Sigma-Delta Converter Calibration Using Out-of-Band Tone Injection
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Solution Overview
Problem
Higher order sigma-delta converters face challenges in maintaining accurate resonator frequencies due to manufacturing variations and temperature dependencies, which affect the signal to quantization noise ratio (SQNR) and require calibration.
Innovation Solution
A calibration system using tone injection, tone detection, and tone level ratio comparison is implemented, where tones are injected into the converter's frequency gaps, and their levels are used to adjust the resonator frequencies through a loop controller, allowing for start-up and background tracking calibration.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If resonator frequencies are set by passive devices (resistors and capacitors) or active components (transistors), then the converter can operate with basic functionality, but the resonator frequencies drift due to manufacturing variation and temperature dependency
Solution Approach 1:
The patent changes the resonator frequency parameter dynamically by injecting calibration tones at specific frequencies and adjusting the loop filter coefficients based on the measured tone levels. This allows the system to compensate for manufacturing variations and temperature drift by adjusting operational parameters rather than relying solely on fixed physical components.
Solution Approach 2:
The patent implements a feedback mechanism where the output of the sigma-delta converter is analyzed to detect calibration tones, and the detected tone levels are used to adjust the loop filter coefficients. This closed-loop feedback system continuously monitors and corrects resonator frequency drift, improving long-term stability and accuracy.
2Manufacturing precision
If calibration is performed to improve resonator frequency accuracy, then the SQNR improves, but the system complexity increases due to additional calibration circuitry and procedures
Solution Approach 1:
The patent makes the calibration system multi-functional by using the same signal path and output for both normal converter operation and calibration measurements. The calibration tones are injected through the same conversion path, and the output is used for both data conversion and calibration analysis, eliminating the need for separate dedicated calibration hardware.
Solution Approach 2:
The patent implements self-service calibration where the converter uses its own output to perform calibration measurements. The calibration tones are processed through the converter's own signal path, and the output contains both the calibration information and normal data, allowing the system to calibrate itself without external test equipment or separate measurement channels.
3Measurement precision
If tone injection is used for calibration, then the resonator frequency can be accurately determined, but the injected tones may interfere with the band of interest (BoI) signals
Solution Approach 1:
The patent applies local quality by selecting calibration tone frequencies that are specifically located outside the band of interest (BoI). The tones are injected at frequencies where they will not overlap with signal channels, ensuring that the calibration measurement can be performed accurately without interfering with normal signal processing in the BoI.
Solution Approach 2:
The patent segments the frequency spectrum by separating the calibration tone frequencies from the signal band frequencies. The calibration tones occupy specific frequency regions outside the BoI, while signal processing occurs in separate frequency regions, allowing both functions to operate simultaneously without interference.
Data Source
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AI summary
A digital conversion system including a sigma-delta converter, a tone generator that generates injects a tone signal into the conversion path of the sigma-delta converter at a frequency that is outside operating signal frequency range, a tone detector that isolates and detects a level of the injected tone signal and provides a corresponding tone level value, a tone ratio comparator that converts the tone level value into a tone level ratio and that compares the converted tone level ratio with an expected tone level ratio to provide an error signal, and a loop controller that converts the error signal to a correction signal to adjust a loop filter frequency the sigma-delta converter. Tones may be serially injected one at a time or simultaneously in parallel for determining a measured tone level ratio for comparison with a corresponding one of multiple stored expected tone level ratios.