PLL and DLL Loop Filter with Variable Resistance for Low Jitter

Resolve Bottlenecks,
Find Innovative SolutionsGenerate Solutions

Solution Overview

Problem

Phase-locked loop (PLL) and delay-locked loop (DLL) circuits face issues with jitter characteristics due to fluctuations in VCO gain and VCDL gain across varying frequencies and delays, leading to increased jitter at low frequencies and wide-range delays, despite efforts to maintain a constant cutoff frequency.

Innovation Solution

The implementation of a low-pass filter with a variable resistor that adjusts according to the control voltage, coupled with transistors configured to maintain a consistent resistance value, effectively cancels out fluctuations in VCO gain and VCDL gain by varying the combined resistor in the filter, ensuring a constant cutoff frequency across the frequency range.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Reliability

If a passive low-pass filter with fixed resistor and capacitor is used, then the circuit structure is simple, but the cutoff frequency fluctuates due to VCO gain variations, causing increased jitter at low frequencies and wide-range delays

Engineering Contradiction:

Improvejitter characteristicsVSAvoidfilter circuit structure

Core Design Contradiction:

ReliabilityVSDevice complexity

Solution Approach 1:

The patent transforms the static fixed resistor into a dynamic variable resistor whose resistance value automatically adjusts according to the control voltage. This dynamic adjustment compensates for VCO gain fluctuations, maintaining a constant cutoff frequency across different operating conditions and reducing jitter without requiring complex external control circuits

Inventive Principle:

Principle #15Dynamics

Solution Approach 2:

The variable resistor is configured to self-adjust its resistance value based on the control voltage signal already present in the PLL/DLL circuit. This self-service mechanism eliminates the need for additional digital control circuits or external amplifiers, maintaining circuit simplicity while achieving stable jitter characteristics

Inventive Principle:

Principle #25Self-service

2Reliability

If the resistor value in the low-pass filter is increased to maintain constant cutoff frequency, then jitter is reduced, but the filter's ability to smooth charge pump current deteriorates

Engineering Contradiction:

Improvejitter characteristicsVSAvoidcharge pump current smoothing

Core Design Contradiction:

ReliabilityVSManufacturing precision

Solution Approach 1:

The variable resistor dynamically adjusts its resistance value based on the control voltage, allowing the filter to maintain optimal performance across different operating conditions. At certain operating points, the resistance increases to reduce jitter, while at other points, it decreases to maintain current smoothing capability, thus resolving the trade-off between these two requirements

Inventive Principle:

Principle #15Dynamics

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 approach maintains low jitter across the entire lock range without requiring digital control circuits or additional amplifiers, allowing for self-recovery during power interruptions and reducing PLL and DLL circuit jitter.

Implementation Method 1

a low-pass filter that includes a resistor and a capacitor and that smoothes the charge pump current and converts the smoothed current into a control voltage

Methodology Applied

Scientific EffectLow-pass filtering: Filter (electronic)

Implementation Method 2

the resistor in the low-pass filter is a variable resistor that is changed according to the control voltage

Methodology Applied

Scientific EffectVariable resistance: Electrical Resistance

Implementation Method 3

a voltage-controlled oscillator that generates an oscillation signal with a frequency according to the control voltage

Methodology Applied

Scientific EffectVoltage-controlled oscillation:

Implementation Method 4

a voltage-controlled delay device that generates a delay signal by providing an amount of delay according to the control voltage to the reference signal

Methodology Applied

Scientific EffectVoltage-controlled delay:

Data Source

PatentUS8085071B2Phase-locked loop circuit and delay-locked loop circuit

Publication Date: 2011.12.27 SOCIONEXT INC

AI summary

A phase-locked loop circuit includes a phase comparator that compares phases between a reference signal and a feedback signal and outputs a phase difference signal indicating a phase difference therebetween; a charge pump that outputs a charge pump current according to the phase difference signal; a low-pass filter that includes a resistor and a capacitor and that smoothes the charge pump current and converts the smoothed current into a control voltage; a voltage-controlled oscillator that generates an oscillation signal with a frequency according to the control voltage; and a frequency divider that generates a frequency-divided signal by frequency-dividing the oscillation signal and outputs the frequency-divided signal to the phase comparator as the feedback signal, wherein the resistor in the low-pass filter is a variable resistor that is changed according to the control voltage.