(WBX+USRP) Unknown signal at the middle of any FFT plot in receive mode


I am currently testing two USRP radios with 3 WBX transcievers.

When I test the receive path, I would use gnuradio companion and go from
USRP source to FFT plot.

However, I see that at every center frequency of the FFT window there is
signal even though I am not inputting anything.

The signal power is about 0 dBm.

Also, if I inject a signal, the power of the signal is shown to be much
higher than the true power I am sending through.

For example, when I send a -50 dBm signal it shows up to be +30 dBm and
tried using multiple signal generators.

So now I’m stuck.

How do I fix this?

Thank you.

On 08/23/2010 01:11 PM, jan acosta wrote:

The signal power is about 0 dBm.

Thank you.

The FFT plot in Gnu Radio is uncalibrated–it’s showing you relative
and NOT dBm

You could perform some calibration and apply calibration constants to
the output of the FFT to turn it into dBm, but
“out of the box”, it is not dBm. It’s rather hard to make it be
dBm, as well, since each daughtercard type is different,
etc, etc.

So, that “signal” at 0dB is actually quite small compared to your -50dBm
input signal. it’s an artefact of the down-conversion

Marcus L.
Principal Investigator
Shirleys Bay Radio Astronomy Consortium

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