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ADF4158 R-Counter on output signal

Question asked by electrosoni on Jun 7, 2016
Latest reply on Jun 21, 2016 by rbrennan

Hi. I am using ADF4158 for a frequency sweep (triangular ramp). Bandwidth is about 110 Mhz and sweep time 4 us or higher. Input Ref_IN clock is 20 Mhz.

When I set R-Counter to 1 for PFD = 20 Mhz, I am able to get the ramp according with the time I need but on the spectrum analyzer I see that the output signal has a ripple that matches with the number of steps I need to reach the final frequency, and CP signal is not as stable as I would like it  to be.

If setting the R-Counter to 2 for PFD = 10 Mhz, the ripple on the signal disappears and the spectrum is flat (which is what I try to get) and the CP signal is fine and stable.

If I try setting both Reference Doubler and RDIV2 (on R2), the ripple and instability are still there.

The question is: why do we have this ripple /  instability when the input REF_in signal is not divided byt the R-Counter?Thanks for your help