[ferret_users] Extracting frequency from FFTA

[David Ogier <david.ogier@xxxxxxxxx>]

Hi there,

I hope you have some extra experience with Fourier analysis as I'm running out of ideas.
I have a time series (hourly for a month) and I have done a FFTA on it with the following

   let FFT_temp = ffta(temp[l=1:744])     !simple enough

and I now need to isolate the 4 hourly frequency and return only it back to the hourly temperature data (ie I need 744 time steps again 24 hours * 31 days)
I've tried FFT_INVERSE but I don't know how to specify the frequency or period I need, 

    let sample_re = FFT_RE(temp[d=1])
    let sample_im = FFT_IM(temp[d=1])
    let invert_ts = FFT_INVERSE(sample_re, sample_im)           ! this returns the full original data set and imposing l limits to the sample_re or sample_im seems to return wrong data

I've also been trying LSL_LOWPASS but I'm not sure on the functioning and is returning strange data that looks incorrect.
Is there something simple that I'm doing wrong?

Many thanks for your time.

Cheers

Dave