NPORT and MTLINE S Parameter File

The nport component is designed to handle any kind of s-parameters, and has no knowledge of what they represent. From the frequency-domain s-parameter description, it has a couple of modelling approaches to build a time-domain approach (convolution-based when using linear or spline modes, or a rational fit when using bbspice mode). However, if the s-parameter file represents a long transmission line, it is hard to accurately model this (particularly the delay that would incur).

A better approach is to use the mtline component which is specifically designed to model transmission lines. In this case, the s-parameters are converted to equivalent per-length frequency-dependent RLGC parameters, but to do this it needs to know the physical length that the line represents (the RLGC input needs this too). The requirement for the length is purely because mtline knows that it's now representing a transmission line, and to accurately do the conversion it needs to know the length (which gets over the challenges that nport can have with long transmission lines).

Andrew

Thanks...I always use MTLINE with length, my feeling has always been this is more accurate.
I see people using NPORT for transmission lines and your post has been a great clarification...thanks.

Thanks...I always use MTLINE with length, my feeling has always been this is more accurate.
I see people using NPORT for transmission lines and your post has been a great clarification...thanks.