ADF5610 Settling time.

The ADF5610 datasheet said :For settling time requirements faster than 50 μs, contact Analog Devices, Inc., applications support. Settling times under 50 μs are possible, but certain limitations on performance may exist.

I will use ADF5610 for 5GHz~7.3GHz , need settling time less 50μs. I want to kown what is the limitations on performance ,and how I can achieve the settling time under  50 μs.