Lock-In Reference Generation for Fast-Switching Gas Sensing
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Solution Overview
Problem
High sensitivity gas sensing systems face challenges with traditional phase-locked-loops (PLLs) in generating reliable local reference signals, especially at low gas densities and when switching between sensing paths, due to signal alignment and convergence issues, which affect noise reduction and measurement frequency.
Innovation Solution
A gas sensing system with a local reference generator that uses a configurable phase shifter to produce a phase-locked local reference signal at the second modulation frequency, doubling the frequency and applying a 90-degree shift, allowing for fast switching between sensing paths and maximizing DC values by scanning phases, using the same base clock for signal generation and detection.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a traditional phase-locked-loop (PLL) is used to generate the local reference signal, then the system can achieve frequency locking, but the convergence is slow and the reference signal has high jitter, reducing system measuring frequency
Solution Approach 1:
The patent replaces the traditional analog PLL mechanism with a digital signal processing approach using a numerically controlled oscillator (NCO) and digital phase-locked loop. This substitution enables faster convergence and lower jitter by utilizing digital computation instead of analog feedback mechanisms, directly resolving the contradiction between locking reliability and measuring frequency.
Solution Approach 2:
The patent dynamically adjusts the phase shift parameter of the local reference signal to maintain optimal alignment with the modulated laser signal. By continuously tuning the phase parameter based on feedback from the demodulated signal, the system achieves fast convergence and stable locking, eliminating the slow convergence and high jitter problems of traditional PLLs.
2Device complexity
If a simple PLL is used at the second harmonic frequency, then the circuit complexity is reduced, but the amplitude variation with gas density causes loss of signal and poor convergence
Solution Approach 1:
The patent introduces an intermediate signal processing stage that includes a band-pass filter centered at the second harmonic frequency and a phase shift network. This intermediary system prepares the signal by isolating the second harmonic component and adjusting its phase, making it suitable for reliable detection even when the amplitude varies with gas density. This resolves the contradiction by adding necessary processing complexity to maintain detection reliability.
Solution Approach 2:
The patent implements a feedback mechanism where the demodulated signal at the second harmonic frequency is fed back to adjust the phase of the local reference signal. This feedback loop continuously optimizes the phase alignment, ensuring reliable signal detection across varying gas densities. The feedback mechanism compensates for amplitude variations and maintains convergence, resolving the reliability issue while keeping the overall system manageable.
3Measurement precision
If the local reference signal frequency is doubled for second order demodulation, then the sensitivity to gas density improves, but the phase alignment becomes more difficult when switching between sensing paths
Solution Approach 1:
The patent performs preliminary phase alignment by storing the phase shift values for different sensing paths in a lookup table. Before switching paths, the system retrieves the appropriate pre-calculated phase shift value and applies it to the local reference signal. This preliminary preparation eliminates phase misalignment issues during path switching, maintaining ease of operation while enabling second order demodulation for high sensitivity.
Solution Approach 2:
The patent dynamically adjusts the phase shift parameter of the local reference signal based on the currently active sensing path. When a path switch occurs, the system dynamically reconfigures the phase shift to match the new path's requirements. This dynamic adaptation allows the system to maintain optimal phase alignment across different paths, resolving the contradiction between sensitivity and ease of operation.
4Stability of the object's composition
If a traditional PLL is used for phase locking, then the system can maintain frequency stability, but the convergence time increases when switching between sensing paths
Solution Approach 1:
The patent pre-calculates and stores the phase shift values for all possible sensing paths in a lookup table during system initialization. When a path switch is required, the system retrieves the appropriate phase shift value instantly from the table rather than performing time-consuming real-time convergence. This preliminary action maintains frequency stability while dramatically reducing convergence time, resolving the contradiction between stability and time loss.
Solution Approach 2:
The patent creates a digital copy of the phase-locked signal by using a numerically controlled oscillator that replicates the phase-locked waveform. This digital copy can be instantly switched between different path configurations without requiring physical reconfiguration or lengthy convergence periods. The copied signal maintains frequency stability while enabling rapid path switching, eliminating the time loss associated with traditional PLL convergence.
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 solution provides a reliable method for second-order lock-in amplifier detection, reduces reference signal jitter, and enables fast switching among different signal paths, improving sensitivity and tolerance to laser fluctuations.
Implementation Method 1
a first phase shifter that is configurable from 0 to 2π and a second phase shifter that shifts 90-degree
Implementation Method 2
a signal detector that uses lock-in detection to detect a second harmonic of the second periodic signal after absorption
Implementation Method 3
the detector uses lock-in amplifier to remove most of the noise while amplifies the signal... picks the second harmonic (i.e., 2× frequency which is 4πθ) of the second modulation signal
Implementation Method 4
The optical signal is generated from a tunable laser whose wavelength changes according to the amplitude of its driving power
Implementation Method 5
High sensitivity optical based gas sensing system detects the absorption of a modulated optical signal after passing it through the gas path... Optical path 106 has spectral-selective absorption for the selected wavelength in accordance with the gas of interest
Data Source
AI summary
A gas sensing system includes a signal generator including a wavelength tunable laser, the signal generator providing a first periodic signal and a second periodic signal, wherein the first periodic signal comprises a wavelength scanning signal and the second periodic signal comprises a modulation signal; an optical signal absorption path which is wavelength selective, wherein the generated signal covers at least one of the absorbance band; a signal detector that uses lock-in detection to detect a second harmonic of the second periodic signal after absorption, the signal detector further including a local reference generator, a multiplier, and a low pass filter; a local reference includes a first path (ref1) that outputs sinusoidal signal with frequency equals to that of the second signal in signal generator, and a second path (ref2) that outputs sinusoidal signal of two times ref1 frequency; and a local reference generator having a first phase shifter that is configurable from 0 to 2π and a second phase shifter that shifts 90-degree, wherein the first phase shifter is for an alignment of ref1 with the modulation signal and the second phase shifter provides 90-degree shifts for ref2 from ref1, wherein the first and second paths (ref1 and ref2) are selected by a switch, wherein the switch uses the first path during initialization and the second path for normal operation.