RE: [Discuss-gnuradio] A medley of questions

[Mike Garcia]

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:address@hidden 
Sent: Friday, March 16, 2007 10:41 PM
To: Mike Garcia
Cc: address@hidden
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