I am working on a receiver design where I am sweeping my 1st LO (ADF4351) from 2375MHz to 4375MHz. I am running into some issues with the spurious performance of the 1st LO where I am thinking some fracN spurs are leaking through the first mixer and mixing with the final IF.
I am using a channel spacing of 5kHz, PFD frequency of 20MHz, and in low-noise mode. I do not see the same issue in intN mode. One thing that was figured out by experimentation, that by increasing the rcounter, in turn, decreasing the PFD frequency, overall, decreasing the magnitude of the spurious components. I am having trouble understanding why this is, or if this data point is true. I attached some screenshots where the rcounter was increased (1,10,20).
Any thoughts/comments would be appreciated.