Hi,
Regarding your latest inquiry on the clock rate vs sample rate, for your information, you can refer to the "Figure 13. Time-Division Multiplexing to Save Hardware Resources" in the FIR II IP Core User Guide for further details. You would need to code additional logic on your own for the serializer and deserializer to work with the FIR II IP. When you are having higher clock rate than sample rate, you are trying to save resource required for FIR II IP.
For example, in your case of 50MSPS and clock rate 100MHz, the FIR II IP is expecting that your serializer will serialize two data stream of 50MSPS into single data stream of 100MSPS. If you do not have the serializer and assert source_valid at 25MSPS, then you should observe your output at 25MSPS.
Please let me know if there is any concern. Thank you.
Best regards,
Chee Pin