What am I really seeing?

I have a GNU Radio setup to look at shortwave. I have my sample size set
50MHz (since I was told I can’t use 30). If I set my FFT to 30MHz with a
baseband of 15MHz, am I actually seeing the shortwave spectrum
Also, any suggestions on setting the FFT Size?

Thank you!

Paul B. Huter

Hello, first I think you meant sample rate, not size. Who said you cant
30Mhz, what kind of hardware are you using? Lastly the FFT algorithms do
not deal with frequency in that way, you are still seeing 0-50Mhz
just the label now says 50Mhz.