Measuring Characteristic Imped

When making highly sensitive measurements on high-speed digital boards,
there is little room for error. Rambus® memory architecture produces
extremely fast signals (250-picosecond risetime) on computer mother-
boards and memory boards and thus requires special measurement tools.
The challenge is even greater for the digital design engineer when faced
with Rambus traces that are only 1-2” in physical length. Smaller form
factor printed circuit boards for new laptop applications and mother-
boards are creating a need for more precise measurement techniques. By
using a test procedure called Time Domain Reflectometry (TDR) normal-
ization, high-speed oscilloscopes can accurately measure the character-
istic impedance of these short traces by removing sources of error. This
application note will describe the test equipment and general method-
ology that is recommended to make these signal integrity measurements.