Both psd and dft in the calculator will re-sample the waveform, because it is the norm for spectre to write out with an uneven timestep - the default is that it writes out any timestep actually simulated.

You can also use the transient option strobeperiod to set a time interval for spectre to place a timestep at, and also to write out the results. This leads to a higher accuracy FFT, since you're minimising the interpolation error if you can arrange the strobeperiod to coincide with the FFT sampling interval.

There are various posts on this in these forums - you should be able to search for strobeperiod to find more info on this.

Regards,

Andrew.