A 3-bit 4th-order Σ-Δ modulator with metal-connected multipliers for fractional-N frequency synthesizer

This paper presents a 3-bit 4^th-order Σ-Δ modulator for fractional-N frequency synthesizer. The modulator employs an interpolative architecture with multiple feedback paths and metal-connected multipliers to implement the feedback coefficients, resulting in a simple hardware complexity. A frequency synthesizer with the proposed modulator exhibits lower out-of-band phase noise, a frequency resolution of 6-Hz, and fast switching time. The experimental results show -82 dBc/Hz in-band phase noise and -140 dBc/Hz out-of-band phase noise at 1 MHz offset frequency. The fractional spurs are less than -80 dBc. A prototype has been implemented in 0.5-μm BiCMOS technology.