Two-Stage PLL Low-Pass Filter for Reference Spurious Jitter
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Solution Overview
Problem
Existing PLL circuits face challenges in reducing reference spurious jitter due to variations in control voltage caused by charge pump current, which conventional low-pass filters are unable to effectively mitigate.
Innovation Solution
The proposed PLL circuit incorporates a low-pass filter with a preceding stage circuit that stores electric charges from a charge pump current and a succeeding stage that generates control voltage, utilizing multiple charge storage circuits and switch control signals to manage the charge transfer and voltage generation, thereby reducing reference spurious jitter.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If a conventional low-pass filter is used to reduce jitter, then power supply noise is reduced, but reference spurious jitter cannot be effectively mitigated due to control voltage variations caused by charge pump current
Solution Approach 1:
The low-pass filter is segmented into a two-stage structure: a first stage charge storage circuit that captures charge pump current during phase comparison, and a second stage charge storage circuit that releases charges gradually. This segmentation isolates the control voltage from direct charge pump current fluctuations, effectively reducing reference spurious jitter while maintaining control voltage stability
Solution Approach 2:
The first stage charge storage circuit acts as an intermediary between the charge pump and the control voltage. It absorbs the charge pump current variations and prevents them from directly affecting the control voltage, thereby mitigating reference spurious jitter without compromising reliability
2Measurement precision
If the control voltage varies greatly at the instant of phase comparison to compensate for VCO control voltage variations, then the charge pump can maintain frequency accuracy, but reference spurious jitter increases
Solution Approach 1:
The first stage charge storage circuit performs preliminary action by capturing and storing the charge pump current during phase comparison before it can cause control voltage variations. This preliminary charge storage prevents the harmful voltage fluctuations that lead to reference spurious jitter while preserving the necessary frequency correction
Solution Approach 2:
The two-stage charge storage circuit operates in periodic cycles: the first stage captures charges during phase comparison, then the second stage releases them gradually in a controlled manner. This periodic operation maintains frequency accuracy through necessary charge transfer while smoothing out the variations that cause reference spurious jitter
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This configuration achieves a further reduction in reference spurious jitter by gradually varying the control voltage, improving the accuracy of the PLL circuit's output clock.
Implementation Method 1
a capacitor, a first switch connected between the capacitor and the charge pump
Data Source
AI summary
A PLL circuit includes a low-pass filter configured to generate a control voltage according to an output current from a charge pump. The low-pass filter includes a preceding stage circuit portion configured to store electric charges according to the output current from the charge pump, and a succeeding stage circuit portion configured to generate the control voltage by receiving the electric charges stored in and transferred from the preceding stage circuit portion. Also, the preceding stage circuit portion includes plural charge storage circuits each including a capacitor, a first switch connected between the capacitor and the charge pump and configured to be driven by a first switch control signal, and a second switch connected between the capacitor and the succeeding stage circuit portion and configured to be driven by a second switch control signal.