Phase-Modulated Signal Demodulation Using Digital FLL and FFT
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Solution Overview
Problem
Existing methods for demodulating phase-modulated signals, such as those from optical displacement sensors, face challenges in continuous tracking of displacement signals while maintaining direction detection, often resulting in interruptions and loss of information due to discrete frequency measurements during periods of increasing and decreasing laser current.
Innovation Solution
A demodulating system utilizing a digital Frequency Locked Loop (FLL) with a controllable reference frequency generator, combined with a digital Fast Fourier Transformation (FFT) device and an analyzing device, which performs complex demodulation and adjusts the reference frequency to correctly lock onto the main signal component, allowing continuous tracking of displacement signals and direction detection.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If discrete frequency measurements are performed during periods of increasing and decreasing laser current, then direction detection is achieved, but continuous tracking of displacement signals is interrupted and information is lost
Solution Approach 1:
The patent implements continuous frequency estimation using a phase-locked loop (PLL) that operates without interruption throughout the entire laser current cycle, rather than performing discrete measurements only during increasing or decreasing current periods. This continuous operation eliminates tracking interruptions while maintaining direction detection capability through the PLL's ability to detect frequency sign.
Solution Approach 2:
The patent introduces an auxiliary frequency component with known frequency and phase that serves as a reference signal. This intermediary signal allows the system to continuously determine the sign of the Doppler frequency without interrupting the tracking, by comparing phase relationships between the auxiliary signal and the measurement signal throughout the entire current cycle.
2Speed
If a phase-locked loop with large bandwidth is used to initially lock onto signal frequency, then frequency acquisition is faster, but the PLL may lock onto incorrect frequency when multiple frequency components are present
Solution Approach 1:
The patent performs preliminary frequency estimation using a fast frequency estimator that operates before the PLL engagement. This preliminary estimation provides an accurate initial frequency value that guides the PLL to lock onto the correct frequency component, preventing false locking even when multiple frequency components are present in the signal.
Solution Approach 2:
The patent maintains continuous frequency estimation throughout the entire signal period using the fast frequency estimator, rather than performing discrete measurements. This continuous estimation provides uninterrupted accurate frequency information that guides the PLL correctly even in the presence of multiple frequency components.
3Measurement precision
If the laser current is triangularly modulated to enable direction detection through frequency peak analysis, then direction information can be derived, but the measurement process is interrupted regularly
Solution Approach 1:
The patent implements continuous direction detection by using a phase-locked loop that operates throughout the entire laser current cycle, rather than performing discrete frequency peak analysis only during specific periods. The PLL continuously tracks the signal phase and frequency, enabling uninterrupted direction determination while maintaining measurement productivity.
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
Enables reliable and continuous demodulation of phase-modulated signals, accurately determining the frequency and direction of movement without interruptions, even when the received signal contains multiple frequency components with small frequency distances.
Implementation Method 1
The laser device 2 receives an electric current, and as a result the laser 2 emits a laser beam 3 with a certain wavelength
Implementation Method 2
the light L0 within the laser medium 13 forms a standing wave. Likewise, light L1 and L3 in the external cavity forms a standing wave which, through the front mirror 11, interferes with the light L0 within the laser medium 13
Implementation Method 3
this light L5 has intensity fluctuations at a frequency fD that is proportional to the velocity of movement of the object 4 with respect to the laser 2, i.e. the component thereof along the optical axis. It should be clear that the measuring beam 5 can be detected by an optical sensor, and that its output signal can be processed by a signal processor in order to process these intensity fluctuations and to calculate the object velocity therefrom. It is noted that said frequency fD is equal to the Doppler frequency
Data Source
Figure 1~2
Figure 3A~3D
Figure 4~5
AI summary
A demodulating system (100) for demodulating a phase-modulated input signal (Si) comprises: a complex demodulator (110), having a first input (111) for receiving the phase-modulated input signal (Si) and being designed to perform complex multiplication of this signal with an approximation of the inverse of the phase modulation; a spectrum analyzing device (130) receiving the demodulated product signal produced by the complex demodulator (110) and capable of analyzing the frequency spectrum of the demodulated product signal.