@herbschwarz. Yes, I agree. This is good method to consider when calculating response for sine wave (single harmonic). However, in my case I have complex wave form due to rectification and exponential rise and decay. FFT of this waveform contains many harmonics. Therefore, I need to make decision to where is my cut off point. Therefore, my approach is to try to find spectrum in which most of signals energy is contained to cut rest of spectrum in frequency domain. Therefore, I suspect convolve filter's and signals response in time domain.