I have been using the IDDATA function on time series data collected at 100hz and using the ETFE function to create bode plots. I would now like to use frequency data instead of time series when I enter it into IDDATA, but was wondering if there is a way to tell what IDDATA and ETFE are doing to the data to convert the time series data to the frequency spectrum? Is this simply taking the FFT(time_series_output) and FFT(time_series_input)? I would imagine it is a little bit more complicated than this?
So I currently do this: DATA=iddata(time_data_output, time_data_input, 0.01)% where (output, input, 1/capture rate) G=etfe(DATA,,length(time_data));%where  describes no smoothing, and NFFT points is the length of time series data bode(G)
I would like to enter frequency data into IDDATA, and the help says to use this format: DAT = IDDATA(Y,U,Ts,'FREQ',Freqs)%where Y and U are now frequency data
However, I was wondering if there was some way to replicate what IDDATA and ETFE data uses to create the frequency spectrum?
If anyone could help, or refer me elsewhere, I would greatly appreciate it!