Charge Pump Error Cancellation for Fractional-N Synthesizer Linearity
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Solution Overview
Problem
Conventional delta-sigma fractional-N frequency synthesizers suffer from noise leakage due to non-linearity, degrading in-band phase noise and spur performance, with existing linearization techniques either worsening reference spur or in-band noise performance.
Innovation Solution
A charge pump linearization technique using an error canceling circuit with matched pairs of charging and discharging current sources, along with an error cancellation circuit that replicates currents to eliminate the non-linear term, ensuring the charge pump operates without increasing in-band noise or adversely affecting reference spur performance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If conventional charge pump is used in delta-sigma fractional-N synthesizer, then the synthesizer can operate in fractional mode, but non-linearity causes noise leakage that degrades in-band phase noise and spur performance
Solution Approach 1:
The charge pump current is segmented into multiple paths: a main charge pump path and a separate error cancellation path. The error cancellation path contains matched current sources that replicate and cancel the non-linear components, separating the linear and non-linear current components for independent handling.
Solution Approach 2:
The non-linear current components, which normally cause noise leakage and performance degradation, are converted into a beneficial cancellation signal. By replicating these error currents through the error cancellation path and injecting them with proper phase and polarity, the harmful non-linearities are transformed into useful correction signals that eliminate the noise leakage.
2Manufacturing precision
If existing linearization techniques are applied, then linearity is improved, but reference spur or in-band noise performance worsens
Solution Approach 1:
The error cancellation current sources are designed to dynamically adjust their current magnitude based on the operating conditions. By changing the current parameter in the error cancellation path to match the non-linear error components, the system achieves adaptive cancellation that maintains linearity across different operating points without introducing fixed spurs or noise.
Solution Approach 2:
The error cancellation mechanism operates as a feedback system where the non-linear error currents are continuously replicated and cancelled. The matched current sources in the error cancellation path provide real-time compensation for the non-linearities, creating a closed-loop effect that maintains linearity while preventing the generation of reference spurs and in-band noise.
Data Source
AI summary
A delta-sigma fractional-N frequency synthesizer having a charge pump with error canceling circuitry eliminates a non-linear term from the charge pump transfer function. The charge pump includes a matched pair of charging current sources, each supplying a first current IP1 to a common node, when enabled. The charge pump also includes a matched pair of discharging current sources, each sinking a second current IN1 from the common node, when enabled. The error canceling circuitry includes a charging current source, which supplies a current equal to the second current IN1 to the common node, when enabled. The error canceling circuitry also includes a discharging current source, which sinks a current equal to the first current IP1 from the common node, when enabled. The charging and discharging current sources of the error canceling circuitry are both enabled when either one of the matched pairs of charging and discharging current sources is enabled.