Why the gxb tx_pll loses lock --Stratix IIGX

:confused:

Hey all:

I 'm using the Stratix IIGX(EP2s30) to realize the high speed serial communication,I met a problem that the GXB tx pll loses lock when simulating with vwf file。The output of epll is fed to the GXB tx pll_inclk, and the tx_pll_cascade is on.

I try to use the refclk pin to fed the pll_inclk, but the tx pll doesn't work due to the noise(Peak to peak 400mv).