Noise Rejection Filter for Trigger Circuit with Slope Detection
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Solution Overview
Problem
Existing trigger circuits for test and measurement instruments face challenges in accurately rejecting noise while preserving signal pulse width and maintaining triggering at signal peaks, leading to errors in pulse width measurement and premature triggering near peak values.
Innovation Solution
A noise rejection filter that compares each sample of the input signal with its filtered output to determine slope changes and hysteresis differences, updating the filter output only when significant changes occur, thereby preserving signal peaks and pulse width by selecting appropriate filtered trigger signals for comparison with a desired trigger level.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If a traditional comparator with hysteresis is used for triggering, then noise rejection is improved, but pulse width measurement accuracy deteriorates and triggering stops near peak values
Solution Approach 1:
The trigger signal generation is divided into multiple independent paths: a first path uses a comparator with hysteresis for noise rejection, while a second path uses a slope detector for accurate pulse width measurement. This segmentation allows each path to optimize for its specific function without compromising the other, resolving the contradiction between noise rejection and measurement accuracy.
Solution Approach 2:
A multiplexer acts as an intermediary to select between the hysteresis comparator output and the slope detector output based on trigger level conditions. When the trigger level is near peak values, the multiplexer switches to the slope detector path, providing accurate pulse width measurement while maintaining noise rejection through the hysteresis path for other conditions.
2Object-affected harmful factors
If a pseudo filter is used to suppress noise, then noise rejection is improved, but signal peaks are knocked off causing triggering to stop near minimum or maximum peak values
Solution Approach 1:
The multiplexer serves as an intermediary that routes different signal paths based on the trigger level relative to peak values. When the trigger level approaches the minimum or maximum peak value, the multiplexer switches to the slope detector path, preserving triggering reliability at signal peaks while maintaining noise rejection through the hysteresis comparator path for other conditions.
Solution Approach 2:
The system changes the operating parameters of the trigger circuit dynamically based on the input signal characteristics. By detecting whether the trigger level is near peak values and switching between different comparison methods (hysteresis-based vs. slope-based), the system adapts its behavior to maintain both noise rejection and peak triggering reliability.
3Object-affected harmful factors
If hysteresis is applied to prevent mistaken triggering on noise, then noise rejection is improved, but rising edge triggers stop occurring when trigger level is near minimum peak value and falling edge triggers stop when near maximum peak value
Solution Approach 1:
The triggering function is segmented into two independent mechanisms: hysteresis-based triggering for normal conditions and slope detector-based triggering for peak conditions. This segmentation allows the system to maintain broad triggering coverage while preserving noise rejection capabilities through the appropriate mechanism for each operating condition.
Solution Approach 2:
The trigger circuit dynamically switches between different triggering mechanisms based on the input signal conditions. The multiplexer dynamically routes signals to the appropriate comparator or slope detector based on whether the trigger level is near peak values, enabling the system to adapt its triggering behavior to maintain both noise rejection and comprehensive triggering coverage.
Data Source
AI summary
A noise rejection filter for a trigger circuit uses an algorithm that updates the filter output monotonically so long as the signal slope remains unchanged, maintains the filter output at a constant level when the signal slope changes but the difference between the sample value and the filter output is less than or equal to a hysteresis value, and changes the signal slope while updating the filter output when the difference is greater than the hysteresis value. This maintains the peaks of the input signal at the filter output. The noise rejection filter may be used in a trigger circuit prior to a comparator so that the trigger signal from the comparator accurately reflects the signal pulse width at a desired trigger level and trigger events are detected when the desired trigger level is near the peaks of the input signal.