# Probes

It is not very common for people to measure jitter using oscilloscope probes, and it is rare for them to measure phase noise using probes. That is because the probe's large attenuation of the Signal Under Test (SUT) significantly reduces the measurement's signal-to-noise ratio (SNR). The two-channel cross-correlation technique, however, removes both the channel's noise as well as the probe's noise. So, high-quality jitter and phase noise measurements can now be made using probes.

To use the two-channel cross-correlation technique with oscilloscope probes, simply double-probe the SUT. In other words, probe the SUT using two different probes as shown in the following figure. This doubles the capacitive loading on the probed voltage node, but that is not usually a problem for clock signals.