Polar Transmitter Amplitude-Aware PLL for Linearity Calibration
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Solution Overview
Problem
Polar transmitters are limited by noise in the phase-locked loop (PLL) and digital power amplifier (DPA) bandwidth, leading to phase/frequency modulation distortions and linearity issues, as well as parasitic amplitude modulation.
Innovation Solution
Integrating a digital amplitude modulator within the PLL and providing a phase error detection unit to detect the amplitude of the feedback signal, allowing for iterative calibration of the transmit signal to suppress PM-to-AM and AM-to-PM distortions, while maintaining phase-noise filtering and on-line calibration of linearity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a phase-locked loop (PLL) and digital power amplifier (DPA) are used for phase/frequency modulation and amplitude modulation, then the transmitter can generate modulated signals, but the noise in the system and PLL bandwidth limit the performance and cause phase/frequency modulation distortions
Solution Approach 1:
The patent implements a feedback mechanism where the transmit signal or a derivation thereof is fed back to the phase error detection unit. This feedback loop enables continuous monitoring and correction of phase errors, allowing the system to overcome PLL bandwidth limitations while maintaining phase-noise filtering and improving modulation accuracy.
Solution Approach 2:
The phase error detection unit performs self-calibration by detecting both phase and amplitude of the feedback signal. The system automatically adjusts for nonlinearities in digital to phase/frequency conversion and digital to amplitude conversion, as well as PM-to-AM and AM-to-PM distortions, without requiring external calibration equipment.
2Adaptability or versatility
If digital amplitude modulation is used in the PLL, then amplitude modulation capability is achieved, but parasitic phase/frequency modulation (AM-to-PM distortion) is induced
Solution Approach 1:
The feedback signal is processed by the phase error detection unit which detects both phase and amplitude components. This dual detection capability allows the system to identify and correct AM-to-PM distortion by generating appropriate correction signals that compensate for the parasitic phase modulation introduced by the digital amplitude modulator.
Solution Approach 2:
The phase error detection unit acts as an intermediary that processes both phase and amplitude information from the feedback signal. It generates correction signals that mediate between the amplitude modulation input and the phase/frequency output, preventing the direct coupling that causes AM-to-PM distortion.
3Adaptability or versatility
If phase/frequency modulation is used, then phase/frequency modulated signals are generated, but unwanted amplitude modulation (PM-to-AM distortion) occurs
Solution Approach 1:
The feedback mechanism enables the phase error detection unit to detect unwanted amplitude modulation caused by PM-to-AM distortion. The detected amplitude variations are used to generate correction signals that compensate for the distortion, allowing the system to maintain pure phase/frequency modulation without spurious amplitude components.
4Extent of automation
If the digital amplitude modulator is integrated within the PLL, then on-line calibration of linearity and suppression of distortion is enabled, but the system complexity increases
Solution Approach 1:
The phase error detection unit is designed to perform multiple functions: detecting phase errors for PLL operation, detecting amplitude of the feedback signal for calibration, and generating correction signals for distortion suppression. This multi-functionality reduces the need for separate calibration circuits and minimizes overall system complexity while enabling comprehensive on-line calibration.
Data Source
AI summary
A polar transmitter provided for transmitting a phase/frequency modulated and amplitude modulated transmit signal and a method for generating a transmit signal using a polar transmitter are described. An example polar transmitter comprises a phase locked loop for generating a phase/frequency modulated precursor of the transmit signal. The phase locked loop comprises at its input a phase error detection unit for detecting a phase error of the precursor fed back from the output of the phase locked loop to the phase error detection unit as a feedback signal. The polar transmitter comprises a digital amplitude modulator for amplitude modulation of the precursor, resulting in the transmit signal. The digital amplitude modulator is arranged within the phase locked loop for amplitude modulation of the precursor before being output by the PLL. The phase error detection unit is further provided for detecting the amplitude of the feedback signal.