What will happen without RREFB reference resistor in the GXB?

Hello,

the GXB refclk input is different from regular Stratix LVDS receivers, I think. According to the manual, it's AC coupled and isn't intended for operation below the minimum PLL input frequency of 50 MHz. I assume that you selected LVDS as I/O standard for the refclk input?

A wrong termination could be normally seen from the input level at the differential pins, with missing termination, the input signal would be to high but still almost symmetrical and the receiver probably operational at lower (< 100 MHz) frequency, perhaps with irregular timing.

But it could be, that the RREFB derived reference current has other uses inside the GXB block, may be also for the differential clock receivers.

Regards,

Frank