Digital Phase Lock System with Dithering Pulse-Width-Modulation Controller

摘要

A Digital Phase-Locked Loop (DPLL) has a digitally-controlled oscillator (DCO) that generates an output clock frequency determined by a digital input with most-significant-bits (MSB's) and a least-significant-bit (LSB). The LSB is generated by a Pulse-Width-Modulation (PWM) controller clocked by a control clock that is the output clock divided by C. A reference clock is compared to a feedback clock that is the output clock divided by M. The PWM controller generates M/C LSB's for each reference clock period and loads them in parallel to a parallel-to-serial shift register that serially delivers the LSBs. The pulse width is determined by a fine digital loop filter that filters phase comparison results using a fine time resolution. A coarse digital loop filter generates the MSB's from phase comparison results using a coarse time resolution. LSB waveforms are dithered by randomly selecting high-going or low-going pulses and randomly adjusting pulse widths.