Phase accumulator with improved accuracy

Abstract

A PLL includes a controlled oscillator, a phase accumulator to measure the controlled oscillator output phase, a phase predictor to calculate the required output phase, and a phase subtractor to calculate the phase difference or phase error. The phase accumulator includes a counter whose output sequence changes only one bit per counted controlled oscillator output cycle, such as a Gray counter. It further includes a register or latches, which sample(s) the counter output value upon receiving a reference clock pulse. The latches output value represents the measured phase. A binary encoder, such as a Gray-to-binary converter, may translate the measured phase to a binary number. The phase accumulator may further include a delay line, second latches, and a delay line decoder to measure a fractional part of the phase. A calibration feedback loop may keep the number of delay line steps per output clock pulse known and stable.