PLL Frequency Synthesizer Calibration for Constant Loop Bandwidth
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Solution Overview
Problem
Wideband frequency synthesizers face challenges in maintaining optimal loop bandwidth, lock time, and phase noise constancy due to variations in VCO gain and division ratio, leading to prolonged calibration times and increased power consumption, especially in fractional-N and integer-N synthesizers.
Innovation Solution
A phase-locked loop based frequency synthesizer with an automatic calibration device that includes a frequency-to-digital converter, frequency difference detector, and loop bandwidth calibration logic to quickly and precisely adjust the charge pump gain and select optimal control codes for the capacitor bank, ensuring frequency resolution below the reference frequency and maintaining constant loop bandwidth.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If a wideband frequency synthesizer is used to provide a wider range of output frequency, then the frequency range is improved, but the loop bandwidth, lock time, and phase noise vary greatly due to changes in VCO gain and division ratio
Solution Approach 1:
The patent implements automatic calibration circuits that dynamically adjust parameters (VCO gain KVCO and division ratio N) based on the output frequency range. The system measures actual frequency values and automatically tunes calibration parameters to maintain constant loop bandwidth, lock time, and phase noise characteristics across the wide frequency band, resolving the contradiction between wide frequency range and parameter stability
2Stability of the object's composition
If an automatic frequency calibration circuit is added to maintain constant loop bandwidth, then the loop bandwidth stability is improved, but the overall lock time is prolonged due to frequency calibration time
Solution Approach 1:
The patent performs frequency calibration in advance during the lock process. The automatic frequency calibration circuit measures the VCO output frequency and determines the optimal control code before final locking occurs. This preliminary calibration action ensures that when locking happens, the parameters are already optimized, maintaining loop bandwidth stability without significantly extending the overall lock time
3Measurement precision
If the frequency resolution of the automatic frequency calibration circuit is made smaller than the reference frequency, then the frequency resolution precision is improved, but the calibration time is extended
Solution Approach 1:
The patent employs a feedback mechanism where the frequency-to-digital converter continuously monitors the VCO output frequency and feeds back the measured value to the calibration logic. The system automatically adjusts the control code based on the frequency measurement feedback, achieving high frequency resolution (smaller than reference frequency) through iterative feedback control rather than exhaustive searching, thus maintaining fast calibration speed
Data Source
AI summary
A frequency synthesizer and an automatic calibration device are disclosed. An automatic calibration device for a phase-locked loop based frequency synthesizer includes: a frequency-to-digital converter for converting a frequency of a signal outputted from a voltage controlled oscillator into a first digital value; a frequency difference detector for calculating a difference between the first digital value outputted from the frequency-to-digital converter and a second digital value corresponding to a target frequency; an automatic frequency calibration logic for selecting an optimal control code for a capacitor bank such that an output frequency of the voltage controlled oscillator is closer to the target frequency; and a loop bandwidth calibration logic for tuning a charge pump gain such that a loop bandwidth is kept constant in the optimal control code using the frequency-to-digital converter. Thus, the calibration speed can be increased, and the loop bandwidth can be kept constant within the output frequency range.