Gary L. Hess

Measurement-Based Modeling and Worst-Case Estimation of Crosstalk Inside an Aircraft Cable Connector

Crosstalk within cable bundles can degrade system performance. In aircraft systems that use shielded twisted pairs, the crosstalk occurs primarily in the connector where individual signal wires are not shielded or twisted. In many cases, the parameters which determine crosstalk within the connector are unknown because the connector is closed and wires cannot be easily accessed. Expanding on prior research [14], a methodology for measuring coupling parameters and modeling crosstalk within aircraft cable connectors at low frequencies (<;400 MHz) was developed. The values of mutual inductance and capacitance were extracted from measurements made with a vector network analyzer (VNA). The characteristics of the individual wires were extracted from VNA-measured TDR response. The accuracy of the model was evaluated through comparison of simulated and measured results. Additionally, a closed-form solution was developed to estimate the worst-case envelope of the differential crosstalk. The calculated results match the measured peak values well. This worst-case crosstalk estimate allows effective evaluation of the impact of crosstalk within different connectors. The developed method can be effective for analyzing complex aircraft cable assemblies and connectors without requiring extensive knowledge of the assembly procedure.

Estimating the radiated emissions from cables attached to a switching power supply in a MIL-STD 461 test

Common-mode currents on cables attached to a switching power supply generate radiated emissions which may interfere with near-by components. A relatively simple equivalent circuit model is developed to predict the radiated emissions measured in a MIL-STD-461 or RTCA/DO-160 test. The intent of this model is to provide an estimate of emissions that allows the designer to better understand the mechanisms behind emissions issues and to rapidly predict the impact of changes to the system, like adding filtering, changing components, or modifying cable connections. The model represents cables connected to the power supply as transmission lines, represents coupling from the cables to the antenna using lumped capacitors, and represents the balun in the antenna using a transformer. The simulated results match the measured results well. This simple SPICE model allows EMI issues to be investigated early in the design of switched mode power supplies.

Prediction of Radiated Emissions From Cables Over a Metal Plane Using a SPICE Model

A method for creating a simple SPICE model is proposed such that the SPICE model allows prediction of radiated emissions in component level tests, such as those specified by CISPR 25 and MIL-STD 461. The model predicts measured emissions when the antenna is in the vertical direction, where emissions are typically worst for such geometry. It is shown that the radiation from the ground connections between the cables and return plane dominates over the radiation from the horizontal cables. The currents in these ground connections are predicted by treating the cables above the return plane as transmission lines and by treating the ground connections as infinitesimal radiating dipoles. The electric fields generated by these infinitesimal dipoles are summed at the antenna, where the antenna factor is then used to predict the received voltage at the antenna. Test results show that this SPICE model is able to predict peak emissions within a few dB over a range from 60 MHz up to 1 GHz for a variety of circuit configurations. This model should help circuit designers to better evaluate the design of their components early in the design process and help them to better understand the mechanisms behind emissions problems.

Measurement-Based Models for Crosstalk Within a Connector Shell

Crosstalk within cable bundles can degrade system performance. In systems that use shielded twisted-wire pairs, the crosstalk occurs primarily in the connector, where individual signal wires are not shielded or twisted. In many cases, the parameters which determine crosstalk within the connector are unknown, in part because the connector is closed and wires cannot be accessed. A methodology was developed for measuring coupling parameters and modelling crosstalk within the cable connector at low frequencies (<; 300 MHz). The values of mutual inductance and capacitance were extracted from measurements made with a Vector Network Analyzer. Values of self inductance or capacitance within the connector for individual wires were extracted from TDR measurements. The accuracy of the model was evaluated through comparison of simulated and measured results. Tests were performed while varying the wire terminations to modify the dominant coupling mechanism. A further simplified model which only takes into account the mutual coupling was also developed to estimate the envelope of the crosstalk. The simulated results match the measured results well. This simple SPICE model allows effective evaluation of the impact of crosstalk within different connectors.

Prediction of radiated emissions from cables with multiple connections to a metal plane

The radiated emissions from a partially shielded cable mounted 5 cm above a metal plane have been found to be dominated by the emissions from the 5 cm vertical conductor segments, which connect the cable shield and the signal wires to the metal plane at various locations. An equivalent circuit model has been developed for predicting radiated emissions from the cable. As part of an extension from the developed model, an additional wire is included and pulled back in the test setup. The methodology of modeling is applied to this new setup by adding an additional transmission line for the wire against the metal plane, and by including the mutual inductance between the transmission lines, which accounts for the locations of the transmission lines. The predicted radiated emissions match the full-wave simulated radiated emissions, further validating the modelling technique that was previously developed. This modelling procedure can be used to determine if a system meets a given radiated emission specification before any actual testing is done. If the radiation is predicted to be excessive then design changes, such as additional filtering, can be modelled before the cabling is built to determine if the changes will allow the system to meet the radiated emissions specification before testing begins.