RMS Detector Settling Time via Clamping Circuit
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Solution Overview
Problem
RMS power detectors exhibit significantly longer settling times for falling input signals compared to rising signals due to the slow discharge of filter capacitors and voltage rail issues when signals exceed the VGA range, leading to inaccurate measurement and prolonged settling times.
Innovation Solution
Incorporating an additional gain stage and a clamping circuit with a precision current mirror and a separate clamping op-amp to regulate voltage swings and shunt excess currents, thereby limiting positive voltage excursions and improving settling times.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a large filter capacitor is used to achieve required RMS accuracy for high crest factor signals, then measurement precision is improved, but settling time for falling edges becomes excessively long
Solution Approach 1:
A clamping circuit is introduced as an intermediary component between the detector output and the filter capacitor. This clamping circuit actively regulates the voltage at the filter input, preventing excessive voltage buildup during high-power signals and enabling faster discharge during low-power signals, thus resolving the contradiction between measurement accuracy and settling time
Solution Approach 2:
The clamping circuit dynamically changes the operating parameters of the detector system by clamping the detector output voltage to predetermined levels. This parameter control allows the system to maintain accuracy for high crest factor signals while enabling faster settling by preventing the filter capacitor from charging to excessively high voltages in the first place
2Measurement precision
If the VGA levels the signal to keep the detector operating in its optimal range, then measurement precision is improved, but when the input signal is too large or too small, the output goes to supply rails causing excessively long settling times
Solution Approach 1:
The clamping circuit provides beforehand cushioning by preventing the detector output voltage from reaching the supply rails. By clamping the voltage to safe levels before it can rail, the circuit prepares the system in advance for signal conditions outside the VGA range, enabling faster recovery and settling when these conditions occur
3Adaptability or versatility
If a set-point approach with a second detector is used to extend dynamic range, then adaptability is improved, but when signal changes quickly from high to low, only the set point current is available to discharge the filter causing asymmetric settling times
Solution Approach 1:
The clamping circuit acts as an intermediary that provides an additional current path for discharging the filter capacitor during high-to-low signal transitions. This supplementary discharge path works in conjunction with the set-point current, significantly reducing the asymmetric settling behavior and enabling faster recovery times while preserving the extended dynamic range capability
Data Source
AI summary
A root-mean-square (RMS) detector includes detection circuitry having as an input a radio frequency signal, target voltage and a set voltage and a RMS signal as an output, and a gain stage within the detection circuitry to produce the RMS signal as an output. The gain stage provides for faster settling times of the detector.