Dual-Path PLL Loop Filter for Low Phase Noise and Jitter
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Solution Overview
Problem
Conventional phase locked loops (PLLs) suffer from excessive phase noise and jitter due to the high VCO gain of CMOS oscillators, which multiplies noise from the low pass filter, limiting their performance.
Innovation Solution
The implementation of a PLL with multiple gain control circuits and a summing circuit, where each gain control circuit has a distinct gain and response, coupled with a voltage controlled oscillator (VCO) that includes a series of delay cells, to manage and reduce noise by applying different gains and bandwidths to the low pass filters and charge pump outputs.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a single low pass filter is used with high VCO gain, then the PLL can achieve lock, but phase noise and jitter are excessive
Solution Approach 1:
The single low pass filter is segmented into multiple parallel low pass filters (first LPF and second LPF) with different bandwidths and gains. This segmentation allows each filter to contribute differently to the control signal, with the narrower bandwidth filter providing stronger noise filtering and the wider bandwidth filter providing faster response, thereby resolving the contradiction between noise reduction and lock capability.
Solution Approach 2:
Different regions of the control signal are processed with different qualities: the first LPF applies high gain and narrow bandwidth for noise reduction in the phase domain, while the second LPF applies lower gain and wider bandwidth for dynamic response in the frequency domain. This local quality differentiation allows simultaneous optimization of both phase noise and lock performance.
2Speed
If the low pass filter bandwidth is increased to improve dynamic response, then lock speed improves, but phase noise increases
Solution Approach 1:
The filtering function is segmented into two parallel paths with different bandwidth characteristics. The first LPF has a narrower bandwidth to filter noise effectively, while the second LPF has a wider bandwidth to provide fast dynamic response. Both paths operate simultaneously and their outputs are combined, allowing the system to achieve both noise reduction and fast response without compromise.
3Measurement precision
If the low pass filter gain is increased to reduce phase error, then phase accuracy improves, but noise from the filter resistor is amplified
Solution Approach 1:
The gain function is segmented and distributed across two parallel low pass filters with different gain values. The first LPF applies higher gain to achieve accurate phase tracking, while the second LPF applies lower gain to minimize noise amplification. The combined output achieves the desired phase accuracy with reduced overall noise compared to a single high-gain filter.
Data Source
AI summary
A method for generating a signal is provided. A control signal is generated in response to a comparison between a reference signal and a feedback signal. Then, charge is provided to first and second low pass filters (LPFs). The first and second LPFs have first and second bandwidths, respectively, and the second bandwidth is greater than the first bandwidth. First and second gains are then applied to the outputs from the first and second LPFs, respectively, so as to generate first and second voltages, respectively. The first gain is also greater than the second gain. The feedback signal is then generated from the sum of the first and second voltages.