Re: [Discuss-gnuradio] A medley of questions

[Eric Blossom]

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