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How to mitigate "walking" spurs on ADF5355

Question asked by jmerti on Nov 10, 2016
Latest reply on Jan 17, 2017 by MRichardson

When using the ADF5355 I see spurs that "walk" across the spectrum.  It looks like they stay within the loop bandwidth and are typically 10-15dB above the phase noise skirts.  I read in a FAQ on this site that reducing FRAC2 and MOD2 can help mitigate these spurs but that isn't always true, I've had the spurs get worse dropping FRAC2 from the automatically calculated value to 0.  Is there anything else I can do to mitigate these spurs?  Is there some sweet spot of FRAC2/MOD2 that changes with frequency?  Does MOD2 matter if FRAC2 is 0?  I managed to grab a screenshot of the spur as it moved across the spectrum while tuned to 4700 MHz.  Ref freq. was 500 MHz, Fpd was 125 MHz, FRAC2 was 0, and MOD2 was 25.  All other parameters were automatically calculated.