Multiphase Fractional Divider for Low-Noise Frequency Synthesis
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Solution Overview
Problem
Fractional-N frequency synthesizers experience noise issues due to periodic changes in divider ratios, leading to frequency and phase jumps that result in undesirable spectral spikes, which are problematic for modern communications systems.
Innovation Solution
A frequency synthesizer circuit utilizing a multi-phase oscillator, delta-sigma modulator, and phase-frequency detector to select phases and control the divider ratio, reducing noise by synchronizing the output with selected phases and using a delta-sigma modulator to attenuate noise, thereby improving spectral purity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If a fractional-N frequency synthesizer uses periodic switching between divider ratios N and N+1 to achieve fractional division, then the desired fractional frequency synthesis is achieved, but periodic frequency and phase jumps occur causing spectral spikes and increased noise
Solution Approach 1:
The patent segments the frequency synthesis process by using a multi-phase oscillator that provides multiple phase outputs (e.g., 8 phases). Instead of switching divider ratios which causes jumps, the system selects different phase segments from the oscillator output based on the fractional requirement. This segmentation allows continuous phase selection without frequency jumps, resolving the contradiction between fractional synthesis capability and spectral purity.
Solution Approach 2:
The patent implements dynamic phase selection where the oscillator phase output is continuously adjusted based on the fractional-N requirement. The system dynamically selects which phase output to use and adjusts the divider ratio accordingly, creating a continuous rather than periodic switching mechanism. This dynamic approach eliminates the periodic jumps that cause spectral spikes while maintaining fractional frequency synthesis.
2Object-generated harmful factors
If a delta-sigma modulator is used to control divider ratio switching in a fractional-N synthesizer, then noise attenuation is achieved, but the complexity of the modulation control circuit increases
Solution Approach 1:
The patent introduces a delta-sigma modulator as an intermediary between the frequency synthesis requirement and the divider control. The modulator processes the fractional-N input and generates optimized control signals for both the divider ratio and oscillator phase selection. This intermediary component manages the complexity by consolidating control logic, achieving noise attenuation through its inherent filtering properties while simplifying the overall control architecture.
3Object-generated harmful factors
If the oscillator phase is selected and synchronized with the divider output, then frequency and phase continuity is improved reducing spectral spikes, but the requirement for precise phase synchronization increases circuit complexity
Solution Approach 1:
The patent implements preliminary phase preparation by providing multiple pre-generated phase outputs from the oscillator before division occurs. The desired phase is selected in advance from these pre-prepared outputs based on the fractional requirement, and the divider is synchronized to this pre-selected phase. This preliminary action ensures phase continuity without requiring complex real-time synchronization circuits, as the phase alignment is established before the division operation.
Data Source
AI summary
A frequency synthesis circuit is disclosed. The circuit includes a phase-locked loop and multi-phase oscillator such as a rotary traveling wave oscillator (RTWO). The oscillator provides a plurality of phases that are applied to a selection circuit. The selection circuit, in response to the output of a delta-sigma modulator, selects one of the phases of the multi-phase oscillator to minimize phase shift noise when the divider ratio in the loop changes, thereby eliminating a source of noise that contaminates the synthesized frequency. This permits the use of the frequency synthesis in applications requiring a high degree of spectral purity.