Sigma-Delta Modulator Impedance Shaping for Out-of-Band Peaking
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Solution Overview
Problem
Continuous-time sigma-delta modulators experience out-of-band peaking in signal transfer function, leading to reduced filtering and potential overload in wireless communications, especially when faced with strong interferers or blockers.
Innovation Solution
Incorporating a signal modulating device with an integrating circuit, resonating circuit, and impedance circuits to alter the location of zeros in the forward-path transfer function, shaping the signal transfer function and reducing or eliminating peaks through the use of frequency-independent and frequency-dependent components.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If feed-forward topology is used in continuous-time sigma-delta modulator, then internal signal swings are reduced and feedback digital-to-analog elements are minimized, but out-of-band peaking in signal transfer function increases
Solution Approach 1:
An impedance circuit is introduced as an intermediary element between the resonating circuit and the integrating circuit. This impedance circuit modifies the forward-path transfer function by adding a zero, which shapes the signal transfer function to reduce out-of-band peaking while preserving the low internal signal swing advantage of feed-forward topology
Solution Approach 2:
The impedance circuit changes the parameter distribution of the system by introducing a frequency-dependent impedance that creates a zero in the forward-path transfer function. This parameter modification effectively reshapes the STF to suppress out-of-band peaking without requiring feedback elements
2Reliability
If out-of-band peaking is present in signal transfer function, then filtering capability of baseband or intermediate-frequency filters is reduced, but this leads to degraded dynamic range of the modulator
Solution Approach 1:
The impedance circuit performs preliminary shaping of the signal transfer function before the signal reaches the baseband or intermediate-frequency filters. By pre-reducing out-of-band peaking through the zero introduced by the impedance circuit, the filtering burden on subsequent stages is reduced, preserving dynamic range
3Reliability
If larger out-of-band signals are present, then filtering is reduced, but this can overload the input of quantizer and lead to unstable modulator
Solution Approach 1:
The impedance circuit applies preliminary anti-action by introducing a zero that actively suppresses out-of-band signals before they can overload the quantizer input. This pre-suppression prevents potential instability without requiring additional feedback stabilization mechanisms
Data Source
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AI summary
A signal modulating device includes: an integrating circuit arranged to generate an integrated signal according to a scaled analog signal and a first feedback signal; a resonating circuit arranged to generate a resonating signal according to the integrated signal; a first signal converting circuit arranged to convert the resonating signal into a digital output signal; a second signal converting circuit arranged to convert the digital output signal into the first feedback signal; and a first impedance circuit having a first terminal receiving an analog signal and a second terminal coupled to the resonating circuit for altering the location of zeros in the forward-path transfer function and consequently shaping the STF of the signal modulating device; and a second impedance circuit having a first terminal receiving the analog signal and a second terminal coupled to the integrating circuit for generating the scaled analog signal.