Second and third-order successive requantizers for spurious tone reduction in low-noise fractional-N PLLs

This paper presents 2
^nd
- and 3
^rd
-order digital requantizers which can be used as drop-in replacements for digital delta-sigma modulators in analog fractional-N PLLs to reduce fractional spurs. The requantizers are demonstrated and compared to conventional delta-sigma modulators in a low-noise 3.35 GHz PLL IC and shown to offer significant reductions in worst-case spurious tones with similar phase noise relative to their deltasigma modulator counterparts.