Multicarrier Transmitter Feedback Loop Linearity
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Solution Overview
Problem
Conventional Cartesian-loop feedback systems struggle to achieve the required linearity for multicarrier transmission, particularly for radio signals with a time-varying envelope, as nonlinearity in the reverse path of the amplifier circuit is not effectively suppressed, leading to signal distortion.
Innovation Solution
A multicarrier-radio transmitter design that avoids I/Q modulators or demodulators in the reverse path of the feedback loop, using a digital signal processor to produce multicarrier signals at an intermediate frequency, which are then amplified and transmitted at radio frequency, with a feedback loop that includes a subtractor, loop filter, and power amplifier to ensure linearity and suppress distortions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If conventional Cartesian-loop feedback systems are used with I/Q demodulator in the reverse path, then the amplifier can be linearized for narrow-band signals, but signal distortion occurs for multicarrier transmission with time-varying envelope
Solution Approach 1:
The feedback loop is segmented into multiple frequency stages: an I/Q demodulator processes signals at the intermediate frequency (IF) stage, while a separate down-converter handles the radio frequency (RF) stage. This segmentation allows each stage to be optimized for its specific frequency range, enabling effective linearization for multicarrier signals without introducing distortion.
Solution Approach 2:
The invention introduces an additional frequency dimension by operating the feedback loop at both IF and RF stages simultaneously. The I/Q demodulator operates at IF while the down-converter operates at RF, creating a multi-dimensional feedback architecture that effectively suppresses distortion across the entire multicarrier spectrum.
2Ease of operation
If I/Q modulators or demodulators are placed in the reverse path of the feedback loop, then the system can process signals, but nonlinearity is not effectively suppressed leading to distortion
Solution Approach 1:
An intermediate frequency (IF) stage is introduced as a mediator between the baseband and RF stages. The I/Q demodulator processes signals at this intermediate frequency, acting as a buffer that enables effective feedback processing without directly interfering with the RF signal path, thereby maintaining linearity while preserving signal processing capability.
3Productivity
If a single transmitter is used to handle multiple communication channels, then frequency economy is improved, but achieving required linearity for all channels simultaneously becomes difficult
Solution Approach 1:
The feedback loop is designed with multi-functionality to handle multiple frequency channels simultaneously. By incorporating both an I/Q demodulator at IF stage and a down-converter at RF stage, the system can process and linearize multiple carrier frequencies through a single unified feedback path, enabling a single transmitter to effectively handle multiple communication channels with required linearity.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach enables improved linearity and suppression of signal distortions, allowing a single transmitter with a single antenna to handle multiple communication channels simultaneously, enhancing frequency economy and reducing the need for multiple transmitters.
Implementation Method 1
an I/Q demodulator in the forward path to down-convert the feedback-corrected multicarrier signal from the IF to baseband
Implementation Method 2
an I/Q modulator in the forward path to up-convert the baseband multicarrier signal from baseband to the RF
Implementation Method 3
a down converter in the reverse path to down-convert the picked-off RF multicarrier signal from the RF to the IF
Data Source
AI summary
A multicarrier-radio transmitter has a digital signal processor to produce a multicarrier signal at IF, and a transmit amplifier circuit to amplify and transmit the multicarrier signal at RF. A feedback loop of the transmit amplifier circuit has a subtractor, an I/Q demodulator in the forward path, a loop-filter system in the forward path at baseband, an I/Q modulator in the forward path, a power amplifier in the forward path, a pick-off node to pick off the multicarrier RF signal, and a down converter in the reverse path to down-convert the picked-off multicarrier RF signal to IF.