Characterization of coupled microstrip structure using FDTD

A new CAD-oriented methodology for the full-wave broadband characterization of coupled microstrip structures for RF/microwave and high-speed digital circuits is presented. The characterization methodology is based on the finite difference time domain (FDTD) technique combined with a systematic extraction procedure using the normal mode approach and multiport network concepts.; The characterization approach is first applied to uniform coupled lines on a lossless substrate to determine their frequency-dependent propagation characteristics. The 2n-port admittance matrix corresponding to n uniform coupled lines is extracted from the port voltages and currents, which in turn provides the frequency-dependent distributed inductance and capacitance matrices. To illustrate the technique, several typical coupled line structures, including an asymmetric three-line structure, are analyzed. Comparison with results obtained with a full-wave spectral domain method shows good agreement.; The new characterization methodology is applied to microstrip lines on lossy silicon substrate. The frequency-dependent propagation characteristics and distributed line parameters for single and symmetric coupled microstrip lines are extracted from FDTD simulation. Results show that the lossy substrate can significantly affect the transmission line characteristics. To reduce the substrate effects on the microstrip characteristics, a new substrate shielding structure consisting of orthogonal grounded lines at the silicon/oxide interface is examined. It is shown for a single and symmetric coupled microstrip lines that the substrate shield leads to a reduction in transmission line loss without significantly changing the characteristic impedance and effective permittivity of the microstrip structure.; Using the FDTD approach combined with a new extraction procedure, the characteristics of several discontinuities in single and coupled microstrip lines are determined. The new extraction procedure for characterizing discontinuities is based on successive excitation of each port and determination of the voltages for the incident and reflected waves. The method is applied to several example structures including coupled step and coupled gap discontinuities.