PLL VCO Gain Calibration via Open-Loop ADC Offset
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Solution Overview
Problem
Phase locked loops (PLLs) in wireless communication devices face significant modulation errors due to variations in the gain of voltage controlled oscillators (VCOs), which are not constant and can vary by up to +/â50% with fabrication process, frequency band, and temperature changes, affecting the dynamic behavior and accuracy of data signal modulation.
Innovation Solution
A method for calibrating the PLL involves open loop calibration of the VCO, determining voltage offset and gain error of the analog to digital converter (ADC), and adjusting the charge pump signal based on the determined gain offset to compensate for VCO gain variations, thereby reducing modulation errors.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If open loop calibration of VCO is performed to account for gain variations, then modulation accuracy is improved, but device complexity increases due to additional calibration circuitry and procedures
Solution Approach 1:
The patent performs open-loop calibration of the VCO before normal PLL operation to pre-determine the gain offset. This preliminary action characterizes the VCO's gain variations across different operating conditions, allowing the system to compensate for these variations during actual modulation without adding complex real-time correction mechanisms.
Solution Approach 2:
The patent introduces an ADC as an intermediary component to measure the control voltage applied to the VCO and determine the gain offset. This intermediary enables the system to quantify VCO gain variations and use this information to adjust the charge pump signal, thereby improving modulation accuracy without requiring direct modification of the VCO itself.
2Manufacturing precision
If VCO gain variations are compensated through calibration, then signal distortion is reduced, but manufacturing complexity increases due to additional calibration steps
Solution Approach 1:
The patent implements a self-calibration mechanism where the system automatically determines the VCO gain offset by measuring its own control voltage and observing its output frequency characteristics. This self-service approach eliminates the need for external manual calibration equipment and procedures, simplifying the manufacturing process while still achieving accurate distortion compensation.
Solution Approach 2:
The patent changes the operating parameters of the VCO across different frequency bands and temperature conditions to characterize its gain variations. By measuring the VCO's response to controlled parameter changes, the system builds a compensation model that reduces signal distortion without requiring physical modifications to the device.
3Reliability
If charge pump signal is adjusted based on gain offset to reduce modulation errors, then communication reliability is improved, but energy consumption increases due to continuous calibration activities
Solution Approach 1:
The patent performs open-loop calibration periodically or at specific intervals rather than continuously. The VCO gain offset is determined during calibration phases and then used to adjust the charge pump signal during normal operation. This periodic approach maintains communication reliability by ensuring accurate compensation while reducing energy consumption by avoiding constant calibration activities.
Solution Approach 2:
The patent performs gain offset determination as a preliminary action before normal communication operations begin. By pre-characterizing the VCO's gain variations and storing this information for later use, the system eliminates the need for continuous energy-consuming calibration during data transmission, thereby improving the energy efficiency of communication operations while maintaining reliability.
Data Source
AI summary
Methods and apparatus for calibrating a transitional loop, such as a phase locked loop, are disclosed. An example method includes performing an open loop calibration of a voltage controlled oscillator (VCO). The open loop calibration includes tuning the output oscillation frequency of the VCO to within a predetermined range of frequencies. The example method further includes determining a voltage offset and a gain error of an analog to digital converter (ADC) coupled with the phase locked loop. The example method also includes determining a gain offset of the open loop calibrated VCO using the voltage offset and the gain error of the ADC. In the example method, a signal provided by a charge pump of the PLL is adjusted based on the determined gain offset.