Cable modeling for high speed data communications

I have not been blogging for a while. The reasons are many, but mostly because of some interesting work that came up. This post is a result of grappling with cable modeling and cable parameter information extraction. The simple comment is, that this is a very tedious affair as very little data is available in the literature. High speed data communication cables come in various disguises. Some of the more popular types are the CATX types and STP ( shielded twisted pair) types. Various gauges are being used. We looked at 24 and 26 AWG types.

The issue is, that if you are designing a cable equalizer for example, you need a cable model. There are a number of ways to do this. The most expensive way is to either buy or rent a laboratory piece of equipment which can perform up to at least 12 Ghz, buy or make connectors, and then make measurements. The other way is to use information available in the literature to build a cable model. The latter is very sparse and difficult to obtain. Anyway both approaches should be tried.

The modeling parameter W in some SPICE based programs is a useful one and with proper manipulation can yield fairly accurate models. It is quite complicated to understand. Simple transmission line models in PSPICE can be used but will only offer very basic models and may not be accurate for design.

There are also cable model ( analytical) parameters available in some text books for coaxial, parallel wire cables, microstrip etc. which can also be used. These can be used to obtain the so-called RLGC SPICE model. Using the RLGC circuit is not enough to model transient response since the cable delay cannot be modeled. A transmission line model is required for this. These too are only first order estimates.

So what is required is more empirical data of cables and cable models. One or two researchers have actually done this. However, the need to model parameters such as attenuation, crosstalk, ISI etc is still an open field, ready for someone to step in and do the needful!