GXB transmitter PLL & GXB receiver PLL Definition

A feature available in supported device ( Arria^® series, Cyclone^® IV , Stratix^® IV GT, and Stratix^® IV GX) family devices that employs a phase-locked loop (PLL). The GXB receiver PLL and the GXB transmitter PLL are special-purpose high-speed PLLs designed for use by the circuitry inside the gigabit transceiver block (GXB).

You can take advantage of the GXB receiver PLL and the GXB transmitter PLL with the ALTGX megafunction.