RF Amplifier Digital Filter Polar Transmitter Linearity
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Solution Overview
Problem
Modern RF power amplifiers in polar transmitters face challenges with amplitude modulation linearity, power control dynamic range, and spectral leakage due to phase and amplitude misalignment, as well as leakage current issues, especially at low power levels, which affect the fidelity of RF signals and spectral purity.
Innovation Solution
A power amplifier design that converts the amplitude component signal into a 1-bit digital signal, processed through a digital finite impulse response filter, ensuring exact and faithful reproduction of the amplitude component, and using a Sigma Delta Modulator for noise shaping to achieve linearity and reduce spectral emissions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If conventional power supply modulation techniques are used for amplitude modulation, then power control can be achieved, but the bandwidth of the amplitude component is limited and cannot support wideband applications
Solution Approach 1:
The patent replaces conventional power supply modulation (analog/physical system) with direct digital modulation of the RF signal. The amplitude component is generated digitally through I/Q modulation, allowing wideband operation without the bandwidth limitations of power supply modulation techniques.
2Measurement precision
If switching power amplifiers are used with conventional power control methods, then power control can be implemented, but there is a trade-off between power control dynamic range and amplitude modulation resolution
Solution Approach 1:
The patent segments the power control function into multiple discrete power levels achieved by selectively enabling different numbers of parallel PA stages. This segmentation allows independent optimization of amplitude resolution (through fine-grained stage control) and dynamic range (through coarse-grained stage selection), eliminating the trade-off present in conventional single-stage approaches.
3Object-generated harmful factors
If LO leakage is reduced by decreasing switch size for low power levels, then spectral purity improves, but the amplifier requires multiple stages which increases device complexity
Solution Approach 1:
The patent divides the power amplifier into multiple parallel stages, each optimized for specific power levels. By segmenting the amplification function, the system can reduce LO leakage at low power levels through multiple smaller stages while maintaining overall system performance, rather than using a single large stage that would generate excessive leakage.
Solution Approach 2:
Each PA stage is designed with local optimization for its intended operating range. The switch sizes and component values are tailored to each stage's specific power level requirements, allowing low-power stages to have smaller switches (reducing leakage) while high-power stages have larger switches (providing necessary drive capability).
4Reliability
If a digital finite impulse response filter is used to process the 1-bit digital amplitude signal, then linearity and spectral purity are improved, but the device complexity increases due to multiple amplification stages
Solution Approach 1:
The patent replaces complex analog filtering mechanisms with a digital finite impulse response filter that processes the 1-bit digital amplitude signal. This digital approach achieves superior linearity and spectral purity through algorithmic signal processing rather than complex analog component networks, reducing sensitivity to manufacturing variations.
Data Source
AI summary
An RF power amplifier for a polar transmitter converts an amplitude component signal into a 1-bit digital amplitude signal, which is fed to a digital finite impulse response filter. Successive taps of the filter each have an RF amplification stage arranged to amplify successively delayed versions of the 1-bit digital amplitude signal, the amplifying being according to a respective tap coefficient, and according to the RF carrier modulated by the phase component. The filter is arranged to combine the outputs of the taps to provide the amplified RF signal. The power amplifier uses a one bit stream which therefore has only two states (2 values), thus achieving linearity in principle. Device mismatch between taps does not lead to non-linearity or distortion.