Dual-Loop RF Amplifier Modulation Control for Stable Envelope Tracking
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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 controllers are insufficient in providing the required control bandwidth and stability.
Innovation Solution
A dual-loop feedback control system is implemented, with a first loop providing a repetitive correction and a second loop addressing residual errors, using an adaptive table to adjust the amplifier control signal based on feedback signals and a reference waveform, reducing the gain requirement and improving stability.
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 provide waveform envelope control, but the system becomes unstable due to nonlinearities and temperature variations
Solution Approach 1:
The patent divides the single control loop into two separate control loops: a first closed-loop control system that provides coarse waveform envelope control, and a second closed-loop control system that provides fine correction. This segmentation allows each loop to operate within stable gain ranges while collectively achieving high precision control, resolving the instability problem of single high-gain loops.
Solution Approach 2:
The first control loop provides partial correction of waveform envelope errors, while the second control loop provides the remaining correction. By distributing the correction burden between two loops rather than requiring one loop to provide all correction, the system achieves the necessary control precision without requiring excessive gain in a single loop that would cause instability.
2Measurement precision
If the closed-loop gain is increased to reduce output distortion, then waveform accuracy improves, but spurious sideband noise increases
Solution Approach 1:
The patent segments the correction function into two loops with different gain characteristics. The first loop operates with moderate gain to provide coarse correction, while the second loop provides fine correction with lower gain. This prevents the excessive gain in a single loop that would amplify spurious sideband noise while still achieving high waveform accuracy through the combined effect of both loops.
3Speed
If a single control loop is used, then the device complexity is low, but the control bandwidth is insufficient to meet fast rise and fall time requirements
Solution Approach 1:
The patent implements two control loops with different bandwidth characteristics. The first loop provides broader bandwidth for fast transient response to meet rise and fall time requirements, while the second loop provides narrower bandwidth for precise steady-state correction. This segmentation achieves the required speed performance without requiring a single excessively complex high-bandwidth loop system.
4Loss of energy
If highly nonlinear class C devices are used to meet efficiency requirements, then power efficiency improves, but the transfer function becomes highly nonlinear making control difficult
Solution Approach 1:
The patent uses two control loops to manage the nonlinearities of class C amplifiers. The first loop handles the gross nonlinear corrections, while the second loop provides fine linear correction. This segmentation allows the system to maintain the efficiency benefits of class C operation while achieving the necessary control precision through the combined action of both loops, without requiring overly complex compensation circuits.
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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.