I cannot get the HMC833LP6GE to lock when with a fractional 'N' divider of 20 or 21. For example, with a reference of 140MHz and 'R' Divider of 2, a VCO frequency of 1501MHz requires an N of 21.44285714 [reg03h = 15h; reg04h = 715F16h; reg06 = 200B4Ah]. However, this does not provide locked signal at 1501MHz. When N is 22.44285714, the VCO does lock to 1571MHz, as expected.
I don't think I've violated the maximum PD frequency.