Optimal Modulator Architectures for Fractional- Frequency Synthesis

This paper presents a comparativestudy of mod- ulators for use in fractional- phase-locked loops. It proposes fa- vorable modulator architectures while taking into consideration notonlythequantizationnoiseofthemodulatorbutalsootherloop nonidealities such as the charge pump current mismatch that con- tributes to the degradation in the synthesized tone's phase noise. The proper choice of the modulator architecture is found to be de- pendent upon the extent of the nonideality, reference frequency, and loop bandwidth. Three modulator architectures are then pro- posed for low, medium, and high levels of nonidealities. Index Terms—Frequency synthesizers, phase-locked loops (PLLs), phase noise, modulation.

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