CML Output Driver Pre-Distortion for Symmetric VCSEL Eye Plots
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Existing VCSEL drivers face challenges in achieving symmetric optical eye plots at high data rates due to high capacitance and asymmetric turn-on/off behavior, leading to increased bit error rates, and existing solutions like pre-emphasis and regulated output impedance fail to provide flexible adjustments for different data rates and VCSEL parameters.
Innovation Solution
A trans-linear driver topology with pre-distortion of the input signal to compensate for output stage distortion, using degeneration resistors and transistor loads to establish a linear relationship between input and output signals, and incorporating a delay buffer and pulse generation stage for independent adjustment of overshoot and undershoot characteristics.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If pre-distorted current signal is superimposed to tail current, then output current peaking is achieved for steeper optical edges, but the solution does not allow flexible adjustment to accommodate different data rates and VCSEL parameters
Solution Approach 1:
The patent implements dynamic adjustment capability by making the peak current amplitude and width adjustable through control signals. The circuit transitions from fixed pre-distortion to dynamically controllable pre-distortion, allowing adaptation to different data rates and VCSEL parameters while maintaining steep optical edges.
Solution Approach 2:
The patent changes the parameters of the pre-distorted signal (amplitude, width, timing) to accommodate different operating conditions. By varying these parameters dynamically, the system achieves both steep optical edges and adaptability to different data rates and VCSEL characteristics.
2Speed
If single-sided fixed peak value implementation is used, then current peaking is achieved, but the output common mode and crossing point are shifted and symmetry is lost
Solution Approach 1:
The patent intentionally introduces asymmetric pre-distortion to compensate for the inherent asymmetric behavior of VCSELs. By applying asymmetric current peaking in a controlled manner, the system achieves symmetric optical output eyes, resolving the contradiction between edge steepness and eye symmetry.
Solution Approach 2:
The patent applies pre-distortion in advance to counteract the expected asymmetric behavior of the VCSEL and transmission medium. This preliminary anti-action ensures that the final optical output achieves both steep edges and symmetry by pre-compensating for anticipated distortions.
3Reliability
If fixed width and height undershoot is superimposed, then some eye opening enhancement is achieved, but the solution fails to maximize both vertical and horizontal eye opening simultaneously
Solution Approach 1:
The patent implements dynamic control of the pre-distorted signal parameters (amplitude, width, timing) to simultaneously optimize both vertical and horizontal eye opening. This dynamic adjustment allows the system to maximize bit error rate performance while adapting to different transmission conditions and data rates.
Data Source
AI summary
An integrated circuit (IC) for driving a light emitting semiconductor device is provided. The IC includes an input stage configured to receive a first input signal with a first differential pair of bipolar transistors and a second input signal with a second differential pair of bipolar transistors and to provide a pre-driver output signal being a superposition of the first input signal and the second input signal and an output stage including a third differential pair of bipolar transistors for receiving the pre-driver output signal of the input stage and for driving the light emitting semiconductor device in response to the pre-driver output signal, wherein the IC is configured to pre-distort the pre-driver output signal of the input stage so as to compensate a signal distortion of the output stage.