FM Reception DC Offset Correction via Phase Variation
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Solution Overview
Problem
Direct conversion FM reception devices face challenges in distinguishing between received signals and DC offsets, leading to degraded reception properties when the received signal is unmodulated and has a frequency identical to the local oscillation signal, resulting in incorrect correction and failure to obtain accurate detection signals.
Innovation Solution
The FM reception device employs a local oscillator, quadrature detection unit, correction unit, DC offset detection unit, FM detection unit, AFC unit, and selection unit to correct the I-phase and Q-phase base band signals, add an offset to the detection signal, and control the local oscillation frequency, ensuring the phase component varies and preventing lock to a constant value, thus enabling effective DC offset correction.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If DC offset correction is performed using the base band Lissajous waveform characteristic, then reception properties are improved, but when the received signal is unmodulated and has the same frequency as the local oscillation signal, the I-phase and Q-phase base band signals remain at constant values, making it impossible to distinguish between the received signal and DC offset, resulting in incorrect correction
Solution Approach 1:
The patent applies preliminary action by performing rectangular to polar conversion on the I-phase and Q-phase base band signals before DC offset detection. This preliminary transformation converts the constant value signals into phase signals that vary with time, enabling the subsequent DC offset detection to distinguish between the received signal and DC offset even when the received signal is unmodulated and has the same frequency as the local oscillation signal.
Solution Approach 2:
The patent changes the parameter representation from rectangular coordinates (I-phase and Q-phase base band signals) to polar coordinates (amplitude and phase signals). This parameter change allows the system to detect DC offset accurately by analyzing the phase variations over time, resolving the ambiguity between received signal and DC offset in the constant value case.
2Stability of the object's composition
If the received signal is unmodulated and has the same frequency as the local oscillation signal, then the I-phase and Q-phase base band signals remain at constant values, but this makes it impossible to distinguish between the received signal and DC offset, causing the corrected signals to become zero
Solution Approach 1:
The patent performs rectangular to polar conversion as a preliminary action before DC offset detection. This transformation converts the constant value signals into phase signals that vary with time, enabling the system to distinguish between the received signal and DC offset even when the received signal is unmodulated and has the same frequency as the local oscillation signal.
Solution Approach 2:
The patent transitions from two-dimensional rectangular coordinates (I and Q components) to polar coordinates (amplitude and phase). This dimensional change adds the phase dimension, allowing the system to detect variations over time that are not present in the rectangular representation, thereby resolving the information loss problem.
3Ease of manufacture
If rectangular to polar conversion is performed on constant I-phase and Q-phase signals, then phase signals are obtained, but these signals remain at constant values and are invariably grouped in a single phase domain
Solution Approach 1:
The patent performs rectangular to polar conversion as a preliminary action that enables subsequent phase domain grouping. By converting to polar coordinates first, the system can then properly group the phase signals into multiple phase domains based on their temporal variations, achieving both processing capability and adaptive phase domain distribution.
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 approach prevents malfunction by allowing the DC offset detection unit to correct only the DC offset voltage, even when the received signal is unmodulated and has the same frequency as the local oscillation signal, thereby maintaining reception properties and avoiding signal suppression.
Implementation Method 1
a quadrature detection unit that subjects an FM signal to quadrature detection using the local oscillation signal output from the local oscillator and outputs an I-phase base band signal and a Q-phase base band signal
Implementation Method 2
If the received signal is unmodulated and the frequency thereof becomes identical to that of the local oscillation signal while a DC offset is being detected by utilizing the characteristic of the base band Lissajous waveform of a constant envelope modulation scheme
Implementation Method 3
an AFC unit that generates a control signal for controlling a frequency of the local oscillation signal based on the detection signal to which the offset is added and feeds back the control signal to the local oscillator
Data Source
AI summary
A quadrature detection unit subjects an FM signal to quadrature detection using a local oscillation signal and outputs a base band signal. A first correction unit and a second correction unit correct the base band signal using a DC offset correction value. A DC offset detection unit subjects the corrected base band signal to rectangular to polar conversion and derives the DC offset correction value such that amplitudes in a plurality of phase domains defined in an IQ plane approximate each other. An FM detection unit subjects the corrected base band signal to FM detection and generates a detection signal. An addition unit adds an offset to the detection signal. An AFC unit generates a control signal for controlling a frequency of a local oscillation signal based on the detection signal to which the offset is added.