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[Discuss-gnuradio] USRP and RFX400

From: TGV
Subject: [Discuss-gnuradio] USRP and RFX400
Date: Tue, 25 Sep 2007 14:44:15 -0700 (PDT)

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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