PLL Feedback Switching With Time Alignment for Phase Stability
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Solution Overview
Problem
Phase-locked loops (PLLs) in radio frequency transmitters face challenges when switching between open loop and closed loop configurations, leading to phase disturbances that can result in amplitude modulation to phase modulation (AM/PM) distortion and interference-induced unlocking, violating specifications such as the ETSI switching spectrum specification.
Innovation Solution
A system and method that includes time alignment circuitry to adjust the phase of feedback signals from either the phase-locked loop or the power amplifier, ensuring minimal phase disturbance during the switch from open loop to closed loop operation by aligning the feedback signals before switching, thereby maintaining the PLL's lock on the desired frequency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the feedback path is switched from the output of the phase-locked loop to the output of the power amplifier circuitry, then the transmitter can operate in closed loop mode to reduce AM/PM distortion, but phase disturbance occurs during switching which may cause the PLL to unlock and violate spectral specifications
Solution Approach 1:
The patent applies preliminary action by time-aligning the feedback signal from the power amplifier output with the PLL output signal before switching occurs. This is achieved by measuring the time difference between the two signals and introducing a corresponding delay to the power amplifier feedback signal, ensuring that when the switch transitions to closed-loop mode, there is minimal phase disturbance and the PLL remains locked
Solution Approach 2:
The patent uses an intermediary approach by introducing a delay element as a mediator between the power amplifier feedback signal and the PLL feedback path. This delay element acts as a buffer that synchronizes the timing of the feedback signal with the PLL output, preventing direct phase conflicts during the transition to closed-loop operation
2Manufacturing precision
If the feedback path is switched to the output of the power amplifier for closed loop operation, then phase modulation accuracy is improved, but the complexity of the switching mechanism and time alignment circuitry increases
Solution Approach 1:
The patent applies parameter changes by dynamically adjusting the delay parameter of the time alignment circuitry based on the operating mode. The system measures the time difference between signals and modifies the delay parameter accordingly, allowing the same hardware to achieve precise phase alignment in both open-loop and closed-loop modes without requiring completely different circuit architectures
Data Source
AI summary
A system and method are provided for switching from one phase-locked loop feedback source to another in a radio frequency (RF) transmitter. The RF transmitter includes a phase-locked loop (PLL) that provides a phase-modulated RF input signal and power amplifier circuitry that amplifies the RF input signal to provide an RF output signal. The PLL includes switching circuitry that couples a feedback path of the PLL to an output of the PLL for open loop operation and couples the feedback path of the PLL to an output of the power amplifier circuitry for closed loop operation. Prior to switching the feedback path from the output of the PLL to the output of the power amplifier circuitry, time alignment circuitry operates to time-align feedback signals from the outputs of the PLL and the power amplifier circuitry such that switching from open loop operation to closed loop operation causes minimal phase disturbance.