Can a PLL work with a sine signal as its XIN?
The FAQ @ https://www.silabs.com/Support%20Documents/TechnicalDocs/Si41xxFAQ.pdf

States that either a sine wave or a square wave is acceptable.

"Q: Should the reference source have a sine wave or a square wave output?
A: The reference source can have either a square wave or sine wave output. The reference amplifier has a large gain that converts the incoming sine wave to a square wave. Therefore, both sine and square wave references are processed identically."

From the data sheet the signal should be at least 0.5V p-p in amplitude and coupled with a 560pF capacitor.

kevin