Charge Pump Buffer for PLL Supply Noise Cancellation

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Solution Overview

Problem

Phase-locked loops (PLLs) are susceptible to supply noise, which can cause jitter in the output frequency and potentially lead to loss of lock with the reference frequency, especially in low bandwidth PLLs.

Innovation Solution

A charge pump buffer is introduced between the charge pump and the voltage controlled oscillator (VCO) to cancel supply noise by generating an inversely proportional voltage change that opposes changes in the supply voltage, allowing the PLL to maintain a stable output frequency.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Ease of operation

If voltage control is used to tune the PLL frequency, then the PLL tuning simplicity is improved, but the susceptibility to supply noise increases

Engineering Contradiction:

ImprovePLL tuning simplicityVSAvoidsupply noise susceptibility

Core Design Contradiction:

Ease of operationVSObject-affected harmful factors

Solution Approach 1:

A charge pump buffer is introduced as an intermediary component between the charge pump and the VCO. This buffer includes a supply noise cancellation circuit that actively compensates for supply voltage variations by generating compensating current changes, thereby isolating the VCO from supply noise while maintaining the simplicity of voltage control tuning.

Inventive Principle:

Principle #24Intermediary (Mediator)

2Speed

If the supply voltage changes rapidly due to noise, then the charging time of the LC tank circuit changes, but this causes jitter in the output frequency

Engineering Contradiction:

Improvecharging speed of LC tankVSAvoidoutput frequency stability

Core Design Contradiction:

SpeedVSReliability

Solution Approach 1:

The supply noise cancellation circuit performs preliminary anti-action by detecting supply voltage changes and generating compensating current changes before these changes can affect the VCO output frequency. The buffer actively opposes supply noise by adjusting the charging current to the LC tank in reverse proportion to supply voltage variations, preventing frequency jitter.

Inventive Principle:

Principle #9Preliminary anti-action

3Reliability

If the charge pump buffer introduces a contrary voltage change to cancel supply noise, then the frequency lock stability is improved, but the circuit complexity increases

Engineering Contradiction:

Improvefrequency lock stabilityVSAvoidcharge pump buffer complexity

Core Design Contradiction:

ReliabilityVSDevice complexity

Solution Approach 1:

The supply noise cancellation functionality is merged with the charge pump buffer circuit rather than being implemented as a separate system. The buffer combines the signal buffering function with the supply noise cancellation function in a single integrated circuit, reducing overall system complexity while maintaining frequency lock stability.

Inventive Principle:

Principle #5Merging (Combining)

Data Source

PatentUS8674753B2Systems and methods for cancelling phase-locked loop supply noise

Publication Date: 2014.03.18 TEXAS INSTRUMENTS INC

AI summary

One embodiment of an apparatus for cancelling supply noise includes an input circuit operable to receive an input from a charge pump and a drive circuit connected to an output of the input circuit. The drive circuit is operable to provide an output matching the input to the input circuit when a voltage source powering the input circuit and the drive circuit is stable, and to introduce a contrary voltage change on the buffered output when the voltage source is noisy, with the contrary voltage change being contrary to a voltage change on the voltage source due to noise.