Abstract: An apparatus and method for measuring delay variation of a regenerated data clock and of the associated data packet has a first-in/first-out (FIFO) buffer to which an input clock associated with the data packet and data from the data packet are input. A sample clock derived from a regenerated data clock is used to clock the data out of the FIFO buffer, the regenerated data clock being a function of a reference frequency signal and a period control parameter. The regenerated data clock and input clock are compared to determine a buffer occupancy for the FIFO buffer, from which the period control parameter is derived. From the period control parameter and the buffer occupancy delay variation of the regenerated data clock and of the associated data packet is calculated.