Comparison of DTC Segmentation Methods in Fractional-N Frequency Synthesizers

Digital-to-time converters (DTC’s) that are based on a controllable delay line are used in fractional-N frequency synthesizers to cancel the quantization error (QE) introduced by the divider controller in order to recover the integer-N phase noise performance. To relax the control complexity and reduce the hardware demand, the DTC delay line can be segmented into a cascade of coarse and fine components. This paper compares two state-of-the-art DTC segmentation topologies and their control methods. We will show that the convergence of the least-mean-square algorithm auto-calibrating the DTC’s gain is an important issue that affects the success of the DTC-based QE cancellation when small fractional frequency control words are used. Behavioral simulations analyzed in the time and frequency domains underpin our study.