ADF4355-3 has spurious at 1.8kHz

Sir:

Our board output of ADF4355-3 has spurious at 1. 8kHz.  The spurious is constant in different output frequency, bleed current and CP current. 

We attempt to replace DC-DC with external power supply,but the spurious still unchanded. 

We have a few questions:

1. Why and how to eliminate the spurious. 

2.  Why and how to Increase output power. 

The specific register configuration and spurious is shown in the figure below