A spur-reduction Delta-Sigma Modulator with efficient dithering for fractional frequency synthesizer

Abstract

A spur-reduction dither structure is presented to achieve low fractional spurs for a wideband frequency synthesizer. The dither structure is added to the digital multi-stage noise shaping (MASH) Delta-Sigma Modulator (DSM) for the wideband fractional-N frequency synthesizer. Traditionally, the MASH can generate spur tone in fractional frequency synthesizers. We propose to add a simple 8-bit linear feedback shift register (LFSR) to reduce spur tone. The MASH Structure is dithered using FPGA to reduce the spur. It improves in-band fractional spur about 13dBc/Hz at 50 kHz offset and achieves high frequency resolution of 0.001Hz.