Ring Oscillator VCO Gain Control for Stable PLL Bandwidth
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Conventional phase locked loops (PLLs) with ring oscillator voltage controlled oscillators (VCOs) experience high variation and instability in loop bandwidth due to temperature, voltage, and process changes, making them unsuitable for applications requiring low variation and stability, especially since they are costly and occupy large layout areas.
Innovation Solution
A PLL design incorporating a ring oscillator circuit with a control circuit that provides two voltage-frequency gains, where the second voltage-frequency gain is lower and more stable, allowing for reduced variation in the VCO gain, thereby stabilizing the loop bandwidth, and is fabricated using a low-cost process with a simple circuit structure and small layout area.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If an LC VCO is used to restrain the variation of the gain (Kvco), then the stability of loop bandwidth is improved, but the layout area and fabrication cost increase
Solution Approach 1:
The patent replaces the expensive LC VCO with a ring oscillator VCO that uses simple CMOS circuits. Although ring oscillators naturally have higher gain variation, the invention compensates for this by using a PLL control mechanism to stabilize the output frequency, achieving the desired stability without requiring expensive components or large layout area.
Solution Approach 2:
The patent changes the operating parameters of the ring oscillator VCO by controlling its tail current with the PLL feedback signal. By dynamically adjusting the current parameter based on frequency error detection, the system compensates for the inherent instability of ring oscillators and achieves stable loop bandwidth without using LC resonators.
2Ease of manufacture
If a ring oscillator VCO is used to reduce cost and layout area, then the fabrication cost and layout area are reduced, but the variation of the gain (Kvco) increases due to temperature, voltage, and process changes
Solution Approach 1:
The patent implements a feedback control mechanism where the phase detector compares the output frequency with the reference frequency, and the resulting error signal adjusts the tail current of the ring oscillator VCO through the loop filter. This closed-loop feedback stabilizes the output frequency and reduces the impact of temperature, voltage, and process variations on the loop bandwidth.
3Reliability
If the gain (Kvco) of the VCO is controlled to reduce loop bandwidth variation, then the stability of loop bandwidth is improved, but the device complexity increases
Solution Approach 1:
The patent extracts the stability control function from the VCO itself and implements it in the PLL control loop. Instead of making the VCO inherently stable through complex internal structures, the invention separates the frequency generation function (ring oscillator) from the stability control function (PLL feedback), achieving stability through the control loop rather than through complex VCO design.
Data Source
AI summary
A phase locked loop (PLL) and a voltage controlled oscillator (VCO) thereof are provided. The VCO includes a ring oscillator circuit and a control circuit. The ring oscillator circuit is used for providing an output clock signal; and the control circuit is coupled to the ring oscillator circuit, and used for receiving an output voltage to respectively provide a first voltage-frequency gain and a second voltage-frequency gain so as to control a frequency of the output clock signal provided by the ring oscillator circuit, wherein the first voltage-frequency gain is larger than the second voltage-frequency gain.