On page 18 of the ADF4356 datasheet: "When the prescaler is set to 4/5, the maximum RF frequency allowed is 7 GHz." So above that frequency the prescaler should be set to 8/9.
However the VCO frequency range for the ADF4356 is only 3.4 to 6.8 GHz, so there would never be a time to set the prescaler to 8/9 if the determining limit is really 7 GHz. Is there a typo in the datasheet on page 18 and another frequency besides 7 GHz should be listed there? (For example, 5.7 GHz?)
For reference we have worked with the ADF4351 previously and its datasheet claims that the max RF frequency allowed for the 4/5 prescaler is 3.6 GHz, and given that its VCO frequency range is 2.2 to 4.4 GHz, that 3.6-GHz limit for the ADF4351 makes sense. Not so as written for the ADF4356.