EXFO FTB-800 Series for FTB-1 - Manual (Page 147)

Test Setup - Test Configurator, Timer, and System
NetBlazer 137
Interface (Ethernet)
TX Frequency

TX Frequency (GHz) indicates the frequency (actual frequency +
Frequency offset) used for transmission.

Offset (ppm) check box, when selected (cleared by default), allows to
set the frequency offset that will be generated. Use the “+” or “-”
button to respectively increment or decrement the frequency offset
value based on the defined Increment/Decrement Size, or directly
type the frequency offset value in the field. The range is ±120 ppm for
Ethernet and Packet Sync test applications.

Step Size (ppm) allows to set the increment/decrement value (from
0.1 to either 50 or 120 depending on the test configuration) that will be
used when changing the frequency offset with the “+” or “-” button.