PWM Loop Filter Phase Shaping for Minimum Aliasing Error
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Solution Overview
Problem
Class D amplifiers suffer from distortion due to high frequency residuals in their feedback loops, limiting their application in high-quality output applications such as high-end audio amplifiers.
Innovation Solution
A loop filter with a transfer function having no real part and a dominant pole placed above the upper frequency range is used to minimize aliasing errors, shaping the feedback signal to resemble an ideal triangular waveform and reducing total harmonic distortion (THD).
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If a typical lowpass filter is used in the feedback loop of a class D amplifier, then the switching frequency components are attenuated, but high frequency residuals and aliasing errors remain in the feedback signal causing distortion
Solution Approach 1:
The patent changes the phase response parameter of the loop filter to achieve a specific phase characteristic (phase response of substantially -90 degrees - N*180 degrees) that minimizes aliasing errors. This parameter change transforms the filter's transfer function to have substantially no real part, which eliminates the harmful aliasing effect while maintaining signal quality.
Solution Approach 2:
The loop filter is designed to pre-shape the feedback signal before it enters the summation node, transforming it into an ideal triangular waveform. This preliminary shaping action prevents aliasing errors from occurring in the first place, rather than attempting to correct them afterward.
2Measurement precision
If the switching frequency is increased to capture more signal information, then the sampling accuracy improves, but the aliasing error and distortion in the output signal increases
Solution Approach 1:
The patent changes the phase response parameter of the loop filter to achieve a specific phase characteristic (phase response of substantially -90 degrees - N*180 degrees) that minimizes aliasing errors. This parameter change transforms the filter's transfer function to have substantially no real part, which eliminates the harmful aliasing effect while maintaining signal quality.
3Device complexity
If a conventional loop filter design is used, then the circuit implementation is simple, but the total harmonic distortion (THD) remains high due to improper phase response
Solution Approach 1:
The patent changes the phase response parameter of the loop filter to achieve a specific phase characteristic (phase response of substantially -90 degrees - N*180 degrees) that minimizes aliasing errors. This parameter change transforms the filter's transfer function to have substantially no real part, which eliminates the harmful aliasing effect while maintaining signal quality.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The solution effectively minimizes aliasing errors and THD, enhancing the linearity of the modulation and improving the quality of the output signal in PWM-based amplifiers.
Implementation Method 1
The loop filter has a phase response of substantially -90 degrees-N*180 degrees, where N is an integer value. The loop filter has a dominant pole placed substantially at or above an upper frequency in the frequency range of interest for the loop filter.
Data Source
AI summary
One embodiment of an apparatus for filtering an electrical signal includes a loop filter with an input and an output that applies a transfer function to a signal at the input. The transfer function has substantially no real part. The loop filter has a dominant pole placed substantially at or above an upper frequency in the frequency range of interest for the loop filter.