IQ data to FFT not displaying correctly

[l_m_s]

I'm following an online tutorial where some one has recorded IQ data of FM broadcasts using a RTL dongle and Matlab. when i follow through the tutorial i can plot an FFT of the spectrum as seen in the attached jpeg. however if i save the raw IQ data to CSV, import it into lab view and do an FFT the plot looks nothing like that in the Matlab plot. my Matlab is very basic so i'm unsure if the commands are doing more then just plotting the IQ data. the webpage with the tutorial is http://aaronscher.com/wireless_com_SDR/RTL_SDR_AM_spectrum_demod.html.

Any pointers would greatly be appreciated.

[AJost · Accepted solution]

Hi I_M_S

The matlab script is plotting the FFT in dB, so it's on a logarithmic scale.

Your labview code is calculating it in absolute values (not dB) and on a linear scale.

I changed the output of the FFT to dB and it produced a plot which is much closer to what you see in matlab.

 

Does this explain the behavior you are seeing?

Andreas

[l_m_s]

Andreas,

Thank you so much for the reply. i knew it must have been something trivial.

you have saved me a lot of head banging.

Liam

[AJost]

Hi Liam

Your are welcome.

Have fun with the further development and don't forget to mark your question as solved

Andreas