First, there is the dielectric loss. This relates to the fact that all insulating materials, when placed in a microwave oven, heat up. The amount of heating may be very great or very little, but the heating effect is always present. The degree of heating in the presence of microwaves is specified by the loss tangent of the material.

For example, you may have purchased some "microwave safe" dishes. These have a relatively low dielectric loss tangent, and so don't heat up very much in the microwave oven. Such a material would make a fair ceramic substrate for the conveyance of high-speed digital signals.

Other materials, like chicken, heat up tremendously when exposed to microwaves. In essence, the chicken absorbs quite a bit of the microwave energy to which it is exposed. That makes chicken a terrible insulating media for use in a high-speed digital product [laughter]. For one thing, it's difficult to get anything to stick to it, because of the grease, and for another the dielectric losses will quickly absorb your fast-moving transients.

For a material with a flat loss tangent (meaning that it does not vary with frequency), you lose a certain fraction of your signal power every time it wiggles up and down. Total losses therefore vary in proportion to how often you wiggle.

Mathematically, we say that dielectric loss (in decibels) grows in direct proportion to frequency.

Materials in popular use for high-speed digital applications are specified with a worst-case (i.e., flat) loss tangent.

The actual loss tangent may not be flat, but from a worst-case design perspective you must assume that it is flat and equal to the worst-case specification at all frequencies.