MZM Predistortion Circuit for Broadband RoF SFDR Linearization
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Solution Overview
Problem
Radio-over-Fiber (RoF) links suffer from low Spurious Free Dynamic Range (SFDR) due to the nonlinearity of electro-optical modulators, particularly Mach-Zehnder Modulators (MZMs), which limits their ability to handle high-frequency signals and is exacerbated by the high power consumption of existing linearization techniques.
Innovation Solution
A predistorter system with low noise RF amplifiers and inductively-degenerated frequency doublers is used to generate and filter second-order intermodulation products, which are then injected into the MZM to linearize the RF signal, achieving broadband compensation and improving SFDR without significant power penalty.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional MZM is used for electro-optical conversion, then device simplicity is maintained, but SFDR is limited due to nonlinearity
Solution Approach 1:
The predistorter applies preliminary nonlinear transformation to the RF signal before it enters the MZM, pre-compensating for the modulator's nonlinear transfer function. This preliminary action linearizes the overall system response without requiring changes to the MZM itself, thereby improving SFDR while maintaining device simplicity.
Solution Approach 2:
The predistorter acts as an intermediary device between the RF signal source and the MZM. It processes the signal to cancel out the anticipated nonlinear distortion that will be introduced by the MZM, serving as a mediator that enables linear operation of the entire electro-optical conversion chain.
2Reliability
If existing linearization techniques are applied to MZM, then SFDR is improved, but power consumption increases significantly
Solution Approach 1:
The predistorter utilizes parameter changes in the RF signal domain by applying voltage-controlled variable gain amplifiers and adjustable delay lines to generate compensating distortion products. By dynamically adjusting these parameters based on the input signal characteristics, the system achieves linearization with minimal additional power consumption compared to conventional linearization techniques.
3Speed
If MZM operates at high frequencies, then bandwidth is increased, but SFDR degrades due to nonlinearity
Solution Approach 1:
The predistorter implements preliminary anti-action by generating distortion products with opposite phase and amplitude characteristics to those produced by the MZM's nonlinear transfer function. This counteracts the harmful nonlinear effects before they manifest in the optical output, enabling high-frequency operation while maintaining high SFDR performance.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The solution achieves a record 120 dB·Hz2/3 SFDR at 10.5 GHz with a 19 dB improvement over previous RoF links, while maintaining low noise figure and reducing power consumption by 60 mW, demonstrating effective linearization across a wide frequency range.
Implementation Method 1
an inductively-degenerated frequency doubler to square and filter IM2 products of the RF input
Implementation Method 2
A Mach-Zehnder Modulator (MZM) is used for electro-optical conversion
Data Source
AI summary
A predistorter for an electro-optical converter includes a plurality of low noise RF amplifiers distributed along a transmission line that receive an RF input. Second order intermodulation injection (IM2) circuitry includes an inductively-degenerated frequency doubler to square and filter IM2 products of the RF input. A Mach-Zehnder Modulator (MZM) is used for electro-optical conversion. Feed forward circuitry injects IM2 to independently propagate RF intermodulation components with velocity matching to the MZM. At least one driver injects the RF input and RF intermodulation components into the MZM.