SMT Power Spectrum to IQ Data

[RF_Buzzer]

I have a 20 MHz IF Signal.
I used SMT Basic Zoom Power Spectrum to zoom in
and capture 5 MHz of that Spectrum.

Can I get back IQ data corresponding to this Zoomed 5 MHz Spectrum.

I tried using IFFT to get back IQ data  from the Spectrum,
but the results are not correct

Which VI should I use to convert Power Spectrum to IQ Data. ?

[Andy_Hinde]

Hi RF_Buzzer,
You cannot re-create the IQ data from the power spectrum because the power spectrum no longer has the phase information a pure complex FFT would still have. You can do an IFFT as long as you have the magnitude and phase data from the FFT. The power spectrum loses the phase info so you can't go backwards.

You can software downconvert the IF data you have to produce an IQ data stream, and then perform a power spectrum on that IQ data too. Look for the software downconversion routines that get installed in the Modulation section of LabVIEW or CVI.

Thanks,
Andy

[walban1974]

I have question about Fourier transform. I have got a spectrum and I can generate raw data in MathCAD using invert fourier transform IFFT, but I need to create different distributions. I need about 20 different distributions with different number of data which have the same spectrum. Any ideas how to do that??

[David L.]

Hi Walban,

I would like to ask for clarification about what you are looking to do. I understand that you have an inverse FFT, but can you expand on the definition of ‘different distributions’ ? I am puzzled by how we can have different data, but the same spectrum. Lastly, what version of LabVIEW are you using? Are you using the Spectral Measurement Toolkit (SMT)? This knowledgebase: How Do I Use the FFT and Inverse FFT VIs in LabVIEW? (http://digital.ni.com/public.nsf/allkb/549BCD6B247012F686256E00002509E5?OpenDocument) may help with using the FFT and Inverse FFT functions in LabVIEW.