Signal Path Linearizer for PAM4 SerDes Nonlinearities
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Solution Overview
Problem
PAM4 signal path nonlinearities cause significant degradation in pulse amplitude modulation 4-level (PAM4) signals due to asymmetrical eye height and width, leading to vertical skew and alignment issues in PAM4 SerDes communications links.
Innovation Solution
The implementation of a signal path linearizer using cascaded differential gm stages to provide a nonlinear wideband gain adjustment by introducing a defined DC gain adjustment, approximating the inverse of signal path nonlinearities, which compensates for nonlinearity across the PAM4 signal path.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If PAM4 signal encoding with four signal levels is used to increase data rate, then productivity is improved, but signal path nonlinearities cause degradation in eye symmetry and vertical alignment
Solution Approach 1:
The patent applies preliminary action by introducing a nonlinear DC gain adjustment stage that pre-compensates for signal path nonlinearities before the signal is transmitted. The gain adjustment circuit modifies the PAM4 signal levels in advance to counteract the expected asymmetry and skew that would occur during transmission, thereby maintaining eye symmetry and vertical alignment while achieving high data rates
Solution Approach 2:
The patent employs parameter changes by dynamically adjusting the DC gain of the differential gm stage based on the input signal level. The gain adjustment circuit varies the transconductance parameter (gm) of the amplifier stage to provide nonlinear compensation, changing the gain parameter across different signal levels to counteract the nonlinearities in the PAM4 signal path
2Productivity
If transitions between non-adjacent signal levels are implemented to encode 2 bits per symbol, then productivity is improved, but transition time increases causing narrowed top and bottom eyes
Solution Approach 1:
The gain adjustment circuit performs preliminary action by pre-shaping the signal levels to account for the longer transition times between non-adjacent levels. By adjusting the DC gain before transmission, the circuit compensates for the extended transition duration, maintaining adequate eye opening and timing margins despite the increased transition time required for 2-bit encoding
3Manufacturing precision
If cascaded differential gm stages with nonlinear DC gain adjustment are added to compensate for nonlinearities, then signal path linearity is improved, but device complexity increases
Solution Approach 1:
The patent applies merging by combining the nonlinear DC gain adjustment function with the existing differential gm stage architecture. The gain adjustment circuit is integrated into the signal path by cascading additional differential gm stages that provide both amplification and nonlinear compensation, merging multiple functions into a unified circuit structure rather than adding separate compensation components
Solution Approach 2:
The differential gm stages serve multiple functions simultaneously: they provide signal amplification, impedance transformation, and nonlinear compensation. The same circuit elements that amplify the PAM4 signal also introduce the controlled nonlinear DC gain adjustment, making the amplifier stage universal in its functionality and reducing the need for separate compensation circuits
Data Source
AI summary
A signal path linearizer for PAM4 SerDes communications compensates (including pre-compensates) for signal path nonlinearities. The linearizer can be configured with first and second differential gm stages, the first differential gm stage to provide a DC gain, and the second differential gm stage to introduce a defined nonlinear adjustment in DC gain by adding to or subtracting from the DC gain of the first differential gm stage. The differential gm stages can be configured to generate a compensated PAM4 signal with the combined DC gain providing a nonlinear wideband gain adjustment to compensate for nonlinearities in the PAM4 signal path. Compensation range can be increased by selective degeneration, and the compensation region can be shifted by selectively introducing input offset(s).