On Fri, Mar 16, 2007 at 08:07:33PM -0700, Mike Garcia wrote: > 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
Hi Mike, thanks for your comments. We do filter when decimating, otherwise nothing would work. The first N/2 decimation is handled with a 4th-order CIC, the final decimation by two is done with a halfband. The Cylone doesn't have any h/w multipliers, so using a halfband at each stage is not feasible. Eric _______________________________________________ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
