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.