Why is the P6139A not advised for use when measuring Rambus clock/data lines or other high-speed digital signals?
The P6139A is a passive voltage probe with an input capacitance of 8 pF and 10 Megohm Input resistance when connected to a 1 Megohm / 10 pF oscilloscope input channel. This essentially applies in parallel to the device under test, an 8 pF capacitance in parallel with 10 Megohm resistance. This combination creates an impedance load of approximately 150 ohms at a frequency of 100 MHz. This load value decreases as the frequency goes up. Improving this loading to the DUT requires decreasing the capacitance loading portion of the probe. Active probes and Active Differential probes provide this additional reduction in capacitance which allows the measurement of signals at higher and higher frequencies, like those in Rambus applications.
