Vias
A via has two main impacts at high frequencies: it can cause signal loss through injection to the planar waveguide and it can act as a resonant stub at microwave frequencies. The coupling to the planar waveguide is linearly proportional to frequency; that is why this phenomenon may not have been noticed by those working in the lower few hundreds of megahertz.

As frequency increases, it becomes a much more serious issue. Again, a major advantage of differential signaling is that differential energy is far less impacted by this than is common-mode. When a differential signal traverses a planar waveguide through symmetrical and closely-spaced vias, it is almost exclusively the common-mode component that loses energy to the waveguide.

A good side to this is if a single-ended signal has to traverse the waveguide, and if the two planes are at the same potential, insert a second via near the signal, using it to short the two planes together.

Image current will flow through the shorting via and largely cancel the fields of the signal via. Losses will decrease. Surprisingly, this works even with differential signals. Adding a shorting via will decrease loss in the signal vias.

A fairly good model of a via can be constructed by R-L-C components where there is capacitor for each surface and reference plane, and an inductor for each region between. Such models can even be constructed with reasonable accuracy through use of lookup tables.

Of course, the entries in the lookup table are calculated with a field solver. Such a model may not show the impact of shorting vias, or their absence, but it can show the impact of resonant stubs in the via.

Electrostatic Discharge
It has already been pointed out that SPICE knows nothing about electromagnetic radiation. Neither does it know anything about electrostatic discharge (ESD). ESD typically involves non-linear phenomena and, though they can be modeled, they are seldom included in models designed for signal integrity applications.

There is a very high significance for signal integrity in one aspect of ESD. Pins that are designed to withstand ESD typically do so through the use of current shunting devices, often diodes. These components typically add significant capacitance to the protected pin. That capacitance shows up in circuit models as a value in parallel with the termination resistor.

This capacitance is often the biggest problem preventing really good values of return loss in microwave frequency ports. In typical systems, the better the ESD protection, the more capacitance. Because of this, very high speed ports often have very poor ESD protection. You need to keep this in mind whenever you handle these devices.

Next  in Part 3:  Modelable  Features of high performance designs
To read Part 1, go to "Unmodelable features of high performance designs"

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.