RF Limiter Gain Compensation for Low AM/PM Distortion
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Solution Overview
Problem
Existing power amplifiers in portable communication devices face inefficiencies when switching between constant and non-constant envelope modulation methods, leading to increased power consumption and instability, particularly in GSM and EDGE communication formats, due to the use of linear power amplifiers which are less efficient and prone to AM/PM distortion.
Innovation Solution
A low distortion RF limiter is implemented using a variable gain amplifier with compensation capacitance and resistance, minimizing parasitic capacitance effects and adjusting gain to maintain constant amplitude, combined with inner and outer AM correction loops and a phase correction loop to stabilize power amplifier output.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If a linear power amplifier is used to implement non-constant envelope modulation (EDGE), then the amplifier can handle amplitude and phase modulated waveforms, but power efficiency significantly decreases and power consumption increases
Solution Approach 1:
The power amplifier is segmented into two independent control paths: an amplitude modulation path that controls the envelope of the signal, and a phase modulation path that controls the phase of the carrier. This segmentation allows a nonlinear power amplifier to be used for amplitude control while a separate phase modulator handles phase information, achieving EDGE compatibility without the power inefficiency of fully linear amplification.
Solution Approach 2:
The amplifier transitions from static linear operation to dynamic nonlinear operation by rapidly switching between different operating states. The amplitude modulation is applied dynamically through a variable gain amplifier or switch, while the phase modulation is applied through a phase shifter, allowing the system to adapt its behavior based on the modulation requirements rather than maintaining constant linear operation.
2Use of energy by moving object
If a nonlinear power amplifier is used with slow gain modulation for power control, then power efficiency is maintained, but the amplifier cannot rapidly change output power to create fast modulation required for EDGE
Solution Approach 1:
The amplitude modulation signal is prepared in advance as a baseband signal that drives the variable gain amplifier or switch. This preliminary preparation of the amplitude envelope allows the power amplifier to respond rapidly to power control requirements without needing slow gain modulation, as the amplitude changes are pre-computed and applied directly through the nonlinear amplification process.
Solution Approach 2:
A variable gain amplifier or electronic switch is introduced as an intermediary between the baseband amplitude modulation signal and the power amplifier input. This intermediary rapidly adjusts the signal amplitude before it reaches the power amplifier, enabling fast power control while the power amplifier itself operates efficiently in nonlinear mode without requiring slow gain modulation.
3Power
If gain is adjusted using variable resistance in the power amplifier, then power output can be controlled, but parasitic capacitance causes phase change and AM/PM distortion
Solution Approach 1:
The harmful AM/PM distortion caused by parasitic capacitance is extracted and separated from the main signal path. A dedicated AM correction loop is implemented that detects the amplitude variations caused by parasitic effects and generates a corrective signal that is subtracted from the original amplitude modulation signal, thereby removing the distortion while preserving the desired power control function.
Solution Approach 2:
An amplitude modulation correction loop is implemented that continuously monitors the output signal for AM/PM distortion caused by parasitic capacitance. The loop measures the actual amplitude variations, compares them with the desired amplitude, and adjusts the gain control signal to compensate for the distortion, thereby maintaining accurate power control while eliminating the harmful phase changes.
Data Source
AI summary
A limiter for minimizing an amount of phase change caused by input amplitude variation includes a variable gain amplifier configured to receive a signal having an amplitude component and a phase component and having a gain controlled by a compensation capacitance and a variable resistance, in which the compensation capacitance minimizes an effect of parasitic capacitance and the variable resistance adjusts a gain in the variable gain amplifier such that the amplitude component at an output of the variable gain amplifier remains substantially constant.