PCB traces, vias, connectors, and cables
all contribute to signal degradation.
Measurements and simulations can
characterize them and software
can remove their effects.
Engineers at FPGA manufacturer Xilinx had a measurement challenge: how to characterize the 28 Gb/s serial links in their devices.
At such a data rate, even very short connections can cause signal degradation and high-frequency content in the signal will be lost. To make matters worse, the data streams must run on differential pairs and even the slightest mismatch in the signal and return paths will cause skew. A paper at DesignCon 2014 explained the problem, offered a new kind of solution, and a hands-on tutorial gave engineers a chance to use simulation and measurement software tools that solved the problem.
It's impossible to get equipment to directly measure high-speed signals because you need a system of interconnects such as a fixture probes, cables, and connectors. The interconnection between the test equipment and the device under test (DUT), however, behaves like a low-pass filter and degrades the signal. For you to effectively evaluate the FPGA transceiver (DUT), you must be able to view the original signal as it appears at the package pin. Using software to remove the effects of interconnects lets you view the original signal. It's a process called de-embedding where you characterize a channel, then apply an inverse filter in software on the captured signal.
Read the full article on EDN.