Re: [Discuss-gnuradio] Remove USRP filters.

[Michael Ossmann]

On Mon, Jul 02, 2012 at 08:27:01PM -0400, Andrew Davis wrote:
>
> 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

That actually doesn't look so bad to me.  Do you know anything about the
transmissions from your target device?  Are they supposed to be in 400
ms long packets?  Do they use a slow FSK modulation?  Are packets
supposed to happen as often as every 600 ms?  If you don't know that
stuff, try looking up the FCC test report for the device.

You can ignore all the spurs and images in the frequency domain that are
20 dB or more below the loudest thing going on at any particular moment.
You'll see stuff like that with a USRP, much more so when you are
intentionally aliasing.

What daughterboard are you using?  Have you modified the FPGA?

> I really can tell what i'm looking at, doesn't look like FHSS to me.

It does to me.  First there is a short burst I not sure about, but then
there are two events of the same duration, the first was received at a
lower power than the second.  The first event could be a packet outside
your band that you are not receiving properly, and the second is within
your band.  Oh, sorry.  I read the time ruler in reverse.

> I did capture what I think is a sync preamble followed by FSK (
> http://i.imgur.com/EpMim.jpg )

That looks beautiful!  Except I think it is the time ruler itself that
is reversed, not my reading of it.  The image is much lower power than
the main signal.  Just ignore it.  That packet is decodable by eyeball
in the spectrogram, which is kind of a rare treat.

Throw that thing into grc and decode it!  :-)

If you need help, post the raw file shown in EpMim.jpg somewhere.