A Differential Digitally Controlled Crystal Oscillator With a 14-Bit Tuning Resolution and Sine Wave Outputs for Cellular Applications Y uyu Chang

Abstract

This paper describes the de sign topologies and con- siderations of a differential sinusoi dal-output digitally controlled crystal oscillator (DCXO) intende d for use in cellular applications. T h eo s c i l l a t o rh a safine-tuning range of 44 ppm, approximately 14 bits of resolution, and an average step size of 0.005 ppm. All sig- nals connecting externally to I/O pins are sine waves for reducing noise, interference, and spurs couplings. The 26 MHz DCXO fabri- cated in 65 nm CMOS achieves a phase noise of 149.1 dBc/Hz at 10 kHz offset measured at the sine wave buffer output. The DCXO is capable of meeting the stringent phase noise requirements for IEEE 802.11n 5 GHz WLAN devices. A typical frequency pulling of 0.01 ppm due to turning on/off the sine wave buffer is measured. The DCXO dissipates 1.2 mA of current, whereas each sine wave output buffer draws 1.4 mA. The DXCO occupies a total silicon area of 0.15 mm .