Bit-Stream RF Power Amplifier for Linear Polar Transmission
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Solution Overview
Problem
Polar transmitters face challenges in achieving phase and amplitude alignment, power control, and linearity, particularly at low power levels, due to local oscillator leakage and device mismatch, leading to spectral leakage and regrowth in neighboring channels.
Innovation Solution
The proposed power amplifier, dubbed 'FIR-PA' or 'bit-stream PA', uses a 1-bit digital amplitude signal processed through a digital finite impulse response filter, achieving linearity and exact time alignment, with noise shaping by a Sigma Delta Modulator and amplification stages designed according to tap coefficients, ensuring faithful signal recombination and reduced cross-talk.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If conventional power supply modulation techniques are used for amplitude modulation, then power control is achieved, but bandwidth efficiency deteriorates because the amplitude component occupies several times more bandwidth than necessary
Solution Approach 1:
The patent replaces conventional analog power supply modulation with a digital direct synthesis approach. A digital modulator generates the amplitude-modulated signal by directly combining in-phase and quadrature components with precise digital control, eliminating the need for analog power supply modulation and its associated bandwidth inefficiency.
2Reliability
If phase feedback loop or pre-distortion filter is used to compensate for AM to PM conversion, then linearity improves, but device complexity increases
Solution Approach 1:
The patent applies pre-distortion filtering to the digital baseband signals before modulation. The filter compensates for anticipated AM to PM conversion and other nonlinearities in advance, so that when the signal passes through the power amplifier, the distortions are already corrected, eliminating the need for complex feedback loops.
Solution Approach 2:
The patent employs a phase feedback loop that monitors the output phase and adjusts the digital modulator accordingly. This feedback mechanism dynamically compensates for AM to PM conversion and maintains linearity without requiring complex pre-distortion filtering.
3Adaptability or versatility
If multiple stages with different switch sizes are used to minimize leakage at low power levels, then power control dynamic range improves, but manufacturing precision requirements increase
Solution Approach 1:
The patent divides the power amplifier into multiple stages with different switch sizes (1x, 8x, 64x ratios) that can be independently controlled. This segmentation allows each stage to operate optimally at different power levels, extending the dynamic range while managing leakage effectively across the entire power control range.
Data Source
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AI summary
An RF power amplifier for a polar transmitter converts (60, 180) an amplitude component signal into a 1-bit digital amplitude signal, which is fed to a digital finite impulse response filter (20). Successive taps of the filter each have an RF amplification stage (40, 120, 130, 140, 150, 160, 165, 170) 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.