Solution: The Group Delay (GD) displayed by the OVA is not the absolute GD, but rather a relative, or offset, GD value. Typically when people are measuring GD for components, they want to compare the GD shape (ripple, slope, etc.) between different components that may have different pigtail lengths. For this reason, it is common when measuring GD to center the GD at some value common to every measurement. Various instruments choose different values for this common GD value - some center it at 0, others place it at the GD of some given length of fiber. The OVA uses the value at the center of the 3 or 6 ns time domain window as this baseline value for the GD. Thus, if you have done a 'Find DUT' such that the impulse response is in the center of the window, the GD value will be centered around 1500 ps for a 3 ns Time Domain window and 3000 ps for a 6 ns Time Domain window, regardless of the actual length of the DUT. This allows components with different pigtail lengths to be more easily compared, as they will not have a large GD offset from each other. The fact that the absolute GD is not displayed is not unique to the OVA, but is common practice for GD measurement in the industry.

Group Delay Centered at 1500 ps

Figure 1: Group Delay centered around 1500 ps

Related Links:
Product Support Home

Document ID: 0607006002
Last Update: 06/14/2006

Other questions, contact LUNA SUPPORT
Phone: 1.866 LUNAOVA (1.866.586.2682)
E-mail: support@lunatechnologies.com