A Low-ppm Digitally Controlled Crystal Oscillator Compensated by a New 0.19-mm2 Time-Domain Temperature Sensor

Abstract

A low-ppm digitally controlled crystal oscillator (DCXO) in a Pierce readout topology with a new 0.19-mm2 CMOS time-domain temperature sensor (TDTS) circuit for temperature compensation is designed and fabricated in this paper. The Pierce readout circuit is designed to output a 16-MHz oscillating signal as an important base clock for mobile devices. To cope with inevitable imprecision caused by environmental temperature variation, the readout should be compensated based on crystal’s frequency-temperature curve via a temperature sensor. To this end, a new low-power, small-sized, CMOS on-chip temperature sensor circuit is successfully synthesized by 20-stages of newly designed delay cells. Each delay cell is compensated by a CMOS-process-compatible varactor, enabling a TDTS consisting of only a low number of delay cells to generate long enough delay that is proportional to temperature and in required precisions. In addition to Pierce and TDTS circuits, the DCXO consists of the logics to compute capacitance correction, a switched-capacitor array to adjust Pierce capacitances, and regulators for DCXO and TDTS. The proposed circuits are fabricated by TSMC 0.18- $\mu \text{m}$ CMOS process, where the active area is 0.516 mm2 for the whole chip, while 0.19 mm2 for the TDTS. With temperature compensation enabled, the best measured frequency deviations of a calibrated TCXO is within ±0.2 ppm from −40 °C to 85 °C. The temperature resolution of the developed TDTS is successfully designed to reach 0.18 °C/LSB with an accuracy of ±1 °C validated by one of the tape-out chips.