Low phase noise, high bandwidth frequency synthesis techniques

Abstract

A quantization noise reduction technique is proposed that allows fractional-N frequency synthesizers to achieve high closed loop bandwidth and low output phase noise simultaneously. Quantization induced phase noise is the bottleneck in state-of-the-art synthesizer design, and results in a noise-bandwidth tradeoff that typically limits closed loop synthesizer bandwidths to be <100kHz for adequate phase noise performance to be achieved. Using the proposed technique, quantization noise is reduced to the point where intrinsic noise sources (VCO, charge-pump, reference and PFD noise) ultimately limit noise performance. An analytical model that draws an analogy between fractional-N frequency synthesizers and MASH A digital-to-analog converters is proposed. Calculated performance of a synthesizer implementing the proposed quantization noise reduction techniques shows excellent agreement with simulation results of a behavioral model. Behavioral modeling techniques that progressively incorporate non-ideal circuit behavior based on SPICE level simulations are proposed. The critical circuits used to build the proposed synthesizer are presented.

(cont.) These include a divider retiming circuit that avoids meta-stability related to synchronizing an asynchronous signal, a timing mismatch compensation block used by a dual divider path PFD, and a unit element current source design for reduced output phase noise. Measurement results of a prototype 0.18/m CMOS synthesizer show that quantization noise is suppressed by 29dB when the proposed synthesizer architecture is compared to 2nd order EA frequency synthesizer. The 1MHz closed loop bandwidth allows the synthesizer to be modulated by up to 1Mb/s GMSK data for use as a transmitter with 1.8GHz and 900MHz outputs. The analytical model is used to back extract on-chip mismatch parameters that are not directly measurable. This represents a new analysis technique that is useful in the characterization of fractional-N frequency synthesizers.