Re: Beginner question about rtlsdr

To get from the time domain (the raw samples) to the frequency domain (waterfall display, etc.), you need an FFT. The FFT operates on some given buffer size, and outputs a buffer of the same size filled with complex frequency levels (where the magnitude squared is the signal energy). The FFT width determines your frequency resolution, not the bandwidth. The bandwidth is determined by the sample rate--in your case, 1.4 MHz. So you choose your FFT based on your resolution and performance requirements. In your case, the FFT will return frequencies from -0.7 to 0.7 MHz around the center frequency--no matter what the FFT width. The FFT results store the positive frequencies from 1..(N/2-1) and the negative ones from (N/2+1)..(N-1). Sample 0 is 0 Hz and N/2 is 0.7 MHz (you can't really use this last one since it aliases). The other frequencies scale linearly between these points (sample N/4 is 0.35 MHz, etc.). For a waterfall display, this generally means you want to swap the left and right halves of a buffer. You can read as many samples as you want from the device. Conceptually, it's similar to an audio device--if you need more samples, you just wait longer. You could collect 64M samples if you wanted, and get some nice sub-Hz resolution, but it would take 45 s to record at your settings. There is of course a tradeoff between time and frequency resolution. I don't know much about radio astronomy, but you'll probably want to figure out what the shortest timescale events you'd like to track are, and choose the FFT width based on that. Incidentally, none of this is peculiar to rtl-sdr--even the fanciest and most expensive units operate the same way, though they have higher sample rates and such. In fact, the stuff I said is fundamental to all signal processing--if you're willing to get your hands dirty with math, you might want to read the wiki articles on Fourier Transforms and other signal processing subjects. -Scott On 10/21/2012 10:25 AM, Michel Pelletier wrote: