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Re: [Discuss-gnuradio] Works with GR 3.6, breaks with 3.7

From: Nick Foster
Subject: Re: [Discuss-gnuradio] Works with GR 3.6, breaks with 3.7
Date: Tue, 4 Nov 2014 17:28:02 -0800

Red face over here.

There was a bug in correlate_access_code_tag -- which only gr-smartnet and gr-ais use, so far as I know. The fix was pulled into master a couple of days ago. This could explain the discrepancy you're seeing in the preamble mark.

Pull latest GR master, build, and see if it fixes what you're seeing.


On Tue, Nov 4, 2014 at 5:19 PM, Andy Walls <address@hidden> wrote:
On Mon, 2014-11-03 at 22:13 -0500, Luke Berndt wrote:
> Thanks Andy! Good catch - I made the changes you suggested. I was just
> doing a simple back of the envelope calculation to come up with
> channel size. It does look like cleans things up and adding the
> waterfall graphs does make it easier to see. Unfortunately, it does
> seem to change the decoding. I am still getting the same amount of CRC
> errors.
> Did the overall signal look right, like something that should be decodable?

So I put the output of the FLL into a quad demod block and compared its
output to the PLL Freq det output.  They are pretty much in line, except
the quadrature demod block output shows occasional burst of noise for a
few symbol times where the transmission must have stopped and restarted.
The PLL freq detect block just makes something up during these times.
So that seems generally OK.

> Anything else I should try?

Your PLL freq detector block output is swinging between -2 and 2.  You
might want to reduce the max and min freq by a factor of 2, so that the
clock recovery block sees inputs limited to +/- 1.0.  See Nick's
recommendation here:

So after i did that, what I really noticed is that the correlator is
just not marking preambles properly.  See the top plot of the attached
to window shots:  Preamble_marked_late_twice.png and

If the deinterleaver and/or crc block isn't searching around for where
the preamble really is, well that would explain it.


>  - Luke
> >
> > Hi Luke,
> >
> > I have not built the SmartNet blocks yet, but:
> >
> > Your low pass filter looks way too wide and you'll get aliases when you
> > decimate by 185.  2 Msps / 185 = 10.81 ksps, so the Nyquist frequency is
> > 5.405 kHz.  I winged this is as a low pass filter and things looked
> > better:
> >
> > firdes.low_pass_2(1, samp_rate, 4500, 1000, 60, firdes.WIN_HANN, 6.76)
> >
> > The 60 dB down may be overkill, you can make it smaller for a filter
> > with less delay.
> >
> > I found an offset slider value of -15k made things look about centered
> > manually.
> >
> > You might want to put a waterfall sink before and after the FLL
> > Band-Edge filter to observe how it is making the spectrum wobble around
> > a little. If you change your offset slider, you can see the FLL
> > band-edge filter centering things back up; so that looks like it is
> > working.
> >
> > Regards,
> > Andy
> >

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