Notch Filter for PWM Audio Phase Preservation
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Solution Overview
Problem
Conventional audio pulse width modulation (PWM) amplifiers suffer from significant phase shifts and reduced sound quality due to the residual switching frequency energy, which degrades the listening experience and fails to meet high-definition audio standards.
Innovation Solution
A modified filter arrangement using a series inductor of 5µH and a shunt capacitor-inductor combination with a high Q factor, creating a notch filter to attenuate the switching frequency without additional phase shift, and additional filtering stages to achieve the desired level of attenuation and phase preservation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If a standard two-pole LC passive filter is used, then switching frequency attenuation is achieved, but significant phase shifts occur in the audio signal
Solution Approach 1:
The filter is divided into multiple independent notches, each targeting specific harmonic frequencies of the switching signal. This segmentation allows selective attenuation of switching harmonics while preserving the audio passband phase characteristics, as each notch is independently tuned to reject only its target frequency without affecting adjacent audio frequencies.
Solution Approach 2:
The filter introduces significant attenuation only at specific localized frequencies (switching harmonics) while maintaining minimal phase shift across the broader audio spectrum. Each notch filter is locally optimized to provide deep rejection at its target frequency while having minimal impact on surrounding audio frequencies, achieving frequency-selective harm reduction.
2Loss of information
If the series inductor value is reduced to minimize phase shift, then audio signal integrity improves, but switching frequency attenuation becomes insufficient
Solution Approach 1:
The filter uses multiple notches with different Q factors and bandwidths to achieve the required attenuation. By changing the parameters of multiple narrow notches rather than relying on a single wide filter, the solution achieves deep switching frequency rejection while keeping the inductor value low enough to minimize audio phase shift.
Solution Approach 2:
The filter combines multiple LC notches with different characteristics into a composite filter structure. This composite approach integrates the advantages of different notch configurations to achieve both high attenuation and low phase shift, which cannot be achieved with a single conventional LC section.
3Object-affected harmful factors
If multiple filtering stages are added to achieve high-definition audio standards, then switching frequency rejection improves, but device complexity increases
Solution Approach 1:
Multiple notch filters targeting different harmonic frequencies are merged into a single integrated filter stage. This combining approach achieves the attenuation performance of multiple cascaded filters while reducing component count and overall complexity, as the notches work together in parallel rather than requiring sequential filtering stages.
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 reduces phase shifts to levels comparable to analogue amplifiers, achieving 61.9dB rejection at the switching frequency while maintaining minimal impact on the audio signal integrity, thereby enhancing sound quality and meeting high-definition audio standards.
Implementation Method 1
A modified filter arrangement using a series inductor of 5µH and a shunt capacitor-inductor combination with a high Q factor, creating a notch filter to attenuate the switching frequency
Implementation Method 2
The output of the power stage 4 is filtered using a passive low pass filter 16 to filter out the switching frequency
Data Source
Figure 1~2
Figure 3
Figure 4
AI summary
An electronic apparatus is described. The apparatus includes a circuit element configured to output a signal comprising a modulated frequency component. The apparatus also includes a filter arrangement comprising first, second and third notch filter arrangements, wherein each of the first and second notch filter arrangements comprise a first series inductor, and a series shunt configuration comprising a second inductor and a capacitor coupled in series and the third notch filter arrangement comprises a series inductor and a shunt capacitor, wherein each of the notch filter arrangements are configured to generate a notch in a frequency response to attenuate the output signal at a frequency of the modulated frequency component.