SiTime's differential oscillators feature excellent phase jitter performance for their respective class of products. For example, the SiT9501 oscillator features 70 fs rms of phase jitter, measured by integrating phase noise over an offset-frequency range of 12 kHz to 20 MHz. SiTime's differential oscillators employ a differential output driver with excellent power-supply noise rejection and an ultra-low output-noise floor. The differential driver also reduces common-mode noise sources internal to the device and power supply noise, as illustrated in Figure 1, leading to better than -170 dBc/Hz at a 156.25 MHz clock frequency for the SiT9501. This document describes test methodologies to measure the true performance of the following ultra-low jitter devices using differential measurements.