On Tue, May 8, 2018 at 12:23 PM, shachar J. brown via USRP-users < usrp-users@lists.ettus.com> wrote:
> 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? > The additional cable length acts as a time delay, so I would calculate the expected phase difference based on the frequency of the baseband signal, not the RF signal. I have a couple different cable lengths, so I can try running an experiment if I get some time later today. > > 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... > The list is copied on each email, so simply reply to all instead of replying to just the sender. Cheers, William > > > > 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 > *>>* > > *>>* > > *>> >>* _______________________________________________* > > > > _______________________________________________ > USRP-users mailing list > USRP-users@lists.ettus.com > http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com > >
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