Data Extraction
Signal de-embedding procedures isolate the electrical behavior of a device by mathematically subtracting the influence of connecting hardware. Engineers use automatic fixture removal to generate accurate s-parameter data for small components that cannot be measured without specific mounting traces or connectors. The process relies on the known characteristics of the fixture to calculate the actual impedance and loss of the internal circuit.
Subtraction Logic
Mathematical algorithms perform the removal by treating the test fixture as a series of cascading network blocks. Because automatic fixture removal assumes the fixture is symmetrical, the software splits the measured data into two halves and negates their impact on the final result. This method eliminates the need for separate calibration standards for every unique fixture design.
High speed digital designs benefit from this approach when characterizing connectors or individual vias within a multilayer stack. Measurement software typically handles the extraction by identifying the fixture delay and loss through a simple through-line measurement.
Accuracy Limit
Precision remains dependent on the initial quality of the vector network analyzer calibration and the physical symmetry of the test leads. If the manufacturing tolerances of the fixture create an imbalance between the input and output paths, the automatic fixture removal routine introduces artificial phase errors. Quality checks often compare the results of this automated technique against a full through-reflect-line calibration to verify the validity of the extracted model.
The technique fails when the fixture includes non-linear components or if the impedance of the fixture traces varies significantly from the nominal system value.
