A high-resolution DLL-based digital-to-time converter for DDS applications

A Digital-to-Time Converter (DTC) based on a Delay-Locked Loop (DLL) for phase interpolation in Direct Digital Synthesis (DDS) applications is described. The conversion is made in two steps using digitally controllable delay cells with configurable shunt-capacitors load. The circuit is able to interpolate a 120 MHz clock, generating a delay proportional to an 8-bit digital control word with 32 ps resolution. The DDS system clock frequency is thus virtually enhanced up to about 30 GHz, achieving a strong reduction of the spurious component level. The 256 level interpolation is achieved using only 35 delay elements (excluding dummy cells).