Cables
Cables are not all that different from what has been already covered. Losses in cables tend to be substantially less than in FR4. Some really good cables are out there, but even the mediocre ones that are practical for use in consumer electronics are really good compared to FR4.

Expect losses in cables to be in the range of cable loss-per-meter equal to FR4 loss-per-inch. If microwave cabling is new to you, you should know some things.

The first goes something like this: if you name a loss figure, a cable can be found that can meet it. This fact, that exceedingly wideband and low-loss cables exist, is not really relevant to the design of circuitry for consumer applications.

It is not an exaggeration to state cables that cost over $1,000 per meter are readily available. I have some in my lab. For their application, they are the right choice. Their application is definitely not consumer electronics.

The $600 and the $30 per meter cables also have valid reasons for existence. In consumer applications, what you need are the cables that are closer to the dollar-or-less per meter items. These, too, exist. In these, the connectors on the ends may cost more than the cable material itself. The nemesis of the engineer with a cable need is the vendor who claims to have a cable that solves all those problems, but the price isn't stated.

Besides that, one of the major differences between cable and trace is that it is quite difficult to get really good length matching on the individual conductors in a cable. As the number of pairs in a cable increase, this problem becomes worse.

In traces on the board, matching lengths is fairly easy and matching velocities more difficult. In cables, matching lengths is the more difficult proposition. It sometimes is also useful to note that the common-mode impedance in a cable may be very different than that on the circuit board. This can be true even though both have precisely the same differential impedance.

It is significant to note that cables can present very severe ESD problems. Those center conductors in cables can sometimes support thousands of volts of charge. The human body ESD model includes a 1,500-ohm series resistor. But when that cable plugs in, the series resistance is in milli-ohms.

So, at least in the lab, always put a terminator on a cable to discharge it before plugging it into your equipment. It is a good idea to lose sleep at night, figuring out how this will be handled by consumers if you have a cable that goes outside your chassis.

The same frequency-dependent-loss transmission lines that were used to model traces are used to model cables in SPICE. Of course, the loss tangents are quite different.

An interesting phenomenon has shown up in cable assemblies designed to meet specific interconnect standards. When the maximum loss allowed for a cable at a specific frequency is specified, all cables, independent of length, tend to have that loss. Consider a cable of some length and loss, cut it in half, and the measured loss will now also be cut in half. That is not what is happening here.

Figure 7.23. Quad and Twin-Ax Cable Constructions

When all else is the same, the cable loss tends to decrease as the cable diameter is increased; the cable cost increases as the cable diameter increases. If the maximum loss is specified, the manufacturer minimizes cost by decreasing cable diameter, increasing cable loss to the specified limit. So it is that in this circumstance, cable diameter tends to decrease as length decreases, rather than cable loss decreasing as length decreases.

Crosstalk in differential cables, both quad construction and twin-ax construction, illustrated in Figure 7.23 above, is typically dominated by the connectors. If the cable length is doubled, the crosstalk does not double, it may even show very little increase. Often an important cable parameter is the quality of the shielding.

Again, it is possible for the connectors to make major contributions to EMI. If there was no common-mode signal entering the cable, radiation would not be a significant problem, but since cable lengths are difficult to match and connectors are not perfect, common mode can be generated by the cable connectors themselves.

Next  in Part 4:  Modeling philosophy
To read Part 1, go to "Unmodelable features of high performance designs"
To read Part 2: go to: "Differiental transmission lines and  receivers."

Dennis Miller has worked in electronics since 1963. His early engineering interests and education centered on control theory and numerical analysis. Now his interests are signal integrity and numerical analysis. Since joining Intel Corp. in 1991, he has been instrumental in the development of Infiniband technology and similar high speed signaling technologies.

This series of articles is based on material from Designing High Speed Interconnect Circuits," by Dennis Miller, used here with the permission of Intel Press which holds all copyrights. It can be purchased on-line.