FSK Receiver DFT Frequency Correction
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Solution Overview
Problem
Conventional FSK radio frequency signal receivers with high bandwidths face challenges in achieving high sensitivity due to frequency errors from inexpensive crystal oscillators, leading to incomplete data reception and high power consumption, especially when using narrowband filters.
Innovation Solution
A receiver that employs a discrete Fourier transform (DFT) on sampled intermediate signals to control and correct the frequency error, allowing for high-sensitivity demodulation by adjusting the local oscillator frequency, using a limited frequency band centered on the central frequency, and subsequent filtering with narrowband digital filters.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If a broadband transmission system with high intermediate frequency bandwidth is used, then the system can support high data rates, but the thermal noise power increases proportionally, reducing receiver sensitivity
Solution Approach 1:
The patent segments the frequency processing into two stages: first a broad bandwidth stage for signal acquisition, then a narrow bandwidth stage for sensitive demodulation. The DFT process divides the frequency analysis into discrete bins, allowing selective processing of only the relevant frequency components containing the FSK signal, thereby reducing the effective noise bandwidth while maintaining data rate capability.
2Ease of manufacture
If an inexpensive crystal oscillator with ±20 ppm frequency stability is used, then the system cost is reduced, but the frequency error of ±100 kHz causes intermediate signals to fall outside narrowband filter bands, preventing high-sensitivity reception
Solution Approach 1:
The patent implements feedback through the DFT-based frequency estimation process. The DFT analyzes the intermediate frequency signal to determine the actual frequency offset caused by the crystal oscillator error. This frequency information is then fed back to adjust the local oscillator frequency, closing the loop and compensating for the initial frequency error, enabling subsequent narrowband filtering and high-sensitivity demodulation.
Solution Approach 2:
The patent dynamically changes the local oscillator frequency parameter based on DFT analysis results. By adjusting the oscillator frequency to compensate for crystal drift, the system adapts to the actual frequency conditions, allowing the use of inexpensive crystals while maintaining the ability to center signals within narrowband filter passbands for high-sensitivity reception.
3Loss of energy
If narrowband bandpass filtering is applied to intermediate signals with frequency errors, then thermal noise is reduced improving sensitivity, but signals with frequency deviations fall outside the filter bands causing data loss
Solution Approach 1:
The patent performs preliminary frequency estimation using DFT before applying narrowband filtering. By analyzing the intermediate frequency signal spectrum in advance and identifying the actual signal frequency components, the system prepares the correct filter center frequencies. This preliminary action ensures that when narrowband filters are applied, they are properly aligned with the signal frequencies, preventing data loss while achieving noise reduction.
Data Source
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AI summary
The receiver (1) has a mixer block (4) mixing radiofrequency signals (FSK) and oscillating signals (So) to produce intermediate signals (INT). A sampler (10) samples the intermediate signals to provide sampled signals. A processing circuit (11) and a selector (12) perform discrete Fourier transform on the sampled signals to determine difference between frequency of a signal amplitude peak above a preset threshold and frequency in the intermediate signals to correct frequency of the oscillating signals and to demodulate data of the sampled signals in a high sensitivity demodulation stage (13). An independent claim is also included for a method for actuating a low power and high sensitivity radiofrequency signal receiver.