Elimination of quantization noise effects at the VCO input of wideband PLLs

An effective mean of eliminating the quantization noise effect at the voltage controlled oscillator input of phase-locked loops using digital filters is proposed in this paper. The solution relies on a simple all-digital sigma-delta modulator to lower the requirements on the digital to analog converter, while keeping the quantization noise effect well under the voltage controlled oscillator inherent phase noise. The proposed solution improves phase noise performances exponentially for lower frequencies and follows a typical sigma-delta noise transfer function for higher frequencies. Applied to a typical example, the solution shows a phase noise performance improvement of 24 dB, without any other modification than an estimated increase in logic complexity of 50 slices on a Xilinx FPGA for a second order phase-locked loop. This is less than 3% logic consumption for small versions of these chips.