Digital RF Power Encoder Using IFPWM for Efficient Linear Transmission

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Solution Overview

Problem

Direct digital-RF transmitters face low power coding efficiency due to noise shaping in delta sigma modulation schemes, especially with non-constant envelope signals, leading to excessive power loss and efficiency degradation, which is challenging to improve with conventional digital implementation methods.

Innovation Solution

The implementation of intermediate frequency pulse-width modulation (IFPWM) via two-stage digital up-conversion at a reduced sampling rate, using a multi-level encoder with non-uniform quantizers and a pre-distortion block, allows for higher time domain quantization and linearity, enabling efficient power coding even with restricted digital processor sampling rates.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Measurement precision

If delta sigma modulation is used for power coding, then linearity specification is met, but power coding efficiency deteriorates due to noise shaping spreading quantization noise throughout the frequency domain

Engineering Contradiction:

ImprovelinearityVSAvoidpower coding efficiency

Core Design Contradiction:

Measurement precisionVSLoss of energy

Solution Approach 1:

The patent segments the modulation process into two distinct stages: first, baseband delta-sigma modulation generates a single-bit digital signal with noise shaping; second, RF pulse-width modulation (PWM) reconstructs the RF signal from this single-bit stream. This segmentation allows the noise shaping function to be separated from the RF signal generation, enabling linearity to be achieved in the baseband while power coding efficiency is improved in the RF domain through PWM's inherent efficiency.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The patent introduces a single-bit digital signal as an intermediary between the baseband delta-sigma modulator and the RF PWM modulator. This single-bit stream serves as a compact representation that carries the essential signal information while eliminating the need for multi-bit digital-to-analog conversion, thereby improving power coding efficiency while maintaining linearity through the noise shaping properties of the delta-sigma modulator.

Inventive Principle:

Principle #24Intermediary (Mediator)

2Loss of energy

If conventional PWM techniques are used to improve power coding efficiency, then digital implementation becomes complex or requires analog/RF high speed comparators

Engineering Contradiction:

Improvepower coding efficiencyVSAvoiddigital implementation complexity

Core Design Contradiction:

Loss of energyVSDevice complexity

Solution Approach 1:

Instead of implementing conventional PWM that compares the input signal with a triangular or sawtooth waveform (which requires complex digital logic or analog comparators), the patent inverts the approach by using the output of a baseband delta-sigma modulator as the input to an RF PWM modulator. This inversion simplifies the digital implementation because the single-bit output from the delta-sigma modulator directly drives the RF PWM, eliminating the need for complex comparison operations.

Inventive Principle:

Principle #13The other way round (Inversion)

Solution Approach 2:

The patent replaces the mechanical/comparison-based PWM implementation (which requires high-speed analog comparators or complex digital logic) with a direct digital modulation approach. The RF PWM modulator is driven by a single-bit digital signal from the baseband delta-sigma modulator, substituting the need for analog comparison circuitry with a simpler digital interface that is easier to implement in modern digital processors and FPGAs.

Inventive Principle:

Principle #28Mechanics substitution (Replace mechanical system)

3Productivity

If sampling rate is restricted by digital processor capabilities, then time domain quantization is reduced, but power coding efficiency and linearity are maintained through IFPWM

Engineering Contradiction:

Improvesampling rateVSAvoidtime domain quantization

Core Design Contradiction:

ProductivityVSMeasurement precision

Solution Approach 1:

The patent introduces an intermediate frequency (IF) signal as a mediator between the baseband signal and the RF signal. The baseband signal is first up-converted to IF, then PWM modulated, and finally up-converted to RF. This intermediate step allows the PWM modulation to occur at a lower frequency where the restricted sampling rate can still provide sufficient time domain quantization, while the final RF signal achieves the desired high frequency and power coding efficiency.

Inventive Principle:

Principle #24Intermediary (Mediator)

Solution Approach 2:

The patent changes the frequency parameter of the signal during processing. By transforming the baseband signal to an intermediate frequency and then to RF frequency through up-conversion, the system achieves high power coding efficiency at RF frequencies while the PWM modulation operates at the lower IF frequency where the restricted sampling rate can maintain adequate time domain quantization precision.

Inventive Principle:

Principle #35Parameter changes

Data Source

PatentUS8953670B1Digital power encoder for direct digital-RF transmitter

Publication Date: 2015.02.10 MITSUBISHI ELECTRIC RESEARCH LABORATORIES INC

AI summary

A transmitter includes a first digital up-converter for converting data to an intermediate frequency (IF) signal, a pulse width modulator (PWM) for encoding the IF signal to an IF pulse train, a second digital up-converter for converting the IF pulse train to a radio frequency (RF) pulse train, a power amplifier for amplifying the RF pulse train; and a filter for reconstructing a RF analog signal from the amplified RF pulse train.