Common Receiver AGC Loop for IQ Gain Balancing
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Solution Overview
Problem
Current receiver automatic gain control (AGC) systems for asymmetrical or unbalanced constellations are inefficient as they require independent AGC loops per tributary, which can re-normalize the I and Q power imbalance, destroying the integrity of signals like BPSK and 4ASK, and are not suitable for high-integration pluggable coherent modems due to complexity and noise introduced by external digital AGC loops.
Innovation Solution
Implementing a common AGC loop for both I and Q tributaries, with an IQ Gain Balancing function to set both tributaries to the same gain using an offset gain control voltage, allowing the receiver to maintain constant total I and Q power and track fast optical power transients, and optionally using hardware or firmware-based implementations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If independent AGC loops are used per tributary, then RF power at ADC input is maintained constant, but the asymmetrical power ratio of signals is destroyed
Solution Approach 1:
The patent combines two independent AGC loops into a single common AGC loop that controls both I and Q tributaries simultaneously. This merging approach maintains the total RF power at ADC input while preserving the asymmetrical power ratio between I and Q channels, thereby maintaining signal integrity for BPSK and 4ASK without requiring complex independent control loops
Solution Approach 2:
The patent introduces an offset gain control voltage parameter that adjusts the gain of one TIA relative to the other. By dynamically changing this offset parameter, the system maintains constant total RF power while preserving the asymmetrical power distribution between I and Q tributaries, solving the contradiction between power stabilization and signal integrity
2Ease of manufacture
If external digital AGC loops are used, then gain control is achieved, but integration and noise performance are degraded
Solution Approach 1:
The patent extracts the AGC loop from the external digital domain and relocates it into the analog domain within the TIA chip itself. This extraction eliminates the need for external digital AGC components and their associated interconnects, thereby improving integration and reducing noise while maintaining gain control functionality
Solution Approach 2:
The patent introduces an analog power detector as an intermediary element that bridges the TIA output and the AGC loop. This analog mediator enables the AGC control voltage to be generated and applied directly within the analog domain, achieving high integration and optimal noise performance without requiring external digital components
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
This solution preserves the asymmetrical power ratio of signals, enhances integration and flexibility in hardware design, and improves the tracking of optical power transients without re-designing existing AGC loops, making it suitable for high-integration pluggable coherent modems.
Implementation Method 1
a loop filter operable for integrating a voltage difference to provide the offset gain control voltage
Data Source
AI summary
An automatic gain control system for a receiver, including: an automatic gain control loop (40) adapted to be coupled to both a first transimpedance amplifier (12) coupled to a first analog-to-digital converter (14) forming a first tributary and a second transimpedance amplifier (12) coupled to a second analog-to-digital converter (14) forming a second tributary; and an offset gain control voltage to gain balance a transimpedance amplifier gain of the first tributary and a transimpedance amplifier gain of the second tributary. The automatic gain control loop can be analog. Also, the automatic gain control loop can be implemented in hardware or firmware.