Sigma-Delta Modulator Dithering for Spurious Tone Suppression
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Solution Overview
Problem
Sigma-delta modulators generate spurious tones in their output signals, particularly when processing dc or slowly varying inputs, which degrade signal quality and reduce signal-to-noise ratio, and existing dithering methods are ineffective in eliminating these tones without increasing noise floor.
Innovation Solution
A signal processor with a sigma-delta modulator that dithers the input signal with a pseudorandom noise sequence, up-converts the signal before processing, and down-converts the output, using a pseudorandom noise generator to spread and despread the signal, while filtering with a low-pass filter to reduce spurious tones.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-generated harmful factors
If dithering is applied to reduce spurious tones, then spurious tone levels are reduced, but noise floor increases
Solution Approach 1:
A pseudorandom noise sequence is introduced as an intermediary signal to modulate the input signal before processing. This intermediary dithering signal breaks up the periodic patterns that cause spurious tones, while the subsequent despreading operation recovers the original signal with reduced spurious content and controlled noise
Solution Approach 2:
The system changes the parameter of the input signal by modulating it with a pseudorandom noise sequence, transforming the signal representation in the time domain. This parameter change disrupts the correlation between the input signal and the modulator's internal states, thereby reducing spurious tone generation
2Measurement precision
If sigma-delta modulator processes dc or slowly varying inputs, then conversion resolution is maintained, but spurious tones are generated
Solution Approach 1:
The pseudorandom noise sequence is applied in advance to the input signal before it enters the sigma-delta modulator. This preliminary modulation action prevents the formation of periodic patterns that would otherwise occur with dc or slowly varying inputs, thereby preventing spurious tone generation while maintaining conversion resolution
3Productivity
If feedback process is used in sigma-delta modulator, then signal processing capability is improved, but spurious tones are generated through interaction with nonlinear or synchronized noise sources
Solution Approach 1:
The pseudorandom noise sequence acts as an intermediary that decouples the feedback process from the generation of spurious tones. By modulating the input signal with this random sequence, the feedback interactions no longer produce correlated spurious tones, as the random modulation breaks the synchronization between the feedback loop and noise sources
Data Source
AI summary
A signal processor includes a sigma-delta modulator. The input signal to the signal-delta modulator may contain a dc component that generates spurious tones in the modulator output. An input signal to the signal processor is multiplied by an output of a waveform generator to produce an up-converted signal prior to processing by the sigma-delta modulator. Preferably, the waveform generator produces a random signal, which may be a pseudorandom signal. However, other waveforms such as a bipolar binary waveform can also be used. The output of the waveform generator is delayed and multiplied by the output of the sigma-delta modulator to produce a down-converted signal with reduced spurious tones. The delay matches the delay of the sigma-delta modulator. The down-converted signal is filtered with a low-pass filter.