Numerical modeling of closely-coupled microstrip transmission lines

As integrated circuit feature sizes decrease and clock speeds increase, the problems of coupling between adjacent microstrip transmission lines and devices become more severe. This is especially true in communication circuits, such as pagers and cellular communications devices. In these cases the simultaneous presence of RF and digital circuitry increases the severity of coupling problems. In the paper an integral equation formulation is used to analyze closely-coupled microstrip transmission lines. A method of moments (MOM) solution is implemented utilizing basis functions which incorporate appropriate edge conditions for current components, allowing for close-form evaluation of spatial integrals. In contrast with earlier subdomain basis MOM solutions, greatly improved accuracy is obtained using far fewer terms. Results in the form of propagation factors and current distributions are presented for several modes, and are compared with the results of other techniques. These results are used as a basis to formulate several "rule of thumb" design guides for RF circuits to minimize coupling and cross-talk problems as far as possible. Other strategies for minimizing coupling in RF circuits are also discussed.