Hi Eric, I just joined the gnuradio discussion but I found your question about the GMSK amplitude distortion interesting. My guess is that it's aliasing. GMSK has good spectral efficiency but it still has spectral sidelobes that get folding into your desired passband when you decimate. I would suggest filtering before decimating. Even better, since your decimating by factors of 2, you can use a half band filter that reduces the sample rate by 2 each time. Or you could consider downsampling via a polyphase filter. I'd be interesting in seeing the phase distortion too. Phase distortion is more of a concern for GMSK demodulation since it's a phase modulation. Take the phase of the input signal and the phase of the output signal and difference the two to get the phase error. You should see higher phase distortion as you decimate more especially if you don't filter.
Mike You're right, I was normalizing for the purposes of plotting. Here's what the raw complex samples look like: http://img490.imageshack.us/img490/3225/rawbasebandvv9.jpg +/- 2400 or so. and here's that other plot regenerated: http://img249.imageshack.us/img249/9520/d256unnormalizedkd4.jpg signal is going in to RX at -10dBm, default gain setting. On 3/14/07, Eric Blossom <[EMAIL PROTECTED]> wrote: On Wed, Mar 14, 2007 at 09:47:20PM -0400, Steven Clark wrote: > Plots of USRP decimation woes: > input is GMSK waveform @ ~30ksym/sec, BT = 0.35, no noise added > using decimation rate of 16: > http://img339.imageshack.us/img339/4467/d16cz9.jpg > top plot is complex baseband, bottom plot is amplitude of top plot > some amplitude variations noticeable, but fairly minor. > > using decimation rate of 256: > http://img256.imageshack.us/img256/2850/d256qh2.jpg > heavy amplitude variations. > zoomed in version of previous plots: > http://img490.imageshack.us/img490/6528/d256zoomedku7.jpg > > Still confused as to what is going on here... Steve, what's the real amplitude of the received baseband signal? I suspect that you are normalizing to 1. Can you regenerate the plots without normalizing so that we've got an idea of the values coming across the USB. Eric _______________________________________________ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
