DC Offset Estimation in Direct-Conversion Receivers
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Solution Overview
Problem
Direct-conversion radio receivers face challenges in accurately processing continuous-wave signals due to DC offsets, which can be variable and interfere with the received signal, especially when receiving continuous-wave signals of the same frequency as the local oscillator, making it difficult to distinguish between the DC offset and the desired signal.
Innovation Solution
A method and apparatus that mix the continuous-wave radio-frequency signal with a local-oscillator signal offset by a frequency offset, allowing for the estimation of the DC offset component using frequency-offset data, enabling accurate compensation and reduction of DC offsets in the down-mixed signal, even in scenarios where the frequency offset is very small.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If a direct-conversion receiver mixes the received signal with a local oscillator signal at the same frequency, then the received signal is shifted directly to baseband, but a DC offset is produced that makes it difficult to distinguish between the DC offset and the desired signal
Solution Approach 1:
The patent applies periodic action by modulating the local oscillator signal with a known periodic sequence (e.g., pseudo-random code or sinusoidal waveform). This periodic modulation shifts the desired signal to a non-DC frequency while the DC offset remains at DC, enabling separation through frequency-domain processing or correlation with the known periodic sequence.
Solution Approach 2:
The patent introduces an intermediary periodic modulation signal that acts as a mediator between the local oscillator and the received signal. This intermediary signal transfers the desired signal to a different frequency domain while leaving the DC offset unchanged, allowing subsequent filtering or correlation to remove the DC component.
2Object-generated harmful factors
If high-pass filtering is used to remove DC offset, then the DC offset is reduced, but the desired continuous-wave signal is also removed
Solution Approach 1:
By periodically modulating the local oscillator signal, the desired continuous-wave signal is shifted to a non-DC frequency component. This allows the use of DC-blocking capacitors or high-pass filters to remove only the DC offset while preserving the modulated signal, which can later be demodulated to recover the original continuous-wave signal.
Solution Approach 2:
The patent changes the frequency parameter of the local oscillator signal by applying periodic modulation. This transforms the mixing product from a DC component to a time-varying signal at the modulation frequency, enabling separation from the static DC offset through frequency-based filtering.
3Measurement precision
If DC offset calibration is performed during radio silence, then DC offset can be measured, but the approach is not robust to interference, power changes, or amplifier gain changes
Solution Approach 1:
The patent employs feedback by correlating the received signal with the known periodic modulation sequence. This correlation process inherently rejects uncorrelated signals including interference and DC offset, providing robust estimation of the desired signal parameters without requiring separate calibration phases.
Solution Approach 2:
The system performs self-calibration by using the known periodic modulation sequence as a reference. The correlation between the received signal and the local copy of the modulation sequence automatically extracts the desired signal while rejecting DC offset and interference, eliminating the need for external calibration procedures.
4Object-generated harmful factors
If analog offset cancellation is used, then DC offset compensation is achieved, but the system becomes complex and requires regular calibration to cope with temperature changes
Solution Approach 1:
The patent replaces complex analog offset cancellation circuitry with digital signal processing. By periodically modulating the local oscillator and using digital correlation or frequency-domain processing, the system achieves DC offset rejection through software-based methods, eliminating the need for complex analog circuits and their associated calibration requirements.
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 allows for robust and rapid estimation of DC offsets, reducing errors from time-varying effects and interference, enabling accurate phase-based ranging and signal processing without the need for separate calibration phases, and can be performed within a single cycle period of the down-mixed signal.
Implementation Method 1
mixing the received radio-frequency signal with the periodic signal to generate a down-mixed signal
Data Source
AI summary
A radio system comprises a radio transmitter apparatus and a radio receiver apparatus. The radio transmitter apparatus is configured to transmit a continuous-wave radio-frequency signal having a first frequency. The radio receiver apparatus comprises: an antenna for receiving the continuous-wave radio-frequency signal; a local oscillator for generating a periodic signal at a second frequency which differs from the first frequency by a frequency offset; a mixer for mixing the received continuous-wave radio-frequency signal with the periodic signal to generate a down-mixed signal; and a processor or other circuitry configured to generate frequency-offset data from the down-mixed signal, wherein the frequency-offset data is representative of an estimate of the frequency offset. The processor or other circuitry is configured to use the frequency-offset data to generate DC-offset data representative of an estimate of a DC offset component of the down-mixed signal.