Fractional Frequency Synthesizers With Low Order Time-Variant Digital Sigma-Delta Modulator

This paper proposes the use of a digital ΣΔ modulator with pseudorandom variation of coefficients. The method improves PLL-based Fractional Frequency Synthesizer´s performance while using a low order digital ΣΔ modulator. The time variant coefficients, in a low order digital ΣΔ modulator, significantly enlarge the pseudorandom output pattern period thus avoiding critical spur tones in the fractional synthesizer´s phase-noise. Behavioral simulations let us verify that the proposed technique operates better than a higher order modulator with additive dither. With a low order ΣΔ architecture, the programmable divider phase-selection logic and frequency range of operation parameters can be relaxed.