Fractional-N PLL Synchronization Using Dual DSM Alignment
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Solution Overview
Problem
Synchronizing large arrays of fractional-N phase-locked loops (PLLs) is challenging due to their fractional nature, leading to difficulties in aligning with a reference clock, which impairs beamforming and reduces operational throughput in wireless communication systems.
Innovation Solution
A dual delta sigma modulator (DSM) system is used to rapidly synchronize fractional-N PLLs by transitioning from coarse to fine alignment, accommodating variations in loop filter bandwidth and temperature, voltage, and process variations, allowing for compact and power-efficient synchronization of multiple PLLs within 120 microseconds.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If traditional synchronization methods are used for fractional-N PLLs, then the PLLs can operate independently, but synchronization accuracy deteriorates due to fractional nature making alignment with reference clock difficult
Solution Approach 1:
The synchronization process is divided into two distinct segments: coarse alignment phase and fine alignment phase. The coarse alignment uses a first delta-sigma modulator to achieve initial synchronization, then transitions to a second delta-sigma modulator for fine alignment. This segmentation resolves the contradiction by making the complex fractional-N PLL synchronization manageable through staged approach, improving synchronization accuracy while controlling complexity.
Solution Approach 2:
The system performs preliminary coarse alignment using the first delta-sigma modulator before transitioning to fine alignment with the second modulator. This preliminary action brings the PLL close to the reference clock frequency and phase initially, making the subsequent fine alignment more effective and reducing the overall synchronization time, thus improving reliability without excessive complexity.
2Productivity
If fast synchronization is implemented, then operational throughput improves, but synchronization precision may deteriorate due to reduced alignment time
Solution Approach 1:
The alignment process is segmented into coarse alignment (fast, less precise) and fine alignment (slower, highly precise). The coarse alignment quickly brings the PLL near the reference clock to enable fast operational throughput, while the subsequent fine alignment refines the synchronization precision. This segmentation allows the system to achieve both fast throughput and high precision alignment.
Solution Approach 2:
The system maintains continuous useful action by transitioning smoothly from coarse to fine alignment without interruption. The dual delta-sigma modulator architecture ensures that the PLL remains locked throughout the transition, maintaining continuous synchronization while improving precision, thus preserving operational throughput while enhancing alignment precision.
3Reliability
If multiple PLLs are synchronized tightly, then beamforming effectiveness improves, but synchronization time increases due to the complexity of aligning multiple fractional-N PLLs
Solution Approach 1:
The dual delta-sigma modulator synchronization system serves as a universal solution for synchronizing multiple fractional-N PLLs simultaneously. Both PLLs use the same two-stage synchronization mechanism, allowing tight synchronization across multiple devices. The multi-functional approach enables the system to achieve effective beamforming across multiple transceivers while managing synchronization time through the efficient coarse-to-fine alignment process.
Data Source
AI summary
An array of devices, such as transceivers on a satellite, each use a phase locked loop (PLL) system to maintain a local oscillator at a particular frequency that is synchronized to a reference clock signal (RCS), maintaining tight timing discipline among the PLL systems. Each PLL system includes a first delta sigma modulator (DSM) and a second DSM. During a first time, a divider uses output from the first DSM to divide output from a voltage controlled oscillator of the PLL system. The output from the divider is provided as feedback to a phase frequency detector (PFD) of the PLL system and is also provided to the clock input of the first DSM. The PFD accepts as input the RCS. The second DSM uses the RCS as clock input. At a second time, the PLL system transitions from the divider using output from the first DSM to the second DSM.