Sigma-Delta Modulator with Feedback Dithering for RF Fractional-N Frequency Synthesizer

A new sigma-delta modulator structure with feedback dither is proposed in this paper, which is simplified by full reusing the self-generating signal of conventional SDM, the filter reshapes this output signal, and feeds back into the conventional SDM as dithering. The validity of the elimination of idle tones with full input range is verified with an eight-bit digital MASH 1-1-1, by both simulation and ASIC implementation. And the circuit exhibits high potential for high frequency, low-power monolithic CMOS fractional-N frequency synthesizer.