I am using the Eval board to generate 0.9 to 1.0 GHz in 12.5KHz steps with the RF out divider set to 4 - this should set the VCO range to 3.6 to 4GHz with a step size of 50KHz. The PFD is 10MHz and I get fractional spurs at the output of 12.5KHz but when I look at the Aux output which is set to give a VCO funadamental out it also has spurs at 12.5KHz. To reduce these 12.5Khz spurs I would have to run a narrow loop around 2KHz to get approx -70dBc spurs.
I had expected to see 50KHz spurs here so I set the Eval board to give 3.6 to 4GHz out at 50KHz steps and behold I get 50KHz fractional spurs!!
My question is how can I synthesize at 3.6 to 4 GHz at 50KHz steps/fractional spurs and divide this by 4 for an output of 0.9 to 1GHz that has lower 12.5KHz spurs?
If this is possible I could put a notch at 50KHz on the 3.6 to 4 GHz VCO loop filter and get much lower spurs at the divide by 4 output and run a wider loop around 15KHz.