Inverse FFT's

[Blackj]

I am using Fourier Transforms to filter a signal I have. I noticed
that for one perticular waveform the waveform returned after the
inverse transform was performed was messed up. In the course of
finding the problem I step up the filter so it would not filter the
signal at all. The waveform returned is still wrong. ie has real and
imaginary part. I have used this program on a number of waveforms and
never encountered this problem before. Anybody have similar
experience?