Re: [Discuss-gnuradio] Funcube dongle issues and a solution

[Gasper Zejn]

Dne Wednesday 30 May 2012 ob 12:01:59 je Alexandru Csete napisal(a):
> On Wed, May 30, 2012 at 8:51 AM, Gasper Zejn <address@hidden> wrote:
> > Hi,
> > 
> > I was using Funcube dongle and found a strange bug. Whenever I used the
> > FCD source in my flow graph and started the flow, the (demodulated)
> > signal came out corrupted. However, if I then just started qthid, it
> > would correct itself and could see bits in waveform perfectly well. This
> > was happening with different firmware versions.
> > 
> > With a bit of experimenting in the earlier out of tree gr-fcd, I found
> > out that reverting two commits[1][2] resulted in resolving the issue.
> > 
> > I've made changes to in-tree gr-fcd and recompiled and it's now working
> > in a way, but issues remain. One is setting frequency at runtime (via a
> > variable), where FCD flips back into spitting out corrupted waveform.
> > 
> > This is where the problem outgrows my knowledge.
> 
> Hi Gasper,
> 
> Sounds to me like you are having some interference on the frequency
> you are trying to use. Or maybe your are using the center of the
> passband where we have a strong DC peak. I'm only guessing since you
> don't give any details about your setup, e.g. what modulation are your
> using, and how do you determine that your signal is corrupted.
>
> The FCD source block in the master branch appears to work correctly
> and I have no issues demodulating narrow band FM. I have no setup to
> try digital modes. The commits you refer to are necessary for correct
> setup and tuning of the FCD. If you revert them the FCD may not have
> the correct initial setup or may have the settings from the last time
> you were using it.
> 
> As said, I suspect you have some noise or interference in your filter
> passband. Please provide more details about your setup, flowgraph,
> etc. I found that plugging the FCD directly into the PC is a bad idea
> causing strong interference. Always use an extension cable and
> preferably also a USB hub between PC and FCD.
> 
> Alex

Hi,

a bit more on my setup: I'm using funcube as a source, tuned to 868.48M, LNA 
gain 20dB, mixer gain 12dB, connected to simple squelch (threshold=-40dB, 
alpha=1) and on to quadrature demodulation block. This block then outputs 
clearly visible binary signal when funcube is initialized properly.

The observed signal is rated at 20kbit, so it's a bit on the upper limit of 
what Funcube can do, but it's still possible to get a decent read. It's a 
burst of bits every 5s from a power meter[1][2], and the first part is a lead-
in and stays the same even if readings change.

Somewhere in the funcube source block there is obviously something wrong with 
initialization. Running qthid after starting flow changes something in funcube 
that makes it output correct signal. Using this and the fact, that the lead-in 
stays the same, it seems the "corrupt" signal (viewed in scope) is sometimes a 
derivative of the expected signal - most of the time on zero, with spikes up 
and down on transitions, with timing corresponding to transitions in expected 
signal.


Does this give any clues?

Kind regards,
Gašper Žejn



[1] http://www.conrad.de/ce/de/product/125353/VOLTCRAFT-ENERGYCOUNT-3000-
ENERGIE-MESSG
[2] http://jeelabs.org/2009/11/14/energy-tracking-with-cost-control/


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