Linkern,

  There two basic non-idealities that analog designers need to consider
when designing: noise and non-linearity. THD is a measure of non-linearity
or distortion. To measure it, input a sine wave and measure the ratio of
the power in the fundamental to the power in the distortion tones. If you
have access to ADE, there is a calculator function to measure THD.

                                                                Best Regards,

                                                                   Sheldon

Linkern,

 Assuming that the simulator tolerances have been set appropriately, then
you are probably seeing interpolation error. The DFT samples the waveform
at regular time intervals. In general, the time point that the simulator choses
are not the same time points that the DFT needs. So the DFT interpolates the
simulator results to estimate the time points required for the DFT. By using
different times steps you are reducing the error in the estimation, interpolation
error. There are many things to try

1) Easy
   Set max_step to less than 1/2 the time step size of the DFT, this can be
   computationally expensive, it makes simulation time longer but usually
   gives good results.

2) More accurate
   add a zvcvs to the output to sample the output and then perform the
   DFT on the sampled output.
   zdft (sampled 0 out 0) zvcvs td=10u ts=10n

   You will need to add a little extra time to the simulation and shift the
   DFT times to compensate for the delay in the zvcvs.

3) Use the Fourier integral it is not sensitive to interpolation error
    fourTransform ( out 0 ) fourier fund=1k points=256

                                                                  Good Luck,

                                                                     Sheldon