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Re: [Discuss-gnuradio] A medley of questions

From: Steven Clark
Subject: Re: [Discuss-gnuradio] A medley of questions
Date: Wed, 14 Mar 2007 21:47:20 -0400

Plots of USRP decimation woes:
input is GMSK waveform @ ~30ksym/sec, BT = 0.35, no noise added
using decimation rate of 16:
top plot is complex baseband, bottom plot is amplitude of top plot
some amplitude variations noticeable, but fairly minor.

using decimation rate of 256:
heavy amplitude variations.
zoomed in version of previous plots:

Still confused as to what is going on here...

Plots showing effects of carrier discrepancy between tx and rx on GMSK demodulation:
These are plots of the output of the fmdemod block (and of the input to the M&M clock recovery block).
Residual carrier manifests as DC offset after fmdemod (look at y-axis):
sending input like this into the clock recovery block -> lots of bit errors, especially when noise is added.

After manually tweaking tx carrier freq by a few kHz:
much better, -> 0 bit errors.

So, that's the motivation for wanting carrier tracking. I think Eric's right that there's a solution involving filters that doesn't require me to leave python-land, but I'm still a bit confused as to how to leverage the IIR block.

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