USRP and RFX400

For a University project, we want to use the USRP for FRS operations.
The project is in C++, so I wrote some c++ drivers for the RFX400 board.

Unfortunately, when it was the time to test the drivers I had some
really
strange result.

Here is the test performed:
I used a wavegenerator to produce an AM signal at 460 MHz of a 1khz
tone.
The generator RF ouput was connected to the RX2 input of the RFX400
dboard.

Then, the USRP/RFX400 were configured.
USRP frequency: 0 Hz
Downsamplig factor: 256 (thus the signal is at 250 kSample/s on the
USB.)
RFX400 frequency: 460 MHz

For the demodulation, only the I sample were used, and the signal was
downsample by 6 (whitout any filter). (foutput = 41,666kHz). I just want
to
look if I was able to select the good frequency with RFX400 dboard.

I was able to hear the tone, but when I changed the carrier frequency
(of
the generator or of the dboard), the tone was still there. For example,
the
generator had fc = 260 Mhz and the RFX400 had fc = 440 MHZ. The tone
was
clearly audible. I can’t understand how it is possible. (I tried many
configurations, the quality of tone changed, but it was still there.)

Then, I try another test. I fixed the RFX400 frequency to 450 MHz and
the
Generator frequency to 460 MHz and I changed the USRP frequency. The
tone
was audible when the USRP frequency was 0 Hz. For other frequencies, I
could
not hear the tone. I can’t not explain that either.

So, do you have any idea what’s going on? Does I have to do special
setting
on the dboard or the USRP?
Maybe I didn’t understand what the dboard is doing.

Thanks for your help.

I developped


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