Adaptive PLL Frequency Calibration for Faster VCO Lock
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Solution Overview
Problem
Conventional frequency calibration methods using fixed frequency step sizes in phase/frequency locked loops are inefficient, especially when the frequency difference between the local oscillator and reference frequency is large, leading to prolonged calibration times and potential overshooting or undershooting, which is problematic in programmable multi-frequency capable circuits.
Innovation Solution
An adaptive calibration method that disconnects the phase correction device to measure and adjust the frequency of the local oscillator in proportion to the relative frequency difference with the reference signal, using frequency counters to determine the step size, thereby reducing the number of tuning iterations and achieving phase/frequency coherence more quickly.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a fixed frequency step size is used for VCO calibration, then the calibration process is simple to implement, but the calibration time increases significantly when the frequency difference between VCO and reference is large
Solution Approach 1:
The patent applies dynamics by making the frequency step size adaptive rather than fixed. The calibration circuit dynamically adjusts the step size based on the measured frequency difference between the VCO and reference clock, allowing larger steps when the difference is large and smaller steps when the difference is small, thereby reducing overall calibration time while maintaining simplicity.
Solution Approach 2:
The patent changes the parameter of frequency step size from a constant value to a variable value that depends on the frequency difference. By measuring the frequency difference and using it to determine the appropriate step size, the system optimizes the calibration process speed without significantly increasing complexity.
2Loss of time
If a large frequency step size is used to reduce calibration time, then fewer tuning iterations are needed, but the system may continuously overshoot and undershoot the target frequency range
Solution Approach 1:
The patent changes the step size parameter dynamically based on the frequency difference. When the frequency difference is large, a larger step size is used to quickly approach the target range. When the frequency difference becomes small, a smaller step size is used to avoid overshooting, thereby achieving both fast calibration and accurate frequency tuning.
Solution Approach 2:
The calibration process becomes dynamic with the step size adapting to the current frequency difference. This dynamic adjustment allows the system to be aggressive when far from target and conservative when close, eliminating the continuous overshooting problem while maintaining fast convergence.
3Loss of time
If a large frequency step size is applied when the VCO frequency is close to the reference frequency, then the calibration process is fast, but the VCO frequency may be tuned further away from the reference frequency
Solution Approach 1:
The patent implements parameter changes by making the step size dependent on the frequency difference. When the VCO is close to the reference frequency, the step size automatically reduces to prevent overshooting and further deviation. This ensures precise frequency tuning while maintaining overall calibration efficiency.
Data Source
AI summary
An open loop frequency calibration algorithm is employed whereby frequency counters are utilized to provide frequency information concerning the difference in frequency between a local oscillator and a reference signal prior to obtaining phase locked operation of a phase locked loop (PLL). The frequency difference is then used to adjust the local oscillator's frequency to be changed by a value that is proportional to the frequency difference measured. Through adaptive calibration of the local oscillator's frequency prior to closed loop PLL operations, a substantial reduction in the amount of time required to obtain phase/frequency coherent operation of the PLL is realized.