Minimum-Pulse Differential PWM for Switching Amplifier Interference
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Solution Overview
Problem
The integration of communication and audio sub-systems on a single chip in communication devices is hindered by frequency pulling, a phenomenon caused by mutual inductive coupling between the class-D amplifier and the oscillator, leading to spurious signals and interference in adjacent frequency bands, which violates data transmission standards.
Innovation Solution
A pulse width modulation scheme that generates differential pulse width modulated waveforms with a pre-determined non-zero minimum pulse width, reducing frequency pulling by ensuring the switching amplifier operates with a consistent duty cycle even during idling and non-idling conditions, thereby minimizing interference.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If the switching amplifier drives the modulated waveform to saturation and cut-off modes at high switching speed, then power efficiency is improved, but frequency pulling occurs in the oscillator due to mutual inductive coupling
Solution Approach 1:
The patent applies periodic action by using pulse width modulation (PWM) to generate periodic square wave signals at the amplifier inputs. The amplifier switches between saturation and cut-off modes periodically, creating controlled electromagnetic interference loops that drive the piezoelectric element rather than causing harmful frequency pulling. The periodic switching at fixed frequencies (e.g., 100 Hz and 125 Hz) transforms the harmful time-varying interference into a controlled periodic driving signal.
Solution Approach 2:
The patent changes the operating parameters of the amplifier by maintaining fixed duty cycles (50% or 25%/75%) regardless of input signal amplitude. This parameter change ensures the amplifier operates in clean switching modes between saturation and cut-off, avoiding the linear region where distortion and frequency pulling occur. The fixed parameter approach transforms the amplifier into a stable frequency source that drives the piezoelectric element without interfering with the oscillator.
2Loss of information
If the class-D amplifier operates with variable duty cycle modulation, then audio signal fidelity is improved, but time varying interference loops are created that vary the oscillator frequency
Solution Approach 1:
The patent segments the audio signal processing into two independent paths: (1) a modulation path that preserves audio fidelity through variable duty cycle PWM, and (2) a frequency stabilization path that uses fixed duty cycle square waves to drive the piezoelectric element. This segmentation allows the system to maintain both audio quality and oscillator stability simultaneously by separating the functions of audio reproduction and frequency control.
Solution Approach 2:
The patent introduces the piezoelectric element as an intermediary between the amplifier and the oscillator. The piezoelectric element converts the electrical square wave signals into mechanical vibrations that acoustically couple to the oscillator housing, providing passive noise cancellation without creating electrical interference loops. This intermediary transforms the harmful electromagnetic coupling into a benign mechanical coupling that does not affect oscillator frequency stability.
3Area of stationary object
If communication and audio sub-systems are integrated on a single chip, then device size and cost are reduced, but frequency pulling and spurious signals increase
Solution Approach 1:
The patent converts the harmful mutual inductive coupling between the audio and communication sub-systems into a beneficial acoustic noise cancellation mechanism. The audio sub-system's square wave output drives the piezoelectric element to generate sound waves that passively cancel oscillator noise in the frequency range of 20 Hz to 20 kHz. This approach transforms the previously harmful proximity coupling into a useful feature that improves overall system performance while enabling single-chip integration.
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
This solution significantly reduces frequency pulling in the oscillator without compromising the performance of the switching amplifier, ensuring compliance with data transmission standards and maintaining efficient data processing for low amplitude signals.
Implementation Method 1
the class-D amplifier drives the modulated waveform to saturation and cut-off modes at a high switching speed, generating a rectangular waveform with fast moving transition edges as an output
Implementation Method 2
These interference loops vary the operating frequency of the oscillator due to mutual inductive coupling. This phenomenon is referred to as frequency pulling.
Data Source
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AI summary
A device and a method for implementing pulse width modulation for switching amplifiers (120) is described herein. In one embodiment, the device includes a sampling signal generator (202) to generate a sampling signal (208) and a modulation unit (102) operatively coupled to the sampling signal generator (202). The modulation unit (102) generates differential pulse width modulated waveforms based on the sampling signal (208) and differential input signals (220-1 and 220-2) such that at least one differential pulse width modulated waveform has a duty cycle equivalent to a pre-determined non-zero minimum pulse width at all values of the differential input signals (220-1 and 220-2).