Compensation Digital Filter for Signal Acquisition Probe Loading
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Solution Overview
Problem
Traditional passive voltage probes load the device under test significantly due to high probe tip capacitance, which affects bandwidth and signal-to-noise ratio, and existing solutions either increase resistance, reducing signal input or require adjustments that compromise frequency response.
Innovation Solution
A signal acquisition system with a signal acquisition probe and a signal processing instrument featuring mismatched time constants, utilizing a compensation digital filter with pole-zero pairs and adjustable gain, along with reduced probe tip capacitance to minimize loading and maintain flat frequency response.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If traditional passive voltage probes are used with high probe tip capacitance, then the probe can provide sufficient signal attenuation and frequency compensation, but the device under test is significantly loaded which reduces bandwidth and signal-to-noise ratio
Solution Approach 1:
The patent extracts the compensation capacitor from the probe tip and relocates it to the oscilloscope input. This separation allows the probe tip capacitance to be minimized for reduced loading, while the compensation function is performed separately at the oscilloscope end, resolving the contradiction between low loading and frequency compensation
Solution Approach 2:
The patent introduces a compensating capacitor at the oscilloscope input as an intermediary element that performs the frequency compensation function without being part of the probe tip loading. This intermediary capacitor, combined with the probe's resistive voltage divider, achieves flat frequency response while the probe itself maintains minimal capacitance
2Object-affected harmful factors
If the probe tip capacitance is reduced to increase capacitive reactance, then capacitive loading is reduced, but the time constant of the probe tip network changes requiring adjustment of other component values which increases attenuation
Solution Approach 1:
By extracting the compensation capacitor from the probe tip network and placing it at the oscilloscope input, the patent allows the probe tip capacitance to be reduced without requiring proportional changes to other probe components. The compensation function is handled separately, preventing the need to increase resistance and thereby avoiding increased signal attenuation
Solution Approach 2:
The patent changes the location of the compensation capacitor from the probe tip to the oscilloscope input, which fundamentally alters the parameter relationships. This allows the probe tip capacitance to be minimized while the compensation capacitor value is optimized for the oscilloscope input impedance, achieving both reduced loading and maintained signal levels
3Stability of the object's composition
If a large shunt capacitance is added to the compensation network to terminate the resistive cable in characteristic impedance, then cable reflections are minimized, but the total input capacitance increases which increases loading on the device under test
Solution Approach 1:
The patent segments the compensation function from the probe tip and places it at the oscilloscope input. The cable termination capacitance is provided by the oscilloscope's input capacitance in combination with an additional compensating capacitor, rather than requiring large capacitance at the probe tip. This segmentation allows proper cable termination without increasing probe tip capacitance
Solution Approach 2:
The patent uses the oscilloscope input capacitance as an intermediary to provide the necessary cable termination capacitance. Rather than adding large capacitance to the probe tip, the oscilloscope's inherent input capacitance serves as the termination capacitor, eliminating the need for large external compensation capacitors at the probe end
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 system achieves reduced capacitive loading of the device under test, increased high-frequency input impedance, and improved signal-to-noise ratio by effectively compensating for mismatched time constants and reducing mid-band and high-band frequency signal current shunting to ground.
Implementation Method 1
A probe tip time constant mismatch with an oscilloscope input time constant is compensated for by pole-zero pairs in a compensation digital filter
Implementation Method 2
The input amplifier has feedback loop circuitry providing adjustable gain
Data Source
Figure 1
Figure 2
Figure 3~4
AI summary
A signal acquisition system has a signal acquisition probe having probe tip circuitry coupled to a resistive center conductor signal cable. The resistive center conductor signal cable is coupled to a compensation system in a signal processing instrument via an input node and input circuitry in the signal processing instrument. The signal acquisition probe and the signal processing instrument have mismatched time constants at the input node with the compensation system having an input amplifier with feedback loop circuitry and a compensation digital filter providing pole-zero pairs for maintaining flatness over the signal acquisition system frequency bandwidth.