLL) with a speculative dual-referenced interpolating time-to-digital converter (DI-TDC). The DI-TDC measures a fractional phase by referencing two adjacent quadrant boundaries which are given by a four-phase digitally controlled oscillator (DCO). It achieves a robust gain matching to the first order without need of any calibration. By predicting a time region of interest for the next TDC conversion, the power and area overheads for DI-TDC is minimized. Except for DCO and a reference delay chain, the PLL is implemented with register-transfer level (RTL) behavioral descriptions followed by an automated synthesis. It is fabricated in 28-nm CMOS with an active area of 0.0043 mm². The PLL shows a wide frequency lock range operating at a supply voltage from 0.3 to 1.2 V, achieving a stable figure-of-merit of better than -220 dB for a supply voltage above 0.6 V.