Low-order approximation method for frequency-dependenttransmission line model

Rational function approximations of characteristic impedance and propagation coefficient are crucial in modeling of frequency-dependent transmission line. TraditionalBode asymptotic method produces lots of pole-zeros. Some of them have no contribution to fitting accuracy and cause unnecessary calculation inelectromagnetic transient simulation. To avoid appearance of redundant pole-zeros, alow-order approximation method of frequency-dependent transmission line model is proposed.Firstly, low-order pole-zeros are located for characteristic impedance and propagation coefficient. Then approximation accuracy is improved withnonlinear least square method.Thus low-order rational function approximationsof characteristic impedance and propagation coefficient are achievedconsequently. Case studies show thatcompared to Bode asymptotic method, fitting with low-order approximation method is more accurate and rational function order is reduced, therefore electromagnetic transient simulation is accelerated.