A Hybrid Spur Compensation Technique for Finite-Modulo Fractional-<i>N</i> Phase-Locked Loops

Abstract

A finite-modulo fractional-N PLL utilizing a low-bit high-order DeltaSigma modulator is presented. A 4-bit fourth-order DeltaSigma modulator not only performs non-dithered 16-modulo fractional-N operation but also offers less spur generation with negligible quantization noise. Further spur reduction is achieved by charge compensation in the voltage domain and phase interpolation in the time domain, which significantly relaxes the dynamic range requirement of the charge pump compensation current. A 1.8-2.6 GHz fractional-N PLL is implemented in 0.18 mum CMOS. By employing high-order deterministic DeltaSigma modulation and hybrid spur compensation, the spur level of less than -55 dBc is achieved when the ratio of the bandwidth to minimum frequency resolution is set to 1/4. The prototype PLL consumes 35.3 mW in which only 2.7 mW is consumed by the digital modulator and compensation circuits.