A Digital-to-Time Converter with Coupled Phase-Rotating LC Oscillators in 90-nm CMOS Technology

This paper presents the design and implementation of a digital-to-time converter with coupled phase-rotating LC oscillators. Unlike conventional quadrature signal generation where circuit symmetry is carefully maintained, in the proposed design, the coupling circuits are intentionally skewed for phase and timing adjustments, while the total phase delay through the coupling circuits is set properly to ensure resonant operation. With 7-bit control across a full cycle, the proposed design can provide unlimited adjustment range. A prototype is designed and implemented with a 90-nm CMOS process, and a phase-locked loop is adopted to stabilize the operating frequency. With the core oscillators operating at 9.87 GHz, measurement results show an effective timing resolution of 1.71 ps. Phase noise at 1 MHz and 10 MHz offset aie -117.7 dBc/Hz and -124.7 dBc/Hz, respectively. The entire DTC design consumes less than 50 mW at all output settings.

I. Introduction

Digital-to-time converters (DTCs) adjust output timing or timing delay with a digital code, similar to variable delay elements. DTCs have been used in many applications such as fractional-N phase-locked loops (PLLs) [1], sampling oscilloscopes [2], and radar systems [3]. In these applications, the timing adjustment resolution, total adjustment range, output signal quality, as well as power consumption, are the key design considerations.