Probing a Midpoint of a Transmission Line that is Not Load Terminated
白皮书
With the advent of variable on die termination (i.e. ODT) many systems like low power double data rate memory (LPDDR) have the ability to either terminate a line in its characteristic impedance for high speed operation or terminate it with an open for lower speed operation. Many times there is a desire to probe the transmission line to monitor the signals on the line. When the transmission line is terminated in close to its characteristic impedance, the signals can be readily observed. However, if the line is terminated in an impedance that is not close to the characteristic impedance then it may not be possible to accurately measure the signals.
This is due to the reflected signal off the termination that will attenuate or even cancel the signal at the probe point at certain frequencies.