Feedback Frequency Measurement Using DDS and Phase Detection
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Solution Overview
Problem
Current frequency measurement methods, such as direct counting and phase coincidence detection, suffer from counting errors, poor measurement stability, and long conversion times, limiting their precision and practical application in high-precision frequency measurement.
Innovation Solution
A phase frequency detector-based high-precision feedback frequency measurement apparatus and method utilizing a reference signal module, FPGA, DDS, phase frequency detector, charge pump, low-pass filter, and A/D converter, which enables precise frequency measurement through negative feedback and PID control, improving measurement precision and stability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If direct counting methods (M method, T method, M/T method) are used for frequency measurement, then measurement speed is high and the process is automatic, but counting error of ±1 word occurs reducing measurement precision
Solution Approach 1:
The patent implements a feedback mechanism where the measured frequency signal is fed back through a DDS (Direct Digital Synthesizer) to generate a reference signal, which is then compared with the original measured signal using a phase frequency detector. The detected phase difference is used to adjust the DDS frequency control word, creating a closed-loop feedback system that eliminates counting errors and achieves high-precision frequency measurement while maintaining automatic operation and high measurement speed.
2Measurement precision
If vernier method is used to overcome counting error, then measurement precision is improved, but design difficulty increases and high precision can only be maintained in short time
Solution Approach 1:
The patent replaces the complex mechanical vernier scale structure with an electronic feedback system using DDS and phase frequency detection. Instead of using physical vernier scales and mechanical interpolation mechanisms, the invention uses digital signal processing, phase-locked loop technology, and software-based frequency synthesis to achieve high precision, thereby eliminating design and manufacturing complexity while maintaining long-term stability.
3Measurement precision
If phase coincidence detection method is used to realize high-precision measurement, then measurement precision is improved, but gate time cannot be too short because phase coincidence point needs to be captured
Solution Approach 1:
The patent employs a self-adjusting feedback system where the DDS automatically generates a reference signal at the measured frequency, and the phase frequency detector continuously compares this reference with the original signal. The system self-corrects any frequency deviations in real-time, eliminating the need for manual phase coincidence point capture and allowing for shorter gate times while maintaining high measurement precision.
4Measurement precision
If analog interpolation method is used to improve measurement resolution, then measurement resolution is greatly improved, but conversion time increases and linearity deteriorates
Solution Approach 1:
The patent replaces analog interpolation circuits with digital signal processing methods. The phase frequency detector outputs a digital frequency difference value that is directly used to adjust the DDS frequency control word, eliminating the need for complex analog-to-digital conversion and interpolation calculations. This digital approach maintains high measurement resolution while significantly reducing conversion time and improving linearity.
Data Source
AI summary
A phase frequency detector-based high-precision feedback frequency measurement apparatus and method: a Field Programmable Gate Array (FGPA) roughly measures a frequency fx of a measured time-frequency pulse by an equal-precision frequency measurement method; a Direct Digital Synthesizer (DDS) automatically synthesizes a frequency fx′ according to the fx roughly measured by the FPGA; the fx and the fx′ are sent to a phase frequency detector for performing phase frequency detection and then sent to the FPGA after passing through a charge pump, a low-pass filter circuit, and an (Analogue-to-Digital) A/D converter; the FPGA processes a frequency difference obtained by the phase frequency detector and then transmits the processed frequency difference to the DDS to form a negative feedback frequency measurement system so that the DDS continuously adjusts the fx′ according to a frequency difference measurement result until the output of the DDS is stable. Therefore, precise measurement of the time-frequency pulse to be measured is realized.