Delta-Sigma Modulator Dithering for Idle Tone Decorrelation
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Solution Overview
Problem
Delta-sigma modulators experience significant idle tone interference due to their nonlinear nature, particularly when signals near the half of the sampling frequency interact with the bit stream, leading to undesirable intermodulation products.
Innovation Solution
Incorporating an extended Barker code generator within the modulator loop, which produces a signal with low autocorrelation values, thereby reducing the correlation between idle tones and other signals, and dynamically switching this code on only for low input signals to minimize operational impact.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a single-bit quantizer is used in the delta-sigma modulator, then the inherent linearity of the feedback digital-to-analog converter is improved, but large idle tones are produced near the half of the sampling frequency due to the strongly nonlinear system
Solution Approach 1:
A dither signal is introduced as an intermediary element into the quantizer input. This dither signal acts as a mediator that breaks up the deterministic limit cycles causing idle tones, converting them into random noise that can be filtered more effectively. The dither signal is added to the quantizer input before quantization, modifying the quantization process to eliminate the harmful periodic idle tones while preserving the linearity benefit of the single-bit feedback DAC.
2Object-generated harmful factors
If the sampling frequency is increased to push idle tones higher, then the idle tones move away from the baseband, but the processing bandwidth and complexity increase
Solution Approach 1:
The order of the noise shaping filter is increased as a parameter change to achieve better suppression of idle tones without necessarily increasing the sampling frequency. By increasing the filter order, the system achieves steeper roll-off characteristics that more effectively attenuate the idle tones in the baseband region, allowing for lower sampling frequencies while maintaining the same level of idle tone suppression.
3Object-generated harmful factors
If dithering is applied to reduce idle tones, then the idle tone content is reduced, but the signal-to-noise ratio deteriorates due to additional noise
Solution Approach 1:
Different dithering strategies are applied to different parts of the signal spectrum. Low-frequency dithering is used to address idle tones in the baseband region, while high-frequency dithering is applied to suppress idle tones near the Nyquist frequency. This localized approach allows each dithering component to target specific frequency regions, minimizing the overall noise impact on the signal-to-noise ratio while effectively reducing idle tones across the spectrum.
Data Source
Figure 1A~1B
Figure 1C~2
Figure 3A~3C
AI summary
A delta-sigma modulator (10) comprises a modulator loop (11) and a code generator (12). The modulator loop (11) comprises a loop filter (18). The code generator (12) is configured to generate a generator signal (BS) that is realized as an extended Barker code. The code generator (12) comprises a generator output (23) that is coupled to the loop filter (18).