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ADF 4351 Phase Noise

Question asked by Chris449 on Jan 22, 2015
Latest reply on Jul 6, 2015 by Chris449


I am starting to use the ADF4351 to generate a LO around 2.3GHz from a 26MHz clock input.

The loop bandwidth is around 10kHz.

ADIsimPLL give me these total phase noise values for 10k and 100k:

10.0k -83.59

100k -112.9

However, it is not what I am measuring (see attached file). At 100kHz, it is pretty good. I can observe a bump at 5kHz from the center frequency. The bump amplitude depends on the charge pump (CP) current value, smaller is the CP current higher is the peak.

I checked the 26MHz that is very clean. I tried to extend the loop bandwidth to 30kHz and the bump amplitude is still the same, just at a different frequency.


There is probably something with the filter that I am doing wrong but cannot figure out what, do you have some track that I should follow?


Thank you.