Figure 2 Coupled microstrip line structure
A model is established in accordance with the structure shown in Figure 2. An excitation signal is added to port 1, microstrip line A is regarded as an attack line, and the adjacent microstrip line B as a victim line. For the convenience of analysis, only the crosstalk caused by the attack line to the victim line is considered, and the secondary crosstalk is ignored. At this time, the crosstalk generated from the attack line port 1 to the victim line port 4 can be expressed as [13]
\begin{equation} \begin{matrix}V_{\text{fext}}=\frac{1}{2}\left(\frac{C_{m}}{C_{T}}-\frac{L_{m}}{L_{S}}\right)\cdot TD\cdot\frac{V_{m}}{T_{r}}\#\left(1\right)\\ \end{matrix}\nonumber \\ \end{equation}
where TD is the time for the signal to pass through the transmission line, \(V_{m}\) is the signal amplitude, and \(T_{r}\) is the rising edge time. When the input signal propagates along the attack line, it will form mutual capacitance and mutual inductance with the victim line, which is recorded as\(C_{m}\) and \(L_{m}\). the crosstalk received by the end of the victim line away from the input port is the far-end crosstalk, denoted as\(V_{\text{fext}}\); \(C_{T}\) represents the sum of mutual capacitance and self-capacitance between the transmission lines, and \(L_{S}\)represents the self-inductance of the transmission line. Equation (1) shows that from the circuit point of view, changing the ratio of the capacitive coupling and the inductive coupling between the coupled microstrip lines is the most direct and effective means to change the size of the crosstalk without changing the rise time of the transmission signal and signal amplitude.