Sigma-Delta Phase Detection Circuit for Noisy Low-Level Signals
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Solution Overview
Problem
Existing phase detection circuits are inaccurate in measuring phase differences between input signals obscured by noise and reference signals due to sensitivity to tolerances, linearity issues, and offset errors, particularly in electrothermal filters where self-heating is a concern and output signals are small and noisy.
Innovation Solution
A phase detection circuit utilizing a sigma-delta modulator with a digitizing circuit and feedback signal generator to determine phase differences between periodic and reference signals, incorporating choppers and integrators to reduce noise and offset errors, and selecting phases from a fixed set to generate a time-average phase measurement.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Temperature
If power dissipation is minimized to avoid self-heating in the substrate, then the filter's output signal becomes small and obscured by noise
Solution Approach 1:
The patent implements a feedback loop where the output signal is fed back through a phase detector and controlled oscillator to the input, creating a self-regulating system that maintains stable phase relationship despite noise and varying signal levels
Solution Approach 2:
The patent introduces an intermediary phase detector that measures phase difference between input and output signals without requiring high signal levels, and a controlled oscillator that generates a clean reference signal to facilitate accurate phase measurement
2Measurement precision
If a voltage-controlled phase shifter is used in the feedback loop, then phase detection is achieved, but the relationship between control voltage and phase difference is ill-defined due to tolerances and linearity issues
Solution Approach 1:
The patent uses a controlled oscillator to generate a copy of the reference signal with controllable phase, replacing the complex analog phase shifter with a simpler frequency-controlled oscillator that has better linearity and fewer tolerance issues
Solution Approach 2:
The patent replaces the mechanical/analog phase-shifting mechanism with an electronic oscillator-based system that achieves phase control through frequency modulation and feedback, eliminating mechanical tolerances and linearity problems
3Measurement precision
If a chopper is used in the feedback loop, then phase detection is achieved, but the circuit becomes sensitive to offset errors caused by asymmetric switching spikes
Solution Approach 1:
The patent extracts and removes the chopper from the feedback loop, replacing it with a controlled oscillator that generates the reference signal directly, thereby eliminating the source of asymmetric switching spikes and offset errors
Solution Approach 2:
The patent converts the potential harm of direct feedback into benefit by using the feedback signal to control the oscillator frequency, transforming a noisy chopper-based system into a clean oscillator-based system where any residual offsets are filtered by the oscillator's natural frequency stability
Data Source
AI summary
A phase detection circuit determines phase difference between a periodic signal and a reference signal of substantially equal frequency. The circuit includes: a source input receiving the periodic signal; a feedback signal generator providing a feedback signal (PFB) with substantially the same frequency as the reference signal; a phase difference circuit coupled to the source input node and a second signal input node coupled to the feedback signal generator, determining an error signal from phase difference between the periodic signal and PFB; an integrator circuit integrating the error signal into an integration signal; and a digitizing circuit digitizing the integration signal. The feedback signal generator is coupled to the digitizing circuit, providing PFB based on the digitized integration signal, and selecting the phase of PFB from a number of fixed phases. The phase detection circuit generates a time-average of the phase of PFB selected from the plurality of fixed phases.