PLL Low-Pass Filtering for Quantization Noise Reduction
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Solution Overview
Problem
Phase locked loops (PLLs) face challenges in achieving reduced noise performance due to quantization noise, which is not effectively suppressed by existing technologies, impacting their stability and performance, especially in applications requiring low out-of-band phase noise.
Innovation Solution
Incorporating a low-pass filter with a cut-off frequency greater than the PLL bandwidth, specifically between the phase detector and loop filter or loop filter and frequency controlled oscillator, to suppress quantization noise without affecting loop stability, using configurations such as first-order or second-order IIR filters or finite impulse response filters, and adjusting gain factors to optimize performance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a low-pass filter is added to suppress quantization noise, then phase noise performance is improved, but device complexity increases
Solution Approach 1:
A low-pass filter is introduced as an intermediary component between the phase detector and loop filter to suppress quantization noise. This mediator selectively attenuates high-frequency noise components while preserving the useful phase error signal, thereby improving phase noise performance without fundamentally altering the PLL's core operation.
Solution Approach 2:
The cutoff frequency of the low-pass filter is carefully selected to be greater than the PLL bandwidth. By adjusting this critical parameter, the filter effectively removes out-of-band quantization noise while allowing in-band signals to pass through unchanged, thus improving phase noise performance without affecting loop stability or requiring complex redesign of existing components.
2Productivity
If the PLL bandwidth is increased to improve response speed, then productivity is improved, but phase noise performance deteriorates
Solution Approach 1:
The low-pass filter acts as a mediator that decouples the relationship between bandwidth and phase noise. By placing the filter after the phase detector, it removes quantization noise independently of the loop bandwidth setting, allowing the PLL to achieve fast response (high productivity) without sacrificing phase noise performance.
Solution Approach 2:
The filter's cutoff frequency is set greater than the PLL bandwidth, creating a parameter relationship that allows independent optimization. This enables the system to operate with higher bandwidth for improved response speed while the filter simultaneously suppresses out-of-band noise, thus resolving the trade-off between productivity and measurement precision.
3Measurement precision
If a low-pass filter with cutoff frequency greater than bandwidth is used to suppress noise, then phase noise performance is improved, but loop stability may be affected
Solution Approach 1:
The critical parameter relationship is established where the low-pass filter cutoff frequency is greater than the PLL bandwidth. This parameter configuration ensures that the filter only affects out-of-band quantization noise while leaving the in-band control signal intact, thereby improving phase noise performance without disrupting loop stability.
Solution Approach 2:
The low-pass filter applies selective attenuation only to specific frequency components (out-of-band quantization noise) while leaving other frequency ranges (in-band signals) unaffected. This localized action improves phase noise performance in the critical out-of-band region without introducing instability into the overall loop system.
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
The additional low-pass filter significantly reduces quantization noise, improving phase noise performance by up to 10 dB in out-of-band frequencies, allowing for higher PLL bandwidths without degrading noise performance and maintaining stability, thus enhancing the overall functionality of PLLs in various applications.
Implementation Method 1
a low-pass filter is provided between the phase detector and the loop filter and/or between the loop filter and the frequency controlled oscillator, to reduce quantization noise from the phase detector
Data Source
AI summary
A phase locked loop, comprising: a phase detector configured to determine a phase difference (Δφ) between a reference signal and a feedback signal; a loop filter configured to perform a filtering operation on a signal derived from the phase difference, and to provide a control signal; a frequency controlled oscillator configured to receive the control signal and provide an output signal with a frequency that varies according to the control signal; wherein a low-pass filter is provided between the phase detector and the loop filter and/or between the loop filter and the frequency controlled oscillator to reduce quantization noise from the phase detector.