A Digital Phase-Locked Loop With Calibrated Coarse and Stochastic Fine TDC

A coarse-fine time-to-digital converter (TDC) is presented with a calibrated coarse stage followed by a stochastic fine stage. On power-up, a calibration algorithm based on a code density test is used to minimize nonlinearities in the coarse TDC. By using a balanced mean method, the number of registers required for the calibration algorithm is reduced by 30%. Based upon the coarse TDC output, the appropriate clock signals are multiplexed into the stochastic fine TDC. The TDC is incorporated into a 1.99-2.5-GHz digital phase-locked loop (DPLL) in 0.13-μm CMOS. The DPLL consumes a total of 15.2 mW of which 4.4 mW are consumed in the TDC. Measurements show an in-band phase noise of -107 dBc/Hz which is equivalent to 4-ps TDC resolution, approximately an order of magnitude better than an inverter delay in this process technology. The integrated random jitter is 213 fs rms for a 2-GHz output carrier frequency with 700-kHz loop bandwidth. The calibration reduces worst-case spurs by 16 dB.

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