A 1.3–4-GHz Quadrature-Phase Digital DLL Using Sequential Delay Control and Reconfigurable Delay Line

A 1.3–4-GHz quadrature-phase digital delay-locked loop (DDLL) with sequential delay control and a reconfigurable delay line is designed using a 28 nm CMOS process. The time resolution of the DDLL is reduced by updating the delay code sequentially. A bidirectional shift register enables this operation with low power, resulting in bang-bang jitter that is three times smaller than that of a conventional DDLL. Conventional delay control is replaced with sequential delay control after a DDLL lock to reduce the locking time. A DDLL with a wide operation range is achieved with a reconfigurable delay line. Unlike the conventional DDLL, the minimum delay difference is adjustable in the proposed structure. To achieve a wide frequency range, the minimum delay difference of the quadrature clock is increased or decreased in three operation modes. To compensate for local variations in the CMOS process, a skew calibration circuit is implemented with the DDLL. The hardware cost of skew calibration is minimized with the proposed DDLL because it shares the subblocks for sequential delay control. The average phase difference from the quadrature clocks becomes the reference for the 90° phase for skew correction. A duty-cycle corrector (DCC) is implemented by collecting the positive edges of the quadrature-phase clocks. The DDLL consumes 6.5 mW at the maximum clock frequency of 4 GHz. The peak-to-peak jitter is improved from 15.6 to 12.5 ps with sequential delay control.