Passive Probe Series Resistor Impedance Matching
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Solution Overview
Problem
High-bandwidth electrical measurements with high sensitivity are challenging when using measuring apparatuses with high input impedance, as conventional probes either have limited bandwidth or reduced sensitivity due to divider ratios.
Innovation Solution
A passive probe with a series-connected resistor, matched to the characteristic impedance of the probe cable and the device under test, allows for high-bandwidth measurements with low attenuation and high sensitivity by adjusting the impedance to match the device under test, suitable for connections via sockets or rigid solder joints, and optionally incorporating inductive components for improved frequency response.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a 1:1 probe is used for high sensitivity measurements, then sensitivity is improved (lower than 10 mV/div), but bandwidth is limited to about 40 MHz
Solution Approach 1:
The patent changes the impedance parameter of the probe system by introducing a series resistor with a specific impedance value (equal to the difference between the probe cable's characteristic impedance and the device under test's output resistance). This parameter change enables the system to achieve both high sensitivity and high bandwidth simultaneously by optimizing the impedance matching conditions.
2Speed
If a 10:1 probe is used for high bandwidth measurements, then bandwidth is improved (more than 50 MHz), but sensitivity is limited to more than 10 mV/div due to divider ratio
Solution Approach 1:
The patent modifies the attenuation parameter by using a 1:1 probe configuration instead of a 10:1 probe, combined with impedance matching through the series resistor. This parameter change eliminates the 10:1 attenuation while maintaining high bandwidth capability through proper impedance matching, thereby achieving both high sensitivity and high bandwidth.
Solution Approach 2:
The series resistor acts as an intermediary component between the device under test and the probe cable. It mediates the impedance mismatch between these two components, enabling optimal signal transmission with minimal reflection and attenuation, thus achieving both high bandwidth and high sensitivity.
3Ease of operation
If a high-impedance measuring apparatus (1 MΩ input) is used, then ease of operation is improved, but measurement quality deteriorates for low source impedance devices due to impedance mismatch
Solution Approach 1:
The series resistor serves as an intermediary that bridges the impedance gap between the low source impedance device under test and the high-impedance measuring apparatus. By matching the probe cable's characteristic impedance through this resistor, the system achieves optimal signal transmission quality while maintaining the ease of operation provided by the high-impedance measuring apparatus.
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
Enables high-quality, high-bandwidth measurements with low attenuation and high sensitivity on devices with low source impedance, suitable for power rails, transistor outputs, and current sense resistors, while minimizing inductance and using standard components to reduce costs.
Implementation Method 1
the resistor having an impedance equal to one of the characteristic impedance of the probe cable and a difference between the characteristic impedance of the probe cable and an output resistance of the device under test
Data Source
AI summary
A probe for measurements on a device under test with low source impedance is described. The probe has a resistor coupled in series between a probe tip and a probe cable connected to a measuring apparatus. The resistor having an impedance equal to a characteristic impedance of the probe cable or equal to the difference between the characteristic impedance of the probe cable and an output resistance of the device under test. Further, a measuring system and a test setup are described.