Study of the FEE64 power supply distribution has yielded the following :-

The most sensitive regulator, as regards the device voltage input, is the TPS51100 that supplies the DDR memory reference voltages. This requires +4.75 to +5.25 volts. This is supplied from the Power input connector +5v input.

The common LT3080 regulator used over much of the board is a Low Voltage Dropout regulator. This requires 0.5v difference between input and output voltage as minimum. This is not a problem with the +4.75v minimum for the TPS51100 requirement setting the voltage for the board.

The supply to the mezzanine is direct from the power connector +5v input. On the mezzanine there is an LT3080 for each ASIC supplying the required 3.3v. These regulators would possibly benefit from a 1uF capacitor at the Control voltage input.

The simplest approach would be to add a capacitor to the bottom layer where the Control voltage enters the mezzanine. 

The power cable has a nominal resistance of 13.3ohms/km. The 3 conductors of the cable are supplying 10A when all is in operation. So the expected voltage drop would thus be ( 10 x 13.3 x 0.007 ) /3 => 0.3v each core. 

The conclusion would be that the voltage at the power supply should drop to 5.25 v thus ensuring the TPS51100 is supplied as required regardless of the operation of the FEE. 

This will be tested at Daresbury.