BasicRX boards and AM transmission

Hi,

I’m still in the exploring phase of my GNU radio work and currently ran
into
an annoying problem. All I want to do is transmit AM from one USRP to
another, which I thought would be the simplest RX/TX chain I could come
up
with for a start.

However, I have not managed to get a clean AM reception with a USRP. I’m
pretty sure the tx component works, because I can listen to my
transmission
with an old kitchen radio and it sounds fine.

RXing with the USRP simply doesn’t work, though, neither with
usrp_rx_nogui.py
nor with anything I could come up with. I get lots more noise than
expected
and usually I get a very loud buzz or whistle which is a lot louder than
the
TXed signal. Has anyone else had this problem?

I’m using the Basic* boards directly connected to an antenna (no
bandpass
filtering). I realize this is not how the BasicRX is intended to be
used, but
can this be the cause of a loud buzz or would it only create lots of
extra
noise? Basically, is this a completely futile attempt?

I also tried the TVRX and then I sometimes get some spectral lines
around
40kHz next to my main signal (this frequency doesn’t occur anywhere in
my RX
chain, it’s audio at 32kS/s interpolated to 320kS/s for the USRP). My
filters
on the TX chain seem OK, at least I haven’t seen those spectral lines on
an
(external) spectrum analyzer. Has anyone else ever seen lines like this?

Thanks for any help,

Martin B.
Institut fuer Nachrichtentechnik
Universitaet Karlsruhe

http://www.int.uni-karlsruhe.de

Martin B. wrote:

RXing with the USRP simply doesn’t work, though, neither with usrp_rx_nogui.py
nor with anything I could come up with. I get lots more noise than expected
and usually I get a very loud buzz or whistle which is a lot louder than the
TXed signal. Has anyone else had this problem?

Hi Martin,

Just a quick “me too!”. I don’t have a USRP yet but I did try using GNU
Radio to receive AM radio signals with a BT878 chip. With a slightly
modified am_rcv.py I could hear the station, but it was very weak and
drowned out by lots of noise. The capture was fine as I was able to get
good audio (as good as AM can be anyway) with the Dream DRM receiver in
analogue mode, using GNU Radio to shift the target carrier down to
12kHz.

-Phil

Martin B. schrieb am 2007-12-10 17:06:

Hi,

RXing with the USRP simply doesn’t work, though, neither with usrp_rx_nogui.py
nor with anything I could come up with. I get lots more noise than expected
and usually I get a very loud buzz or whistle which is a lot louder than the
TXed signal. Has anyone else had this problem?

I’m using the Basic* boards directly connected to an antenna (no bandpass
filtering).

Hello Martin!

The BasicRX has no aliasing filter. You get a lot of mirrored
frequencies from above 30MHz. It is possible to receive FM Radio…

Check your selected frequency for mirror-frequencies from the UKW-Range.
Maybe you can use FM demodulation to identify a interferring FM radio
station. Just a guess.

Patrick

Engineers motto: cheap, good, fast: choose any two
Patrick S.
Student of Telematik, Techn. University Graz, Austria

“Philip Heron” [email protected] wrote:

modified am_rcv.py I could hear the station, but it was very weak and
drowned out by lots of noise. The capture was fine as I was able to get
good audio (as good as AM can be anyway) with the Dream DRM receiver in
analogue mode, using GNU Radio to shift the target carrier down to 12kHz.

-Phil

I don’t think it is the USRP and I don’t think it is anything inherent
in
GNU Radio. I have been using the USRP (with LFRX) and GNU Radio for
HF/MF/LF reception for a while now with great success. My recollection
is
that am_rcv.py had some problems (e.g., specifying the if_rate in the
design
of the audio filter); I have not looked at usrp_rx_nogui.py.

Unless your hardware is broken, I’ll bet you can find a remedy in the
software.

– Don W.

Don W. wrote:

I don’t think it is the USRP and I don’t think it is anything inherent
in GNU Radio. I have been using the USRP (with LFRX) and GNU Radio for
HF/MF/LF reception for a while now with great success. My recollection
is that am_rcv.py had some problems (e.g., specifying the if_rate in the
design of the audio filter); I have not looked at usrp_rx_nogui.py.

Unless your hardware is broken, I’ll bet you can find a remedy in the
software.

– Don W.
Hi all,

I advise to try hf_explorer or hfradio in the gnuradio-example/apps
folder.

For experimenting with am reception I just added an usrp_am_mw_rcv.py
example script in the style of usrp_wfm_rcv.py to svn.
You can find it at:
http://gnuradio.org/trac/browser/gnuradio/trunk/gnuradio-examples/python/usrp/usrp_am_mw_rcv.py

Or by just updating your subversion checkout.

It is nothing fancy.
But this is good I think for understanding the script and settings used.

No AGC is implemented (for that see usrp_rx_nogui.py or hf_radio or
hf_explorer)
This means you have to adjust the volume for very strong, and very
weaks stations.
If you have the volume set too high, the output will clip, and you hear
nothing or just crackles.

It does however seem to work on both strong and relatively weak
stations.
(You do need to have an antenna usable for MW connected to your
basic_RX, input RX-A. Try the longest wire you can find or better, build
a loop
antenna)

Greetings,
Martin Dudok van Heel

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