Two channel Tx sample alignment USRP-294xR

[rkossler1]

Hi,

I am trying to use a single USRP RIO (2942R) to transmit two channels.  I am getting the desired signal on both channels, but the two channels are not sample aligned.  If I simultaneously sample the output of the two channels (using another RIO for RX capture) and then perform a cross-correlation, I can see that they are not aligned.  The number of samples misalignment seems to be random from run to run (i.e., restarting the TX VI).

The TX VI that I am using is a modified example VI.  I am not able to use the example VI directly because it transmits only a single tone on both channels.  With such a signal, it is impossible to tell the difference between a random phase shift between channels or a sample misalignment between channels.  So, I modified the example to transmit a multi-tone waveform which enables me to detect this sample alignment.

Looking at the various "write Tx *" VIs, it is not obvious to me how to get the two channels to be sample synchronous.  Please let me know if you have any ideas.

Thanks.

Rob

[rkossler1]

The attached JPG shows the output as measured by another USRP RIO.  I modified the example program which produces a sine wave such that the signal would be pulse modulated (1us pulse every 10us).  The middle plot in the attachment shows the "sample misalignment" that I'm fighting.  Why do the pulses on the two channels not coincide?  It is the same signal that is sent to both channels via the Tx Write VI.

Rob

[lucaso3]

Hello  rkossler1,

Thank you very much for your question. I was looking into some examples and I believe that I found one that might be doing exactly what you require. I hope that this one might clarify more were the error might be located. The example is called niUSRP EX TX Multiple Outputs (Single Device, Multiple Channel).vi.

You might be able to find this example in a Windows 7 computer by going to Start > All Programs > National Instruments > NI USRP > Examples.

Please let me know if this was the example you looked up and were trying to modify, because if this is the one you modified I can try searching some other examples or recommendations that we might need to look at.

Regards,

Luis

[rkossler1]

The example you mentioned is one of the examples using the Ettus FPGA and Ethernet connectivity.  I am using one of the USRP RIO examples that uses LabVIEW FPGA (unmodified from stock image) and PCIe connnectivity.  The example I modified is "Tx Streaming (host).vi".  So, I don't think I can use the one you mentioned as an example because of the significantly different synchronization architecture between the two "modes" of operation.

Rob

[Danan]

Hi, Rob.  I want to write a project that has similar function with yours, How is your progress?

[rkossler1]

I abandoned the effort.  Sorry.