Sigma-Delta Modulator Dither Shaping Without Extra Loop Headroom
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Solution Overview
Problem
Sigma delta modulators operating at low input values or with DC inputs suffer from tonal behavior, leading to degradation in signal-noise ratio (SNR), and the introduction of dither requires additional headroom in the loop filter, increasing power consumption.
Innovation Solution
Introducing dither at the input of the quantizer and removing it at the output, using first-order digital processing to shape the dither, which allows for increased power efficiency by avoiding the need for additional headroom in the loop filter.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If dither is added to the quantizer input to break tonal behavior, then signal quality is improved, but additional headroom in the loop filter is required, increasing power consumption
Solution Approach 1:
The patent segments the dither signal processing into two distinct parts: (1) adding dither at the quantizer input to break tonal behavior, and (2) subtracting dither at the quantizer output to remove it from the feedback loop. This segmentation allows the system to enjoy the benefits of dither for signal quality while avoiding the penalty of increased loop filter headroom requirements and power consumption.
Solution Approach 2:
The patent applies the principle of discarding and recovering by introducing dither noise into the system at the quantizer input, allowing it to serve its purpose of breaking tonal behavior, and then deliberately removing (discarding) the same dither signal at the quantizer output. This ensures the dither does not propagate through the feedback loop and consume additional power in the loop filter.
2Measurement precision
If dither is added to the feedback loop to improve signal quality, then tonal behavior is reduced, but loop filter headroom must be increased, requiring more power
Solution Approach 1:
The patent segments the dither signal path by adding dither only to the forward path at the quantizer input while preventing its entry into the feedback loop through subtraction at the quantizer output. This segmentation allows the system to achieve tonal behavior reduction without increasing loop filter headroom or complexity.
Solution Approach 2:
The patent extracts the harmful component (dither) from the feedback loop by subtracting it at the quantizer output. This extraction ensures that while dither serves its useful purpose at the quantizer input, it does not propagate into the feedback loop where it would require additional headroom and increase device complexity.
Data Source
AI summary
A sigma delta modulator may include a loop filter and an adder configured to accept an output of the loop filter and a dither input signal. The adder may be further configured to combine the output of the loop filter and the dither input signal into a combined output signal. The sigma delta modulator may further include a quantizer configured to accept the combined output signal from the adder, and quantize the combined signal into a quantizer output signal. The sigma delta modulator may further include a first subtractor configured to accept the quantizer output signal and subtract the dither input signal from the quantizer output signal.