A 2.4-8 GHz Phase Rotator Delay-Locked Loop Using Cascading Structure for Direct Input-Output Phase Detection

This paper presents a phase rotator (PR)-based delay-locked loop (DLL) for a dynamic random-access memory interface in 28-nm CMOS technology. A direct input-output comparison using a sub-sampling technique reduces the effect of a timing mismatch of a replica delay line for synchronizing an input clock and output strobe clock. A divided clock samples the input clock for phase alignment. A 4-b analog-to-digital converter was used for input phase acquisition. The output phase is aligned with respect to the sampling clock phase using a bang-bang phase detector. Two DLLs for the input-output synchronization are implemented in a cascade structure, and the loop delay of the replica delay line is considerably reduced. In the proposed DLL, a PR delay line using an 8-phase clock generator is adopted for linearity to reduce the phase difference of the phase interpolation from π/2 to π/4. The resolution of the PR-DLL is 7-b of 2π, which consists of a 3-b coarse and a 4-b fine control. The DLL operates from 2.4 to 8 GHz, and the power dissipation at the maximum input frequency is 20.2 mW. The measured clock root mean square jitter was 1.68 ps. According to the simulation results, the phase mismatch between the input and output clocks was reduced by 80.5%.