UG-570 document provides guidelines on a reference clock source. You can use a crystal or some other type of an oscillator. You should ensure that you are using a high quality source with low phase noise. Figure 2 in UG-570 provides a phase noise plot that will allow you to get datasheet specification from the part. If you are using single ended XTALN pin for reference clock input you cannot exceed 1.3Vpk-pk swing. It is recommended to maximize input swing without exceeding 1.3Vpk-pk maximum input level. Please see UG-570 for more details.
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