Fractional-N Synthesizer Timing Shift for Lower LO Noise Correlation
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Solution Overview
Problem
In 5G cellular systems using millimeter waves, implementing local oscillator (LO) generation circuitry for beamforming systems is challenging due to stringent phase noise requirements and the need to distribute LO signals efficiently without excessive power consumption, while maintaining low phase noise correlation across different transceivers.
Innovation Solution
The use of a modulator circuit with memory elements to generate and shift a sequence of control words for frequency dividers in fractional-N frequency synthesizers, allowing for time-shifted synchronization of LO signals across different transceivers, reducing phase noise correlation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If fractional-N frequency synthesizers are used to generate LO signals with high frequency resolution, then frequency resolution is improved, but phase noise correlation between different transceivers increases
Solution Approach 1:
The patent applies asymmetry by introducing different time shifts to the modulator circuits of different fractional-N frequency synthesizers. While the synthesizers use identical modulator circuits (symmetry in design), the deliberate asymmetric time shifting creates uncorrelated phase noise sequences, solving the correlation problem while maintaining high frequency resolution.
Solution Approach 2:
The patent employs dynamics by making the time shift parameter variable and controllable for each synthesizer. The time shifts can be dynamically adjusted to optimize phase noise correlation reduction while maintaining frequency resolution, allowing the system to adapt to different operating conditions.
2Productivity
If LO signals are distributed to all transceivers in a beamforming system, then system range and capacity are improved, but power consumption increases
Solution Approach 1:
The patent segments the LO signal generation function by distributing fractional-N frequency synthesizers to each transceiver rather than using a centralized LO distribution system. This segmentation enables independent local generation, reducing the power required for signal distribution across the beamforming system while maintaining enhanced range and capacity.
Solution Approach 2:
Each transceiver becomes self-sufficient by generating its own LO signals locally through dedicated fractional-N frequency synthesizers. This self-service approach eliminates the need for power-hungry centralized LO distribution infrastructure while preserving the system's range and capacity benefits.
3Speed
If the loop bandwidth is widened to achieve fast frequency switching, then switching speed is improved, but phase noise performance deteriorates
Solution Approach 1:
The patent applies partial action by using multiple modulator circuits with different time shifts rather than a single wide-bandwidth modulator. Each modulator operates with a more moderate bandwidth, but the combined effect of multiple shifted sequences achieves the fast switching performance while maintaining better phase noise characteristics than a single wide-bandwidth design would provide.
Data Source
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AI summary
A fractional-N frequency synthesizer circuit (20, 20a-c) is disclosed. It comprises a frequency divider circuit (70) configured to receive a first oscillation signal having a first frequency, to receive a control word indicating a divisor, and to frequency divide the first oscillation signal with the divisor to generate a second oscillation signal having a second frequency, lower than the first frequency. It also comprises a modulator circuit (80) configured to generate a sequence of control words to the frequency divider circuit. The modulator circuit (80) comprises a set of memory elements (M, M1-MN), configured to store an internal state of the modulator circuit (80) in response to a first control signal and to restore the internal state of the modulator circuit (80) in response to a second control signal, thereby enabling a time shift of the sequence of control words. A communication circuit, a communication apparatus, and a method are also disclosed.