Managing Electrical Parasitics and Capacitance in Extended Bus Sensor Harnesses
Managing sensor harness parasitics demands fluoropolymer dielectrics, matched line termination, active buffer acceleration, and strict shield grounding topology.
Transfer Impedance
Shielding Effectiveness
Shielding effectiveness in a coaxial or multi-conductor cable depends on the relationship between the current flowing on the shield surface and the voltage induced on the internal conductors. Transfer impedance is a frequency dependent parameter measured in ohms per meter that indicates how well the shield blocks electromagnetic interference. A lower value signifies a more effective shield that provides better signal protection.
Coupling Mechanism
Measurement of transfer impedance reveals the efficiency of the shield in preventing external fields from penetrating the cable. Braid density and the presence of foil layers significantly influence this value. In high quality cables, the internal signal is isolated from the currents generated by external electrical noise.
Laboratory Testing
Standardized tests for transfer impedance involve injecting a known current into the shield and measuring the resulting internal voltage. A triaxial test fixture is often used to maintain a controlled environment for these high precision measurements. The data is plotted over a wide frequency range to identify any resonances or weak points in the shield construction.
Results allow engineers to predict the performance of a cable in an environment with high levels of radio frequency interference.
Quality Specification
Procurement documents for aerospace wiring often include a maximum allowable transfer impedance to ensure the safety of critical flight systems.