Hi:
There are quite some tools can convert s-parameters into RLC or SPICE netlist in a wide frequency band. However, none of them can guarantee the RLC are always positive or the SPICE netlist to be of passive components only. It seems impossible to avoid negative RLC or active devices inside the SPICE net list. If possible, I do suggest people try to perform time transient analysis directly from s-parameters without converting them to RLC or control sources. It should avoid the problems involved in negative RLC or control sources in the SPICE netlist. The MDSPICE from ZELAND can perform time transient analysis directly from s-parameters and it can also convert s-parameters into RLC with perfect matching in a wide frequency range. I am from Zeland. Regards.