Optical Receiver AGC Using Split VOA and PGA Control Loops
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Solution Overview
Problem
In multi-chip receiver systems, providing an automatic gain control (AGC) loop between chips with programmable gain elements on one chip and an analog to digital converter (ADC) on another chip is challenging due to slow and difficult control without dedicated feedback lines.
Innovation Solution
An AGC loop is implemented using a controller to manage a variable optical attenuator (VOA) and programmable gain amplifier (PGA) on separate integrated circuits, optimizing signal-to-noise ratio (SNR) by controlling attenuation and gain stages with independent loop update times, and utilizing local and external feedback for stability and accuracy.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If an AGC loop is implemented between separate chips with programmable gain elements on one chip and ADC on another chip, then gain control functionality is achieved, but the control speed becomes slow and the system becomes difficult to control
Solution Approach 1:
The system divides the AGC control into two independent segments: a fast coarse control loop implemented in the optical receiver circuit that provides rapid initial gain adjustment, and a slower fine control loop implemented in the digital signal processor that provides precise final adjustment. This segmentation allows each loop to operate at its optimal speed without being constrained by the other, resolving the contradiction between achieving comprehensive AGC functionality and maintaining fast control response.
2Device complexity
If an AGC loop is implemented between separate chips without dedicated feedback lines, then device complexity is reduced, but control precision and stability deteriorate
Solution Approach 1:
The patent introduces digital signal processing as an intermediary mechanism that receives digital samples of the output signal, performs precise measurement and analysis, and generates control commands that are sent back to the programmable gain elements. This intermediary digital processing layer compensates for the lack of direct analog feedback paths, maintaining high control precision while avoiding the complexity of dedicated feedback lines between chips.
3Speed
If independent loop update times are used for coarse and fine control loops, then control speed is improved, but loop synchronization becomes more difficult
Solution Approach 1:
The system implements periodic control cycles where the fast coarse loop operates continuously at a high update rate, while the slow fine loop operates periodically at a lower update rate. Each loop is synchronized to specific phases within the overall control cycle, with the fine loop waiting for appropriate timing windows to update its control parameters. This periodic coordination maintains loop synchronization while preserving the speed advantages of independent operation.
Data Source
AI summary
Provided herein are techniques to facilitate automatic gain control for an optical receiver. In one example, a method may include for a first mode, first adjusting, for a received optical input signal, at least one of: a coarse gain of a programmable gain amplifier (PGA), a fine gain of the PGA, or an attenuation of a variable optical attenuator (VOA) until a voltage swing peak value of an output signal of the optical receiver satisfies a first voltage swing peak range. The method may further include, through a second mode, triggering second adjusting of at least one of the VOA attenuation or the PGA fine gain based on determining that the voltage swing peak value of the output signal does not satisfy a second voltage swing peak range or that another voltage swing peak value obtained from an external signal processing device does not satisfy another voltage swing peak range.