Re: [Discuss-gnuradio] Remove USRP filters.

[Andrew Davis]

Really cool presentation!  Thanks for the info. Now i'm running into
another problem, I sample at about 4MSPS for a bit and try to capture
the signal as it passes though my window, but I never seem to get it,
just a huge mess of noise, aliasing and ghosts.
http://i.imgur.com/w3oBP.jpg as you can see it looks like the are
about 400ms long bursts that just shake in frequencies, It also raises
the noise floor about 10db and throws images all over some lines break
up, I really can tell what i'm looking at, doesn't look like FHSS to
me. I think it is a really strong carrier just outside of the band
that causes all these images. I did capture what I think is a sync
preamble followed by FSK ( http://i.imgur.com/EpMim.jpg ) but as you
see it is mirrored many times all the way to the noise floor. Could
someone with more DSP background help me figure out what I am looking
at?

Thank you all
~Andrew

On Sun, Jul 1, 2012 at 4:10 PM, Michael Ossmann <address@hidden> wrote:
> On Sun, Jul 01, 2012 at 04:01:48PM -0400, Andrew Davis wrote:
>>
>> So my plan is to somehow remove at least one stage of filtering from
>> the FPGA so I can sample at ~4MSPS and have all out of band signals
>> aliased into that bandwidth. My questions is can this be done and is
>> my DSP info correct ( Can this work? ).
>
> I used that trick to sniff all Bluetooth channels with a single USRP2 a
> few years back.  I described the technique in this talk at ShmooCon 2009
> (179 MB):
>
> http://shmoocon.org/2009/videos/Bluetooth-Ossman.m4v
>
> I don't think you'll have to hack a daughterboard like I did, so you'll
> only need to work on the FPGA.  My old FPGA mod is here:
>
> http://gr-bluetooth.git.sourceforge.net/git/gitweb.cgi?p=gr-bluetooth/gr-bluetooth;a=blob_plain;f=doc/usrp2_alias.patch;hb=HEAD
>
> It is out of date, but you should be able to get the idea.  I just had
> to zero out some filter coefficients.