Spread Spectrum PLL Calibration for Wide-Frequency Jitter Control
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Solution Overview
Problem
Conventional spread spectrum control PLL circuits face challenges in maintaining optimal loop bandwidth over a wide range of input frequencies, leading to increased jitter and distortion, which are exacerbated by process variations and the need for complex analog circuits that increase circuit area and design complexity.
Innovation Solution
A spread spectrum control PLL circuit with a calibration circuit that adjusts the charging current, filter capacitance, and voltage-controlled oscillator gain based on the input frequency, using a linear-gain VCO and digital control signals to absorb process variations and adapt the loop bandwidth.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the loop bandwidth is fixed to an optimal value, then the jitter and distortion are reduced at a specific input frequency, but the jitter or distortion increases when the input frequency varies over a wide range
Solution Approach 1:
The patent implements dynamic loop bandwidth adjustment by detecting the input frequency and selectively activating different loop bandwidth control modes (first mode for lower frequencies, second mode for higher frequencies). This allows the PLL circuit to adapt its loop bandwidth to the current operating frequency, maintaining optimal jitter and distortion performance across a wide frequency range rather than being fixed at a single optimal point
Solution Approach 2:
The patent changes the loop bandwidth parameter based on the input frequency by using a frequency detection circuit to measure the input frequency and a control circuit to adjust the loop bandwidth accordingly. This parameter adaptation ensures that the PLL maintains optimal performance characteristics across different operating conditions
2Adaptability or versatility
If an analog circuit with nonlinear VCO gain is used to adjust loop bandwidth, then the loop bandwidth follows the input frequency change, but the circuit area increases and design complexity increases
Solution Approach 1:
The patent replaces the analog nonlinear VCO gain adjustment mechanism with a digital control system. Instead of using complex analog circuits with resistors to achieve nonlinear gain characteristics, the invention uses a frequency detection circuit and control logic to detect the input frequency and selectively activate different loop bandwidth modes, thereby reducing circuit area and simplifying design while achieving the same adaptive functionality
Solution Approach 2:
The patent segments the loop bandwidth control into distinct modes (first loop bandwidth control mode and second loop bandwidth control mode) that are activated based on the detected input frequency range. This segmentation allows the use of simpler control logic rather than a continuous nonlinear analog adjustment, reducing overall circuit complexity
3Adaptability or versatility
If an analog circuit with nonlinear VCO gain is used, then the loop bandwidth can be adjusted, but variations in process cause variations in VCO gain leading to variations in loop bandwidth
Solution Approach 1:
The patent replaces the analog VCO gain adjustment mechanism with a digital control system that detects the input frequency and selectively activates predetermined loop bandwidth modes. This digital approach eliminates the sensitivity to process variations that plagues analog nonlinear gain circuits, as the loop bandwidth is determined by digital control signals rather than analog component characteristics that vary with manufacturing processes
Data Source
AI summary
A calibration circuit (19) adjusts at least one of one of a charging current of a charge pump circuit (12) and a capacitance value of a filter capacitor in a loop filter circuit (13) and a gain of a voltage controlled oscillator (14), depending on a frequency of a reference clock signal input to a calibration circuit (10).