How can I derive 1.6 - 1.8 volts for the VTRX supply without a separate regulator?
Because of the very small current draw of the VTRX pins, you can use the following circuit to derive 1.78 V from the 2.5 V analog supply for the VTRX pins on the Virtex-II Pro device. This configuration has been verified on the ML321 RocketIO Characterization Board. Using a separate voltage divider for each VTRX pin, a ferrite bead is not necessary for acceptable filtering, as a result of sufficient inductance available from the resistor package.
Voltage dividers can be shared across multiple transceivers. In this case, the values of the divider should be scaled down accordingly. For two VTRX pins, scale the divider resistors 201 ohm and 500 ohm; for eight VTRX pins, make the divider resistors 50 ohm and 125 ohm. In this setup, each transceiver's VTRX pin must have its own bypass network, which always includes a ferrite bead and, depending on package, a bypass capacitor. (For more information on the bypass capacitor, see (Xilinx Answer 18872)).