Second and Third-Order Noise Shaping Digital Quantizers for Low Phase Noise and Nonlinearity-Induced Spurious Tones in Fractional- $N$ PLLs.

Noise shaping digital quantizers, most commonly digital delta-sigma $(\Delta\Sigma)$ modulators, are used in fractional- $N$ phase-locked loops (PLLs) to enable fractional frequency tuning. Unfortunately, their quantization noise is subjected to nonlinear distortion because of the PLL's inevitable non-ideal analog circuit behavior, which induces spurious tones in the PLL's phase error. Successive requantizers have been proposed as $\Delta\Sigma$ modulator replacements with the advantage that they reduce the power of these spurious tones. However, the quantization noise from previously published successive requantizers is only first-order highpass shaped, so it usually causes more PLL phase noise than that from the second-order and third-order $\Delta\Sigma$ modulators commonly used in PLLs. This paper presents second-order and third-order successive requantizers to address this limitation. Additionally, successive requantizer design options are presented that result in either lower-power spurious tones or lower phase noise compared to $\Delta\Sigma$ modulators when used in PLLs. [ABSTRACT FROM PUBLISHER]