Crosstalk in Coupled Microstrip Lines With a Top Cover

In this paper, we investigate the influence of the excitation of improper radiating modes (leaky modes) in the propagation characteristics of coupled microstrip transmission lines. We calculate the crosstalk noise excited in the victim line and evaluate the impact of the presence of a metallic top cover due to the circuit packaging. The time domain propagation of pulses of different widths and rise times are computed in order to study the effect of the separation between the coupled strips on the crosstalk noise currents. We show that both even and odd leaky modes can be simultaneously excited on the covered microstrip structure at low frequencies. This can seriously compromise the signal integrity on the line because of the appearance of effects such as radiation, power loss, and interference, which are usually found in microstrip transmission lines at high frequencies. Our results are compared with those provided by a conventional analysis based on the quasi-TEM approximation and transmission line theory. For an open microstrip structure, we have found that the transmission line model accounts properly for the crosstalk current on the victim line. However, on covered coupled microstrip lines with a sufficiently low top-cover height, the crosstalk noise is higher than expected and cannot be accurately predicted by the transmission line model, even at frequencies where the cross section of the line is much smaller than the wavelength.