Thanks Eric. That makes sense. I'm still learning about gnuradio so excuse my naivete. Could the distortion be cause by the CIC filters? I understand a CIC filter needs some amount of bit growth. If the LSBs are truncated instead, then that amounts to adding noise to the system. This may be unavoidable but might explain the distortion you're seeing.
Mike -----Original Message----- From: Eric Blossom [mailto:[EMAIL PROTECTED] Sent: Friday, March 16, 2007 10:41 PM To: Mike Garcia Cc: discuss-gnuradio@gnu.org Subject: Re: [Discuss-gnuradio] A medley of questions 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
