Fixture De-embedding for Microwave Impedance Measurement
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Solution Overview
Problem
High impedance probes used in measurement systems face challenges in accurately measuring small changes at microwave frequencies due to test fixture effects, which are not accounted for in existing calibration methods, leading to inaccuracies in voltage and transfer characteristics.
Innovation Solution
A method and system that measure the scattering parameters of an unloaded test fixture, apply a splitting algorithm to calculate transmission line leg parameters, and de-embed test fixture effects from combined measurements to isolate measurement system effects, allowing for precise impedance measurement and improved accuracy.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If measurement systems use high impedance probes to measure voltages at test points, then the probe impedance can be assumed insignificant compared to test fixture impedance, but at high frequencies the probe loading impedance alters the voltage present at the probing tip contact point
Solution Approach 1:
The patent replaces direct electrical measurement with a computational approach. Instead of relying on the high impedance probe to directly measure voltage without loading effects, the system measures scattering parameters (S-parameters) of the test fixture and uses mathematical de-embedding algorithms to calculate and remove the fixture's influence, thereby obtaining accurate DUT characteristics without being affected by probe loading at high frequencies.
Solution Approach 2:
The patent introduces an intermediary computational model (de-embedding algorithm) that acts as a mediator between the raw measurement data and the final DUT characteristics. The algorithm uses measured S-parameters of the test fixture as intermediate data to mathematically separate the fixture effects from the DUT response, enabling accurate measurement without direct electrical contact that would cause loading effects.
2Measurement precision
If calibration standards are used to remove measurement system effects, then accuracy can be improved, but existing methods require breaking the test fixture connection to insert multiple standards
Solution Approach 1:
The patent performs preliminary measurement of the test fixture's scattering parameters before connecting the DUT. By pre-characterizing the fixture's electrical properties (S-parameters) across the frequency range, the system creates a computational model that can be used during actual measurements to de-embed fixture effects, eliminating the need for repeated physical reconfigurations or breaking connections during calibration.
Solution Approach 2:
The patent creates a computational copy (model) of the test fixture's electrical characteristics through S-parameter measurements. This virtual model is then used in de-embedding calculations to represent and remove fixture effects, replacing the need for physical calibration standards and manual reconfigurations of the test fixture.
3Measurement precision
If traditional calibration methods are used to account for measurement system effects, then some accuracy can be achieved, but test fixture effects are not fully accounted for leading to residual errors
Solution Approach 1:
The patent segments the measurement system into distinct components: the test fixture and the DUT. By measuring the S-parameters of the test fixture separately and using de-embedding algorithms to mathematically separate the fixture response from the DUT response, the system preserves and utilizes information about fixture effects rather than treating them as a single inseparable measurement system, thereby eliminating residual errors.
Data Source
AI summary
A method and system for measuring the input (loading) impedance of measurement systems using a test fixture. This is done by first measuring the characteristics of an unloaded test fixture to obtain scattering parameters of the test fixture and using a splitting algorithm to calculate the scattering parameters of each transmission line leg of the test fixture. The test fixture is then measured with a measurement system attached. The test fixture effects defined by the scattering parameters are then removed from the measurement to yield the scattering parameters of the measurement system alone (measurement system effects).