[Discuss-gnuradio] Change in Phase on a USRP2 configured as a VNA

[Justin Bracken]

Hello Everyone,

 

I have put together a simple vna setup using a USRP2. For right now Im using the loop back cable that ettus sells (SMA-SMA) and a 3dB pad to connect the RX to the TX port. While running the attached setup in GRC I see a couple of things that concearn me and I was hoping to get some help on them.

 

The first thing is the startup phase. If Im not mistaken without calibration there is no way to determine what this should be even though the cable thru does not change substantially each time I restart the grc file. From what I read I believe this just has to do with the phase of the internal clock in the usrp when I execute a file in grc. I think I can live with this since I ll have a reference in my measurement that I can set the phase off of.

 

The second thing that I see though that is more troublesome is that when I start the file and just allow it to run the phase changes over time. It will be stable for like .5 minute or so and then jump in value and then stay stable again for another interval. This pattern repeats. Im runnig at 200k sample frequency and 500 decimation so I don't think Im pushing the bounds of gig-e by any means. Plus not that Im sure that this has anything to do with it but the decimation is a multiple of 4 so Im getting the full benefit of the hardware. Am I wrong in thinking this should be stable over time while the hardware is running? Is this indicative of a drop in communication to the USRP2 which causes it to shut down and then its merely the first problem again, that is a random intial startup phase. I don't see any U of O s in the log so I don't think Im having that problem. 

 

That third thing that I m curious about is the affects of changing transmit frequency (LO Frequency) , at run time, of the USRP2, and its affect on the measurement. Im assuming that there is some sort of settling time before I should assume the measurment is valid what is this? I m guessing this will also change the phase value. But how. I saw that matt asked in a previous thread why the user didn't just transmit the bandwidth they needed to the USRP2 instead of using the LO. In this case since Im using the WBX board and I can't use an LO of 0 MHz (I assume) it makes sense to use the LO of the WBX board as the RF source. It would allow me to use the full bandwidth of the WBX board 68MHZ to 2.2GHz without the gig-e limitations.

 

The final thing is which is better as a source. A constant or a sinusoid? I settled on a sin function by trial and error. But is there a better rationale for selection?

 

Thanks,

Justin

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