Power Encoder Pre-Distortion for PWM Linearity and Efficiency
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Solution Overview
Problem
Conventional direct digital-RF transmitters using delta-sigma modulators face challenges in achieving high power coding efficiency due to noise shaping, leading to power inefficiency and linearity issues, especially when encoding non-constant envelope signals, and existing linearization methods are limited by the complexity of inverse function derivation for multi-level encoding.
Innovation Solution
A look-up-table (LUT) based digital pre-distortion method is employed to compensate for the nonlinearity of pulse width modulators, allowing for experimental determination of non-linear mappings and pre-distortion of input signals to achieve linear output, applicable to multi-level PWM encoders without restrictions on level complexity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If delta-sigma modulator is used for power encoding, then in-band signal-to-noise ratio is improved, but power coding efficiency deteriorates
Solution Approach 1:
The patent changes the fundamental parameter of the power encoding scheme from delta-sigma modulation to pulse-width modulation (PWM). This parameter change fundamentally alters the noise characteristics and power efficiency properties of the system. PWM encoding directly generates pulse widths proportional to the input signal amplitude without noise shaping, thereby achieving both high power coding efficiency and acceptable in-band noise performance.
2Use of energy by moving object
If pulse-width modulation is used for power encoding, then power coding efficiency is improved, but linearity performance deteriorates
Solution Approach 1:
The patent applies preliminary action by introducing a pre-distortion block before the PWM encoder. This pre-distortion block pre-compensates for the inherent nonlinearity of the PWM quantization process. By applying the inverse of the PWM transfer function to the input signal before encoding, the system achieves linear overall performance while maintaining the power efficiency benefits of PWM.
Solution Approach 2:
The patent implements feedback through an iterative pre-distortion algorithm that measures the actual output of the PWM encoder and adjusts the pre-distortion parameters accordingly. This closed-loop approach allows the system to adapt to variations in the PWM characteristics and achieve accurate linearization while maintaining high power coding efficiency.
3Manufacturing precision
If pre-emphasis block with inverse function is used for linearization, then linearity is improved, but device complexity increases for multi-level encoding
Solution Approach 1:
The patent uses copying by creating a simplified model or lookup table that represents the inverse PWM transfer function. Instead of implementing the complex analytical inverse function directly, the system pre-calculates and stores the pre-distortion values in a lookup table, which can be easily implemented in digital logic. This copying approach maintains linearity performance while significantly reducing device complexity for multi-level encoding.
Data Source
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AI summary
A power encoder includes an amplitude-phase splitter for splitting an input signal into an envelope signal and a phase modulated signal, and a pre-distortion unit for distorting the envelope signal using a look-up table (LUT) to produce a distorted envelope signal. The power encoder also includes a digital converter for combining the distorted envelope signal with the phase modulated signal to produce a distorted input signal, a pulse width modulator (PWM) for modulating the distorted input signal according to the transformation function to produce a modulated signal, and a switch mode power amplifier for amplifying the modulated signal. The look-up table stores a non-linear mapping of a transformation function and a relationship between the distorted input signal and the modulated signal is non-linear.