Dual-Loop RF Envelope Control for Nonlinear Power Amplifiers
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Solution Overview
Problem
RF amplifiers in nonlinear RF transmitters face instability due to severe temperature stresses and nonlinearities, making it difficult to match desired waveform envelopes, especially in applications like TACAN and TDMA, where existing single closed-loop controllers are insufficient in terms of gain and bandwidth.
Innovation Solution
A dual closed-loop modulation controller system that includes a first closed loop for stationary corrections and a second loop for reducing residual errors, using a feedback mechanism to adjust the RF amplifier control signal based on a reference waveform and adaptive table updates to stabilize the output envelope.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a single closed-loop controller is used to control RF amplifier output, then the system can be kept simple, but the controller becomes unstable over portions of the output envelope due to nonlinearities and absence of closed loop between pulses
Solution Approach 1:
The control system is divided into two separate closed-loop controllers: a first closed-loop controller that operates during the RF pulse to provide rapid correction, and a second closed-loop controller that operates between pulses to provide gradual adaptation. This segmentation allows each controller to be optimized for its specific operating condition, resolving the instability problem of single-loop designs.
Solution Approach 2:
The system dynamically switches between two control modes depending on the operational phase. During RF pulses, the first controller with higher gain and bandwidth active. Between pulses, the second controller takes over to maintain stability. This dynamic adaptation allows the system to maintain both stability and performance across different operating conditions.
2Manufacturing precision
If the closed loop gain and bandwidth are increased to minimize output distortion, then waveform accuracy improves, but spurious RF sideband output levels are degraded (increased)
Solution Approach 1:
The correction function is segmented between two controllers with different gain characteristics. The first controller handles rapid corrections during pulses with moderate gain, while the second controller provides gradual adaptation between pulses with lower gain. This prevents the excessive gain that would otherwise be needed in a single-loop system, thereby reducing spurious sideband generation while maintaining waveform accuracy.
Solution Approach 2:
The second closed-loop controller performs preliminary adaptation between RF pulses by gradually adjusting the control signal based on accumulated error information. This preliminary action prepares the system for the next pulse, reducing the correction needed during the pulse itself and allowing the first controller to operate with lower gain, thus minimizing sideband distortion.
3Use of energy by moving object
If highly nonlinear class C devices are used to meet frequency range and efficiency requirements, then power efficiency improves, but the RF input to output amplitude transfer function becomes highly nonlinear making envelope control difficult
Solution Approach 1:
Two closed-loop feedback controllers are implemented to compensate for the nonlinear transfer function of class C devices. The first controller provides immediate feedback correction during RF pulses, while the second controller provides gradual feedback adaptation between pulses. This dual feedback mechanism enables precise envelope control despite the highly nonlinear characteristics of class C amplifiers, maintaining both efficiency and accuracy.
Solution Approach 2:
The second closed-loop controller performs preliminary correction between RF pulses by gradually adjusting the control signal based on measured output errors. This preliminary action pre-compensates for nonlinear distortions before the next pulse occurs, reducing the burden on the first controller during pulse transmission and improving overall envelope accuracy while maintaining class C efficiency.
Data Source
AI summary
In accordance with various exemplary embodiments of the present invention, systems, methods and devices are configured to facilitate RF envelope amplitude control. For example, a RF envelope amplitude control system comprises: a RF amplifier, wherein the RF amplifier is associated with a feedback device that is configured to create a first feedback signal representing the power in an RF output signal; a transmit waveform generator configured to generate a reference waveform signal; an adaptive table waveform generator configured to compare the reference waveform signal and the first feedback signal and to create a second feedback signal based on that comparison; and a loop filter configured to combine the reference waveform signal, the first feedback signal, and the second feedback signal to form an amplifier control signal, wherein the amplifier control signal is provided to the RF amplifier to adjust the RF output signal to conform to a specified RF envelope.