LMX2491: Some question about the phase noise of LMX2491

Part Number:LMX2491

Hi all

 i  have a question about the phase noise of LMX2491

my parameters ;

          pdf = 50MHz

          vco = 1500MHz   and  1507.8125MHz   

           R = 1

          Frac Delta_sigma = 4

          Frac  denominator = 2^24(the register of denominator is  all zero)

          loop filter = 100KHz

when i config the chip to 1500MHz, the phase noise  of output signal  is good. the 10KHz offset is -85dBc/Hz

but when i config the chip to 1507.8125MHz, the phase noise of output signal is bad . the 10KHz offset is -72dBc/Hz

So i think it may be something about the frac mode . 

I want to know how can i debug the register to make the signal better at 1507.8125MHz. 

Thank you.