On 2/2/18, Steve <coupaydevi...@gmail.com> wrote: > > If you look at FM radio stations on your SDR, you see the signal > duplicated on both sides of the carrier, and also a large carrier when > the modulation is low. I guess you could say FM has built-in > diversity. >
Hi Steve, The FM signal does not contain duplicated information in the same sense that AM does. The signal can't be mathematically described just by using the real part of the signal. If you think of the trig. circle, both the negative part and the positive part contain only part of the information. In continuous time, you can think of the samples as continuously following the circle (constant amplitude modulation) and wrapping at the maximum deviation of the FM modulator, with the center at 0 and the half bandwith point at PI. But, I think I understand what you're trying to do. So you want to simulate what would happen if you put a QPSK signal through an FM transmitter right? That's easy to accomplish using the GNU radio frequency modulator block. What you need to do is oversample the QPSK signal by a factor of 2, then rotate the samples by 2 * PI * bandwidth / 2 / sample_rate, take just the real part of the signal and feed it to the frequency modulator. The imaginary part will not contain any information at this point. On the RX side, you need to do the inverse, so FM demodulate and derotate the samples with the same but negative coeff before feeding them to the QPSK demodulator. Hope it helps, Adrian ------------------------------------------------------------------------------ Check out the vibrant tech community on one of the world's most engaging tech sites, Slashdot.org! http://sdm.link/slashdot _______________________________________________ Freetel-codec2 mailing list Freetel-codec2@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/freetel-codec2
