I'm choosing the Crystal oscillator for DDR（C10GX/CV）
could you help to confirm the jitter of the ddr2/3 reference clock?
Welcome to Intel Community.
In the EMIF handbook Vol. 2, below is the spec for PLL reference clock jitter:
The clock source of the PLL reference clock must meet or exceed the following jitter requirements:
You may also refer to Cyclone 10 GX datasheet under section Table 43. Memory Output Clock Jitter Specifications for Intel Cyclone 10 GX Devices as shown below for more details. --> https://www.intel.com/content/dam/www/programmable/us/en/pdfs/literature/hb/cyclone-10/c10gx-51002.p...
It is recommended to use the fastest possible PLL reference clock frequency because it leads to better jitter performance.
Hope this helps.