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how to run the loopback test with a NI-USRP 2921?



I have two NI-USRP 2921, when I configure one NI-USRP in TX and the other one in RX, I have no problem, the reception is good. The problem arrives when I want to run a loopback test on a NI-USRP (with one computer and one NI-USRP, I want simulate a TX and a RX on the same NI-USRP). The transmission works perfectly (I check with a spectum analyzer) but the side of the reception I see just the noise.

I already change the parameters values (IQ rate(100k to 5M) , waveform size(1000 to 10000), number of sample 1024 to 16384,...).


Have you an explication?

I saw that this configuration is possible :



        frequency 2,4Ghz to 2,5 Ghz

        niUSRP EX Rx Continuous

        niUSRP EX Tx Continuous

        Labview 2011





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Message 1 of 6

Hello Fab,


It is important to use the 30 dB attenuator when directly connecting a transmitter to a receiver such that no damage will occur to the receiver.


Do both USRP display noise or just one USRP? If connecting the TX from one USRP to the RX of another USRP, does it also read only noise?

Anthony F.
Staff Software Engineer
National Instruments
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Message 2 of 6
The USRP-2921 is half duplex. Two units are required for simultaneous transmit and recieve. We will note this in the tutorial on the link above.
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Message 3 of 6

Thanks for the response  (not half duplex).

Maybe some people can explain me why?

For me, the USRP 2920 and the 2921 are the same box, and it's just the frequency which change.

The FPGA are not the same? I don't understand....





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Message 4 of 6

Hi Fab,


While the 2920 and 2921 have the same casing, the boards that are actually inside of them are different.  For these USRPs, there is a motherboard which is the same for the 2920 and 21.  However, the 2920 and the 2921 each have daughterboards as well which are different for the two different models.  The 2920 has a WBX daughterboard and the 2921 has a XCVR daughterboard.  You can find more information about the daughterboards by clicking on the links.  This is the reason for the differences in performance between the 2920 and 2921 that you are seeing.  Let me know if this doesn't answer your question and I'll do my best to help.

Sarah Y

SDR Product Manager
National Instruments | Ettus Research
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Message 5 of 6

hello, I have the same problem with you, did you figure out whether it can run the loopback test with 2921? We have made sure  just as Sarah said , the  XCVR daughterboard is  half duplex , so the application is very limited . 

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Message 6 of 6