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Re: [Discuss-gnuradio] Bug in gr_quadrature_demod_cf, or scheduler?
From: |
Matt Ettus |
Subject: |
Re: [Discuss-gnuradio] Bug in gr_quadrature_demod_cf, or scheduler? |
Date: |
Fri, 21 Sep 2007 10:04:18 -0700 |
User-agent: |
Thunderbird 2.0.0.0 (X11/20070326) |
Steven Clark wrote:
> Hi all-
> please look at this sequence of eye diagrams:
> http://picasaweb.google.com/steven.p.clark/GMSKFmdemodGlitches
>
> These are from a gmsk mod/demod pair, showing the output of the TX's
> gaussian filter (blue) overlaid with the output of the RX's fmdemod
> (red). BT = 0.35.
>
> At 8 samples per symbol, everything looks ok. Red is pretty much right
> on top of blue, as we'd expect.
> As I increase samples per symbol, however, something strange happens,
> shown in plot sequence. The RX's fmdemod output gets successively more
> "glitchy".
One other thing -- you really shouldn't be going to more than 8:1
oversampling on GMSK anyway -- all the timing recovery stuff will have
trouble with that high of an oversampling rate when you have noise on
the signal. We still need to fix this problem, but I would advise
staying at 8:1 or lower. Your bandpass filtering also becomes harder at
the higher oversampling ratios.
Matt