Dual-Loop AGC for Radio Receivers With Fewer DC Transients
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Solution Overview
Problem
Radio receivers, particularly zero-IF receivers, face performance degradation due to DC current transients caused by automatic gain control (AGC) gain step transitions, which cannot be effectively filtered or cancelled by existing DC offset compensation circuits, leading to saturation and noise in the baseband signal.
Innovation Solution
Implementing a dual-AGC loop system where a slower analogue AGC loop controls signal amplification in the analogue domain and a faster digital AGC loop controls amplification in the digital domain, with the digital loop having a cycle time significantly shorter than the analogue loop, to reduce the occurrence of DC transients.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a single AGC loop with discrete gain steps is used to control signal amplification, then the system can maintain signal dynamics within ADC range, but DC transients occur at gain transitions causing saturation and performance degradation
Solution Approach 1:
The AGC system is divided into two separate loops: an analog AGC loop that controls the analog gain before ADC, and a digital AGC loop that controls the digital gain after ADC. This segmentation allows each loop to operate independently with different cycle times, preventing the DC transients that occur when a single loop switches gain steps.
Solution Approach 2:
The system dynamically adjusts the cycle time of each AGC loop based on operating conditions. The analog AGC loop uses a longer cycle time to avoid generating high-frequency DC transients, while the digital AGC loop uses a shorter cycle time to maintain precise signal dynamics control. This dynamic timing adjustment eliminates the harmful DC transients while preserving reliability.
2Object-affected harmful factors
If DC offset compensation circuits are used to remove DC components, then DC offsets can be filtered, but DC transients from gain switching cannot be effectively cancelled
Solution Approach 1:
The analog AGC loop updates gain at a slower rate with longer cycle time, preemptively preventing the generation of high-frequency DC transients before they can affect the signal. By controlling the timing of gain transitions in the analog domain, the system avoids creating the problematic DC transients that digital filtering would otherwise need to handle.
3Object-generated harmful factors
If gain control is implemented in the digital domain only, then DC transients are avoided, but signal dynamics control before ADC is insufficient
Solution Approach 1:
The gain control function is segmented between analog and digital domains. The analog AGC loop handles coarse gain adjustment before ADC to maintain signal dynamics within the converter's range, while the digital AGC loop handles fine gain adjustment after ADC to maintain precision and avoid DC transients. This segmentation allows both functions to operate optimally in their respective domains.
Solution Approach 2:
The analog-to-digital converter acts as an intermediary between the two AGC loops. The analog loop controls the signal level entering the ADC, while the digital loop controls the signal level exiting the ADC. This intermediary structure allows the system to benefit from both analog gain control (for dynamic range) and digital gain control (for precision and transient avoidance).
Data Source
AI summary
An automatic gain control (AGC) method and system for a radio receiver are proposed in which the ACG comprises two AGC loops; a first loop controlling signal gain in the analogue portion of the radio receiver, a second loop controlling gain in the digital domain after digitization of the received signal. The analogue AGC loop has a slower response time than the digital AGC loop. When applied to a multi-branch diversity receiver, each branch has its own digital AGC loop, but the analogue gain can be common to all branches, based on measurement of the analogue signal in each branch.