Dear,

 I plotted the psd of ideal sine wave using psd function in the calculator. (in log scale)
 There seems to be noise floor and the difference between the peak signal frequency and the noise floor is less than 70dB. I don't know why I got this high noise floor even when I used ideal sine wave.
 Is it because the psd function uses interpolated points when they do fourier transform? (I am thinking the normal spectre simulation time step might not be evenly spaced. So I guess in this case to get evenly spaced points, psd function might do some interpolation to the waveform.)

Thank You