VXS Impedance Measurements

At higher clock speeds, the PCB requires cleaner signal transmission without compromising the stability of the system. Signal integrity issues such as reflections, cross talk, frequency dependent transmission line loss and dispersion can significantly lead to poorer system performance propagating through the interconnect. As VXS is a relatively new specification, we are first measuring the impedance of the PCB.

Depending on the configuration, routing a VXS backplane with superior performance can be challenging. In the higher slot sizes, the number and length of the traces can have an effect on the signal integrity. Particularly with larger backplane the number of traces and lack of physical space, it takes creative and intelligent routing schemes from an experienced designer. Avoiding undesirable stubs for upper layer backplane traces presents some tough choices. One option would be to have these worse case vias back-drilled -- a costly fabrication process which removes the unused portion of the plated via structure below the layer at which the signal is terminated. Another possibility is to minimize the length of via stubs by choosing a laminate with a lower dielectric constant as Bustronic did with its 12-slot Dual Star Backplane.

To provide an illustration, let's look at two VXS backplanes - the 5-slot switchless Mesh (where the Mesh spans 3 of the slots) and the 12-slot Dual Star. The smaller backplane with traces that spanned 3 slots was done using FR-4. The larger backplane with traces spanning 8 slots was built using Nelco4000-13SI - a high-grade material.

To ensure a clean signal, it is necessary to understand and control impedance in the transmission environment through which the signals travel. Impedance mismatches (due to vias and connectors) and variations can cause reflections that decrease signal quality as a whole. Time Domain Reflectometry (TDR) measures the reflections that result from a signal traveling through a transmission environment like-a circuit board trace, cable, connector. The impedance values of typical transmission lines as a function of the trace geometry and the dielectric constant of the surrounding environment. (-More-)