Time variant digital sigma-delta modulator for fractional-N frequency synthesizers

A digital sigma-delta modulator whose noise transfer function changes randomly in time is presented. The spectrum of the output bit-stream is tone free for any constant input value. The time-variant behaviour causes a white floor in the noise output spectrum, but the effect is negligible if the levels is kept well below other white noise sources. The scheme preserves the high frequency phase noise even at critical inputs. The hardware cost is affordable and the system complexity is lower than using dither.