How to calculate fractional spurs?

[zeshan102]

i am using ADF4153 to generate 3262.5 MHz signal with pfd freq of 20 Mhz and i am getting spurs at around 1.25Mhz, is this fractional spurs?if yes how to calculate it.

 

[Mazz]

fractional spurs

From my calculations the frac spur should be 2.5MHz.
The formula I use is:
Fref*K/((2**Nbits)-1)
where K is integer number that gives you the fractionality (32768 in your case), Nbits are the DSM bits (I use 18).
Of course I'm not sure that those numbers are correct, the formula is correct.
Mazz