[SOLVED] THD distortion of a filter , worst case @ fc/3 why ?

[MohEllayali]

hello everybody,

i designed a filter, and my boss gave me in the specification that the THD should be less than such amount at fc/3
by curiosity i swept the frequency and plot the THD, and it turned out indeed that the worst case for THD is for fc/3.
does somebody know why

here is the result of my simulation
actually the specification is less than 45 dB THD, so for each freq i draw the maximum swing allowing to meet this specification

 

[MohEllayali]

i have part of the response , for freq > fc/3, the 3 harmonics are out of band , so they are canceled, so the THD get better, but i dont understand why thd worsen whith increasing frequencies