PLL Charge Pump Calibration for VCO Gain Variation Control

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Solution Overview

Problem

Phase lock loop (PLL) circuits face challenges in tolerating variations in voltage-controlled oscillator (VCO) gain due to process, temperature, and frequency changes, leading to increased phase noise and requirements for larger circuit area and higher power consumption to maintain jitter specifications.

Innovation Solution

A calibration method is introduced that involves applying a sweeping control voltage to the VCO in discrete steps, measuring frequencies, determining VCO gain, and calculating a calibrated magnitude for the charge pump current to maintain a constant product with the VCO gain, using a look-up table to correlate measured frequencies with corresponding charge pump current magnitudes.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Reliability

If the PLL circuit uses a fixed charge pump current to tolerate VCO gain variations, then the circuit area and power consumption increase, but phase noise performance deteriorates

Engineering Contradiction:

Improvetolerance to VCO gain variationsVSAvoidpower consumption

Core Design Contradiction:

ReliabilityVSUse of energy by moving object

Solution Approach 1:

The charge pump current is made dynamic rather than fixed. The calibration circuit measures the actual VCO gain and adjusts the charge pump current magnitude accordingly to maintain a constant product (Icp × KVCO). This dynamic adjustment allows the system to tolerate VCO gain variations without requiring excessive power consumption or circuit area.

Inventive Principle:

Principle #15Dynamics

Solution Approach 2:

The system changes the parameter of charge pump current magnitude based on measured VCO gain characteristics. By measuring the VCO gain at different control voltages and calculating the appropriate current magnitude, the system adapts the charge pump current parameter to compensate for VCO gain variations, resolving the contradiction between reliability and power consumption.

Inventive Principle:

Principle #35Parameter changes

2Reliability

If the PLL circuit increases charge pump current to maintain jitter specifications under VCO gain variations, then power consumption increases, but phase noise increases

Engineering Contradiction:

Improvejitter specification complianceVSAvoidphase noise

Core Design Contradiction:

ReliabilityVSObject-generated harmful factors

Solution Approach 1:

The calibration circuit implements a feedback mechanism that measures the actual VCO gain and uses this information to adjust the charge pump current magnitude. This feedback loop ensures that the product of charge pump current and VCO gain remains constant, maintaining jitter specifications while avoiding excessive phase noise that would result from simply increasing the current.

Inventive Principle:

Principle #23Feedback

3Device complexity

If the PLL circuit uses a fixed charge pump current, then the circuit design is simpler, but it cannot tolerate VCO gain variations without increasing area and power

Engineering Contradiction:

Improvecircuit design simplicityVSAvoidtolerance to process, temperature, and frequency changes

Core Design Contradiction:

Device complexityVSReliability

Solution Approach 1:

The system performs preliminary calibration by measuring the VCO gain at different control voltages before normal operation. This preliminary action captures the VCO gain characteristics and stores them in a look-up table, enabling the system to tolerate process, temperature, and frequency variations during actual operation without requiring complex real-time adjustments.

Inventive Principle:

Principle #10Preliminary action

4Object-generated harmful factors

If the PLL circuit implements calibration to maintain constant Icp×KVCO product, then phase noise is reduced, but device complexity increases

Engineering Contradiction:

Improvephase noiseVSAvoidcircuit complexity

Core Design Contradiction:

Object-generated harmful factorsVSDevice complexity

Solution Approach 1:

The calibration process is segmented into discrete steps where the control voltage is swept through a range of values and the VCO gain is measured at each step. These measurements are stored in a look-up table, which can be queried during operation. This segmentation approach reduces phase noise while keeping the circuit complexity manageable by avoiding continuous complex calculations.

Inventive Principle:

Principle #1Segmentation

Data Source

PatentUS11418204B2Phase lock loop (PLL) with operating parameter calibration circuit and method

Publication Date: 2022.08.16 STMICROELECTRONICS INT NV

AI summary

A calibration scheme is used to control PLL bandwidth and contain its spread. In open loop, the VCO control voltage is swept over a range of values and VCO output frequency is measured at each control voltage level. The gain KVCO is determined for each measured output frequency and a corresponding current magnitude for the variable magnitude charge pump is calculated from a ratio of a constant to the gain KVCO and correlated in a look-up table to the measured output frequency. Once calibration is completed, the PLL loop is closed and a calculated current magnitude is fetched from the look-up table based on a desired output frequency for the PLL circuit. The variable magnitude charge pump circuit is then controlled to generate a charge pump current with a magnitude corresponding to the fetched charge pump current magnitude.