I am designing an acquisition system that has two parts, a hub and an acquisition part that collects the signals. The two parts communicate through FPGA Serdes.

The hub has a main clock from 16MHz crystal oscillator later multiplied to a higher frequency and the acquisition part has another FPGA that I am trying to choose a crystal oscillator for. The two parts are connected through long shielded cables and share the same power, my question is what considerations should I keep in mind for choosing the frequency of the second oscillator? does it have to be within the 16MHz range or around 20MHz.

The reason for this is because the system has to pass EMC and I am afraid if the crystal frequencies are close, this can cause strong harmonics problem in radiated or conducted emission. Therefore, what are some design considerations for EMC when choosing multiple crystal oscillators in a design.



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