Polar Loop RF Transmitter Amplitude Control for Wider Dynamic Range
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Solution Overview
Problem
Existing GSM transmitter hardware is inefficient in handling signals with both phase and amplitude modulation due to the need for highly linear power amplifiers, which consume more power and reduce battery life, and struggles to maintain dynamic range control for output power levels, leading to undesirable AM to PM conversion.
Innovation Solution
A closed-loop power control system with variable gain elements that receive power level, error, and amplitude signals, using a reference VGA to maintain constant AM signal levels and extend dynamic range, allowing for non-linear power amplifiers to be used efficiently.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a highly linear power amplifier is used to transmit signals with both PM and AM components, then transmission quality is improved, but power consumption increases and battery life decreases
Solution Approach 1:
The power amplifier is divided into two separate functional blocks: a linear power amplifier for PM signal amplification and a non-linear power amplifier for AM signal amplification. This segmentation allows each amplifier to operate in its optimal mode, with the linear amplifier ensuring transmission quality for phase modulation while the non-linear amplifier maintains high efficiency for amplitude modulation, thereby resolving the contradiction between transmission quality and power consumption.
Solution Approach 2:
The system dynamically switches between linear and non-linear power amplifier modes based on the signal type being transmitted. A control mechanism determines whether the signal requires PM or AM amplification and routes it through the appropriate amplifier path. This dynamic operation allows the system to maintain high transmission quality when needed while maximizing power efficiency during normal operation, thus resolving the contradiction.
2Use of energy by moving object
If a non-linear power amplifier is used for efficient transmission, then power consumption is reduced, but the ability to transmit signals with AM components is lost
Solution Approach 1:
The transmission system is segmented into two parallel amplification paths: one through a non-linear power amplifier for AM signal transmission (maintaining power efficiency) and another through a linear power amplifier for PM signal transmission (maintaining modulation capability). This segmentation allows the system to preserve both power efficiency and signal modulation versatility by selecting the appropriate path based on the modulation type.
Solution Approach 2:
The dual-amplifier configuration creates a universal system that can handle both AM and PM modulation schemes. The non-linear power amplifier provides efficient AM transmission, while the linear power amplifier ensures accurate PM transmission. By making the system multi-functional with two amplifier types, it resolves the contradiction between power efficiency and modulation capability.
3Reliability
If a closed feedback loop is used for power control, then output power stability is improved, but AM signal variations are attenuated
Solution Approach 1:
The feedback control system is segmented to apply different control strategies to different signal components. The closed feedback loop maintains stable average output power while a separate AM control mechanism preserves amplitude variations. This segmentation allows the system to achieve both power stability and AM signal fidelity by handling each aspect independently.
Solution Approach 2:
An intermediary control mechanism is introduced that mediates between the closed feedback loop's power stabilization function and the AM signal's amplitude variation requirements. The intermediary allows the feedback loop to control average power while permitting AM variations to pass through, resolving the contradiction between power stability and AM signal preservation.
4Speed
If the power control loop time-constant is reduced for fast response, then power control speed is improved, but dynamic range control is degraded
Solution Approach 1:
The power control system uses dynamic time-constant adjustment where the feedback loop time-constant is adapted based on operating conditions. During rapid power transitions, a shorter time-constant provides fast response, while during steady-state operation, a longer time-constant ensures precise dynamic range control. This dynamic adjustment resolves the contradiction between response speed and control precision.
Solution Approach 2:
The system changes the time-constant parameter of the power control loop based on the operational phase. A variable time-constant is implemented that switches between fast-response mode for power ramping and precision-control mode for steady-state operation. This parameter change strategy allows the system to achieve both fast response speed and accurate dynamic range control at different times.
Data Source
AI summary
A closed loop power control system for a radio frequency (RF) transmitter comprises a first variable gain element located in a power control loop and configured to receive a power level signal and an inverse representation of a power control signal, a second variable gain element located in the power control loop and configured to receive an error signal and the power control signal, and a third variable gain element configured to receive an amplitude modulated (AM) signal and the power control signal, the third variable gain element having a gain characteristic configured to operate to reduce the gain applied to the AM signal when the power control signal falls below a minimum predetermined value, and to provide the AM signal as a reference signal.