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The CC1020 can be operated with crystal reference frequency in the range 4 – 20 MHz. The reference design (CC1020EMX) is optimised for 14.7456 MHz. Chipcon recommends that this crystal frequency is used for optimum performance using the reference design. In applications where a different crystal frequency than 14.7456 MHz is required, some precaution and optimisation must be done. This application note discusses issues related to using other than 14.7456 MHz crystals.
http://www.ti.com/litv/pdf/swra070