A reference clock output with a differential swing of 600 mV to 1200 mV as defined in Figure 1.1 below is becoming a common requirement for optical modules and is driven by the chipset input requirements. This definition of differential swing is defined as OUT+ minus OUT- and is how the differential receiver will process the waveform. The OUT+ minus OUT- subtraction effectively doubles the amplitude of waveform relative to an oscilloscope observation when measuring each of the differential outputs in single-ended mode as shown in Figure 1.2 on page 2 for example purposes. Because different differential output formats can have different definitions for differential waveform swing levels, it is very important to confirm the exact requirements with the chipset vendor, preferably with a waveform diagram.
