When the CDCE(L)913 is in crystal oscillator or in VCXO configuration, the internal capacitors require different internal capacitance. The following steps are recommended to switch to VCXO mode when the configuration for the on-chip capacitor is still set for XO mode. To center the output frequency to 0 ppm:

1. While in XO mode, put Vctrl = Vdd/2
2. Switch from X0 mode to VCXO mode
3. Program the internal capacitors to obtain 0 ppm at the output.