Approximation of Worst Case Crosstalk at High Frequencies

Predicting system-level crosstalk problems early in the design process is increasingly important to today's automotive industry. Closed-form estimates of worst or nearly worst case crosstalk can be useful in system analysis, since these estimates can quickly identify problem areas that require close inspection and they show a clear relationship between design parameters and crosstalk that allows intelligent modification of the system. In this paper, simple formulas are derived to estimate worst case crosstalk between circuits for weak coupling at high frequencies (i.e., where the length of the circuits is comparable to or greater than the wavelengths of the signals of interest). Derivations are based on transmission line theory for general (inductive, capacitive, or resistive) sources and loads, assuming losses at the terminations dominate losses in the transmission lines. Formulas are verified with bench-top and in-vehicle experiments and are shown to predict maximum crosstalk levels within about 6 dB.