Dual-Loop RF Amplifier Envelope Control for Stable Nonlinear Output
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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-loop control solutions are insufficient and prone to instability and sideband noise degradation.
Innovation Solution
A dual-loop RF envelope amplitude control system that uses a first closed loop for stationary corrections and a second loop for adaptive adjustments based on a reference waveform and feedback signals, reducing the gain requirement and improving stability by minimizing error between the RF output and desired waveform.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a single closed-loop control system is used to control RF amplifier output, then the system can maintain waveform envelope accuracy under certain conditions, but the system becomes unstable when operating conditions change (temperature, frequency, power level) or when requiring high bandwidth for fast rise/fall times
Solution Approach 1:
The control system is divided into two separate closed-loop control systems operating at different stages. The first control loop controls the modulator output envelope, while the second control loop controls the RF amplifier output envelope. This segmentation allows each loop to be optimized for its specific function, with the first loop handling bandwidth-critical modulation and the second loop handling power amplification, thereby resolving the stability-accuracy tradeoff.
Solution Approach 2:
An intermediate control stage is introduced between the modulator and the RF amplifier. The first control loop outputs an intermediate envelope signal that serves as the input to the second control loop. This intermediary allows the system to decouple the tight bandwidth requirements from the high power amplification requirements, enabling both accuracy and stability.
2Manufacturing precision
If high closed-loop gain is used to minimize output distortion, then waveform accuracy improves, but spurious sideband noise increases
Solution Approach 1:
The total required gain is segmented between two control loops rather than concentrated in one loop. The first control loop operates with moderate gain to control the modulator, while the second control loop provides additional gain for the RF amplifier. This distribution prevents excessive gain in a single loop, thereby reducing sideband noise while maintaining waveform accuracy.
3Loss of energy
If class C nonlinear devices are used to meet efficiency and power requirements, then energy efficiency improves, but the control transfer function becomes highly nonlinear requiring complex control
Solution Approach 1:
Two closed-loop feedback systems are implemented to control the nonlinear class C amplifier stages. The feedback measures the actual output envelope and compares it to the desired envelope, generating error signals that drive the control loops. This feedback mechanism compensates for the nonlinearities of the class C devices, maintaining waveform accuracy despite the inherent nonlinearity and enabling efficient operation.
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.