Pulse Area Modulation for Linear RF Amplifier Harmonic Reduction
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Solution Overview
Problem
Conventional high-efficiency linear power amplifier systems using pulse width modulation (PWM) suffer from significant power losses and efficiency degradation due to the inability of high-Q band pass filters to perfectly eliminate harmonic components, which are generated adjacent to inband signals in the frequency domain.
Innovation Solution
The implementation of pulse area modulation (PAM) in a high-efficiency linear power amplifier system, where the area of the modulated signal is proportional to the amplitude of the input signal, reduces harmonic components without requiring a high-Q band pass filter, by varying both the width and height of the modulated pulse based on the input signal's amplitude, thereby reducing energy losses and enhancing efficiency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If pulse width modulation (PWM) is used to amplify signals with varying envelopes, then the power amplifier can operate in high-efficiency switching modes, but harmonic components are generated that require high-Q band pass filters to eliminate
Solution Approach 1:
The patent changes the modulation parameter from pulse width (PWM) to pulse area (PAM). By varying both the width and height of pulses such that the area is proportional to the envelope amplitude, the system maintains switching amplifier efficiency while significantly reducing harmonic components generated by PWM modulation.
Solution Approach 2:
Instead of using fixed-height pulses with variable width (PWM), the patent inverts the approach by using variable-height pulses with variable width (PAM), where the product of height and width (area) represents the envelope information. This inversion fundamentally changes the spectral characteristics and reduces harmonics.
2Manufacturing precision
If high-Q band pass filters are used to eliminate harmonic components from PWM output, then signal linearity is improved, but the system complexity and filter requirements increase
Solution Approach 1:
The patent extracts the harmful harmonic components at the source by changing the modulation technique from PWM to PAM. Instead of relying on filters to remove harmonics after generation, the modulation method itself prevents significant harmonic generation, eliminating the need for complex high-Q band pass filters.
3Manufacturing precision
If class A or class AB linear amplifiers are used to amplify signals with varying envelopes, then good linearity is achieved, but power efficiency deteriorates with great power losses
Solution Approach 1:
The patent segments the amplification process into two independent stages: envelope detection (separating amplitude information) and phase modulation (separating frequency/phase information). This segmentation allows the use of efficient switching amplifiers for the phase component while the envelope is handled through pulse area modulation, avoiding the need for inefficient linear amplification of the entire signal.
Data Source
AI summary
A linear power amplifier system using pulse area modulation includes: an envelop/phase decomposer for decomposing an input signal into an envelop signal and a phase signal; a pulse area modulator for modulating the envelop signal such that an area of the modulated envelop signal is proportional to an amplitude of the envelop signal; a control signal generator for converting the modulated envelop signal into a control signal; an automatic gain adjuster for equalizing pulse height of the modulated envelop signal; a mixer for mixing the phase signal with the output of the automatic gain adjustor to produce a RF pulse train; a power amplifier for amplifying the RF pulse train, to generate an amplified RF pulse train; and a band pass filter for restoring the original input signal from the amplified RF pulse train. The output level of the power amplifier is controlled by the control signal.