Laser Ranging Interferometer Signal Acquisition Using Wavelet Packet Decomposition
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Solution Overview
Problem
Current laser ranging interferometers face challenges in initial signal acquisition due to limited pull-in ranges of Phase-Locked-Loop (PLL) systems, which restrict frequency offset and require extensive FPGA resources for accurate frequency detection, making it difficult to implement a full phasemeter in a single FPGA.
Innovation Solution
A signal acquisition and distance variation measurement system using a frequency detection subsystem with wavelet packet decomposition and a phase/frequency detector PLL, allowing for discrete time signal processing and reducing FPGA resource usage, enabling accurate tuning of the PLL VCO frequency without FFT-based algorithms.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If FFT-based algorithms are used for frequency detection in signal acquisition, then frequency detection accuracy is improved, but FPGA resource consumption (logic elements and memory) increases significantly
Solution Approach 1:
The patent replaces the computational FFT-based frequency detection algorithm with a hardware-based phase-locked loop (PLL) system that uses wavelet packet decomposition for signal filtering. This substitution transitions from software-intensive frequency analysis to a hybrid hardware-software approach, reducing FPGA resource requirements while maintaining frequency detection capability through the PLL's natural frequency locking mechanism
Solution Approach 2:
The patent extracts the frequency detection function from the main PLL and implements it as a separate frequency detection subsystem using wavelet packet decomposition. This extracted subsystem pre-processes the signal to identify frequency components, allowing the main PLL to lock onto the correct frequency more efficiently without requiring the full computational power of FFT algorithms
2Adaptability or versatility
If the pull-in range of the PLL is increased to handle larger frequency offsets during signal acquisition, then signal acquisition capability is improved, but measurement precision deteriorates
Solution Approach 1:
The patent segments the frequency acquisition process into two distinct stages: a coarse acquisition stage using a wide-pull-in-range PLL with wavelet packet decomposition to quickly identify the frequency range, and a fine measurement stage using a narrow-pull-in-range PLL for precise phase measurements. This segmentation allows each stage to be optimized for its specific function without compromise
Solution Approach 2:
The patent implements dynamic switching between different PLL configurations during operation. The system transitions from a wide-pull-in-range mode during initial signal acquisition to a narrow-pull-in-range mode during precise measurement, allowing the system to adapt its characteristics based on the operational phase and signal conditions
3Adaptability or versatility
If a wide pull-in range is used in the PLL for signal acquisition, then frequency offset tolerance is improved, but lock time increases
Solution Approach 1:
The patent applies wavelet packet decomposition as a preliminary action before the PLL locking process. This pre-processing step decomposes the incoming signal into different frequency sub-bands, allowing the system to quickly identify which band contains the target signal. This preliminary frequency identification enables the PLL to start locking from a closer frequency point, significantly reducing lock time while maintaining wide frequency offset tolerance
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This system improves the initial signal acquisition phase by reducing FPGA logic element and memory requirements, allowing for accurate frequency detection and phase locking of laser signals, even at low SNRs, and enables operation in a broader frequency band with reduced faults.
Implementation Method 1
an acquisition subsystem with a wavelet packet decomposition unit and a phase/frequency detector PLL
Implementation Method 2
a phase/frequency detector PLL; a main PLL unit being coupled to the frequency detection subsystem
Implementation Method 3
the interference of a reference beam with a measurement beam allows performing range measurements with a precision equal to a fraction of the laser wavelength
Data Source
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AI summary
The invention refers to a signal acquisition and distance variation measurement system for laser ranging interferometers, in particular for use in a space application, having a signal acquisition unit. The signal acquisition unit comprises a frequency detection subsystem (100) for detecting the frequency of a measurement signal (MS) of a received laser beam which is buried in noise wherein the frequency detection subsystem (100) comprises an acquisition subsystem (110) with a wavelet packet decomposition unit (1) and a phase/frequency detector PLL (120); a main PLL unit (200) being coupled to the frequency detection subsystem (100) for receiving the detected frequency of the measurement signal (MS) for phase estimation of the measurement signal (MS); and a phasemeter band detection subsystem (300) for detecting whether the frequency of the measurement signal (MS) is higher or lower than the frequency of a reference signal of a reference laser beam by operating a known change to the frequency of the reference signal and measuring the consequent change in the frequency of an interference signal for having the phase of reference signal locked to the phase of the measurement signal.