It is possible to use an external TCXO instead of a crystal.
This option does not require production line trimming, but it will draw more current and hence significantly degrade sleep performance. The TCXO should be disabled when in Shutdown mode to reduce current consumption. It is not possible to dynamically control the TCXO when transitioning between Sleep and Active states. The TCXO should have a typical clipped sine wave output of 0.4 V peak (maximum 0.6 V peak) and be connected through a series capacitor (typically 10 nF).