Differences in Signal Spectra between Daughtercards

GNU Radio
1

Hi,

I’m working on a project that requires using two receivers tuned the
same frequency simultaneously. Ideally, I would use identical receivers,
but I am using a TVRX card and a DBSRX card becuase those are what I
have available.

I’m curious why the spectra of signals received with the TVRX seem to be
more spread out than with the DBSRX. An example is shown in the link
below. The upper plot is from the TVRX, and the lower plot is from the
DBSRX. This signal happens to be an ATSC carrier. Both daughtercards are
tuned the same frequency, and the FFT parameters and so forth are
identical.

http://img23.imageshack.us/img23/3206/screenshotusrpfft2.png

Does anyone know what causes this apparent spreading and if there is
anything I can do about it?

I purposely left out details of my configuration to keep this message
short, but feel free to ask for more information if this isn’t enough to
answer my question.

Thank you,

Thomas

2

On Mon, Jan 11, 2010 at 08:58, Thomas [email protected] wrote:

I’m curious why the spectra of signals received with the TVRX seem to be more spread out than with the DBSRX. An example is shown in the link below. The upper plot is from the TVRX, and the lower plot is from the DBSRX. This signal happens to be an ATSC carrier. Both daughtercards are tuned the same frequency, and the FFT parameters and so forth are identical.

It looks like the gain on the TVRX board is set too high.

Johnathan

3

On Mon, Jan 11, 2010 at 08:58:16AM -0800, Thomas wrote:

Hi,

I’m working on a project that requires using two receivers tuned the
same frequency simultaneously. Ideally, I would use identical
receivers, but I am using a TVRX card and a DBSRX card becuase those
are what I have available.

I’m curious why the spectra of signals received with the TVRX seem
to be more spread out than with the DBSRX. An example is shown in
the link below. The upper plot is from the TVRX, and the lower plot
is from the DBSRX. This signal happens to be an ATSC carrier. Both
daughtercards are tuned the same frequency, and the FFT parameters
and so forth are identical.

http://img23.imageshack.us/img23/3206/screenshotusrpfft2.png

Does anyone know what causes this apparent spreading and if there is
anything I can do about it?

I purposely left out details of my configuration to keep this
message short, but feel free to ask for more information if this
isn’t enough to answer my question.

Thank you,
Thomas

Try setting the gains differently on the two boards.

Eric

4

I’ve tried different gains on both boards, but it doesn’t make a
difference. The frequency I am tuning to is 560.31 MHz, which is below
the specified frequency range of the DBSRX. My DBSRX tunes to that
frequency without any errors, but perhaps tuning it out of range it is
causing a problem. I will try to compare with a spectrum analyzer to see
which card is giving the most accurate representation of the signal.

Thanks,

Thomas

On Mon, Jan 11, 2010 at 08:58:16AM -0800, Thomas wrote:

Hi,

I’m working on a project that requires using two receivers tuned the
same frequency simultaneously. Ideally, I would use identical
receivers, but I am using a TVRX card and a DBSRX card becuase those
are what I have available.

I’m curious why the spectra of signals received with the TVRX seem
to be more spread out than with the DBSRX. An example is shown in
the link below. The upper plot is from the TVRX, and the lower plot
is from the DBSRX. This signal happens to be an ATSC carrier. Both
daughtercards are tuned the same frequency, and the FFT parameters
and so forth are identical.

http://img23.imageshack.us/img23/3206/screenshotusrpfft2.png

Does anyone know what causes this apparent spreading and if there is
anything I can do about it?

I purposely left out details of my configuration to keep this
message short, but feel free to ask for more information if this
isn’t enough to answer my question.

Thank you,
Thomas

Try setting the gains differently on the two boards.

Eric