Thanks everyone for trying to sort this out! Robbin - as you stated yourself, spliiters and channel coupling should have minor effect, especially in our case where the wavelength is larger than the whole system.
Ian - I am actually using a proper external signal generator. No loopbacks. Anyone here ever done such an experiment and can share his results? And what are your thoughts about my assumption, that measurment of the phase is done on the baseband and therefore resulting in different phase-diff's than the actual input? Thanks again, Steve P.S. How do you respond within the mailing list? Each time I send a new mail with similar subject and copying the previous replies manually... Steve , I got the feeling from your original message, though you didn’t say it outright, that you might be using the same USRP its self as the signal generator with loopback cables. If so, beware “self receive” via paths other than your calibrated length cables confusing the results, a sensitive full duplex radio like this hears its self via various leakage paths especially with RX and TX tuned to the same freq. (I’m assuming in the original email the cable you added was 50.8cm or 0.508m, right?) -Ian >* On May 8, 2018, at 9:34 AM, ROBIN TORTORA via USRP-users <usrp-users at >lists.ettus.com ><http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com>> wrote: *> >* some small things: *> >* I am pretty sure you cables are not phase matched (costs about 5K for a 26” matched pair), so you will get some difference there. Not sure how to quantify. *>* Splitters have a phase mismatch, I think its called phase unbalance, proportional to cost :), but can be multiple degrees. *>* even between 2 channels on the same device, there will be some phase noise *>* Still seems far away from 18 degrees, so cant help more than above... *> >>* On May 8, 2018 at 11:17 AM "shachar J. brown via USRP-users" <usrp-users at lists.ettus.com <http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com>> wrote: *>> >>* Hi Jeff, *>>* Thanks for your response, but you understood me completely wrong. *>>* Of course I have set the RF freq in the source block. Furthermore, I have fully analized the signal from both chanels, and it appears crystal clear in all the sinks throughout the flow graph (e.g., a pure max at the correct bin after the FFT in the vector sink, and a beautiful phase gain in the time sink after the phase extraction). *>>* My problem isn't receiving the signal or analizing it. My problem is that the phase difference between the two channels does not match the theory. The wire to one channel is longer than the other by at least 1/6 of a wavelength, whilst the phase diff was only 1/10 of 2*pi. *>>* Am I understood? *>>* Does anyone have a clue what's going wrong? *>>* Thanks again, *>>* Steve *>> >> >>* On 05/07/2018 11:11 AM, shachar J. brown via USRP-users wrote: *>>* > Hi All, *>>* > *>>* > I am trying to measure the phase difference of a received signal between *>>* > the two RX ports of the B210. (In the grc I simply ran each of the two *>>* > received signals through an FFT, took the bin with highest amplitude and *>>* > extracted it's phase, and finaly - subtracted the two). *>>* > *>>* > I experimented with a single signal source generator split in two. I *>>* > first connected both RX ports with matching wires and received zero *>>* > phase difference as expected. *>>* > *>>* > Though when I added a wire of some length to one of the ports, the *>>* > received phase difference was NOT as expected by theory. *>>* > *>>* > (In short, I sent 100[Mhz] pure sine wave, thus wavelength of 3[m] or *>>* > shorter, the extra wire was 0.508[cm] long, thus I would expect a phase *>>* > diff of about 60 deg or more. Frankly I received a phase diff of about *>>* > 18 deg). *>>* > *>>* > What am I doing wrong? *>>* > *>>* > I thought maybe the phase calculation of gnuradio is done on the *>>* > baseband frequency and not on the RF, and therefore the phase diff would *>>* > be different. Is this my problem? (e.g. if the baseband is only 30[Mhz], *>>* > then expected phase diff would be 18 deg). If that is the case - how can *>>* > I know which baseband frequency the AD9361 has chosen? *>> >>* GNU Radio works with signals at baseband. It sounds like you might not *>>* have set a RF frequency in the USRP source block. The default is 0, and *>>* I'm not sure what the B210 would tune to in that case. *>> >>* I don't know whether this experiment actually works, but to do it you *>>* would tune to 99M, set the sample rate to 2M, and see what happens. The *>>* peak should be right in the middle. *>> >>* Also, make sure your signal generator is sending out a very low signal - *>>* try something like -40dBm. Max is higher, but there's no need. *>> >>* > *>>* > Thank you all for your time, *>>* > Steve *>>* > *>>* > *>> >>* _______________________________________________*
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