Cyclone V native PHY rx_is_lockedtoref problem

Dear all,

I'm trying to reproduce the Altera Example design "Cyclone V Native PHY with external fPLL design example" on my custom board.

(https://fpgawiki.intel.com/wiki/Cyclone_V_Transceiver_PHY_Basic_Design_Examples#Cyclone_V_Native_PHY_with_external_fPLL_design_example)

The only change I made to the project is the fPLL source clock period.

Unfortunately, I have some problem with the clock management.

In particular, I in my custom board the PHY signal "rx_is_lockedtoref" is not stable to '1'.

What can be the problem? My fPLL clock source is a 50MHz oscillator, this clock could be with a too high jitter?

Thank you