Passive Capture Adapter Circuit for Distortion-Free Serial Bus Probing
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Solution Overview
Problem
Conventional oscilloscope probes are ineffective for sensing high-speed digital signals due to loading issues, which distort waveforms and make it difficult to analyze high-speed serial bus circuitry, especially in environments where limited space restricts the placement of external test equipment.
Innovation Solution
A multi-stage passive capture adapter (PCA) circuit with a differentiator and dual-slope comparator/driver stages that use high-pass filtering and low-voltage differential signaling to capture and reconstruct high-speed digital signals, providing a high-impedance interface that minimizes loading and allows for remote analysis.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional oscilloscope probes are used to sense high-speed digital signals, then the signals can be captured for analysis, but the probes load the circuit and distort the waveform
Solution Approach 1:
The patent introduces a passive capture adapter circuit as an intermediary device between the high-speed circuit and the oscilloscope probe. This adapter includes a differentiator stage with a small capacitor that presents a high impedance to the circuit under test, minimizing loading effects while still capturing the essential edge information of the digital signals for accurate waveform reconstruction and analysis.
Solution Approach 2:
The patent transforms the signal parameters by using a differentiator circuit that converts the original digital signal into its derivative form. This parameter transformation allows the capture of signal edges with minimal loading on the circuit, as the differentiator's small capacitor presents high impedance while still detecting the rapid transitions characteristic of high-speed digital signals.
2Measurement precision
If external test equipment is placed close to the circuit for signal sensing, then accurate measurement is possible, but there is limited space on fine pitch serial bus implementations
Solution Approach 1:
The patent divides the test system into two separate segments: a passive capture adapter that interfaces with the circuit under test and occupies minimal space on the circuit board, and a remote oscilloscope or analysis equipment that processes the captured signals. This segmentation allows accurate signal capture in limited space while enabling comprehensive analysis remotely.
Solution Approach 2:
The patent extends the testing capability from the spatial dimension to the temporal dimension by capturing signal edges instantaneously at the circuit location and allowing remote analysis at a different time. The passive capture adapter captures the essential signal characteristics in-situ, enabling remote equipment to analyze the signals without requiring physical proximity to the circuit.
3Object-affected harmful factors
If the differentiator uses a small capacitor to form a fast time constant, then only edges of the signal are sensed, but this provides high impedance to minimize circuit loading
Solution Approach 1:
The patent extracts only the essential information from the signal - the edges or transitions - using a differentiator with a small capacitor. This extraction approach captures the critical timing and frequency characteristics of high-speed digital signals while ignoring the intermediate voltage levels, thereby minimizing circuit loading while preserving the most important signal features for analysis.
Solution Approach 2:
The differentiator creates a simplified copy of the original signal that contains only the edge information. This copied signal, while not identical to the original waveform, preserves the essential timing and transition characteristics needed for debugging and analysis, while requiring minimal interaction with the circuit under test.
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
The PCA circuit effectively senses and recovers high-speed digital signals with minimal distortion, enabling accurate analysis and debugging of high-speed serial bus circuits without disrupting the signal paths, allowing for convenient testing from a distance.
Implementation Method 1
a differentiator having a small capacitor coupled to a plurality of resistors to form a high-pass filter with a substantially fast time constant
Implementation Method 2
a dual-slope comparator/driver of the first stage that illustratively includes a plurality of high-speed comparators and drivers
Implementation Method 3
The outputs of the comparators are illustratively differential signal pairs that are transmitted/driven by drivers over a dual-pair interconnect cable
Implementation Method 4
The differential receivers are illustratively paired and receive the differential signal pairs transmitted over the cable from the dual-slope comparator/driver
Implementation Method 5
each of which is formed of a resistor and a capacitor arranged in parallel and coupled to a totem pole of resisters
Implementation Method 6
The signal restorer is an edge reconstruction flip-flop configured to respond to the input signals to reconstruct the edges of the original input signal waveform
Implementation Method 7
a driver of the driver/shaper combines the input signals to an output that is fed to a third shaper network
Implementation Method 8
which illustratively includes a plurality of resistors and a capacitor configured to substantially match an output signal of driver/shaper to the input signal waveform
Data Source
AI summary
A multi-stage passive capture adapter (PCA) circuit is configured to sense and recover digital signals present on a high-speed serial bus for capture and analysis in external test equipment. A first stage of the PCA circuit includes a differentiator that functions as a high impedance probe that contacts the serial bus to capture an original input signal waveform of the high-speed digital signals. The signal waveform is fed to a dual-slope comparator/driver that includes a plurality of high-speed comparators and drivers. The second stage includes a differential receiver/shaper that converts logic levels of differential receiver outputs to input signals that set and reset a signal restorer whose output signals are fed to a driver of a driver/shaper. The output of the driver is then fed to a shaper network configured to substantially match an output signal of driver/shaper to the input signal waveform sensed from the high-speed serial bus.