On Tue, May 8, 2018 at 1:51 PM, Marcus D. Leech via USRP-users < usrp-users@lists.ettus.com> wrote:
> On 05/08/2018 02:17 PM, Reinhold Frederick William Hollender via > USRP-users wrote: > > > > 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 > > >> >> The phase offset at baseband should be the same as at RF. If it wasn't, > then things like PSK modulators that do their modulation at baseband would > not > work correctly at RF. That clearly isn't the case. > > I do phase-sensitive interferometry for radio astronomy. My fringe rates > are exactly what theory predicts from the *RF* frequency and antenna > spacing. If there was "phase scaling", that would not be the case. > > Doh! You are correct of course. That's what I get for relying on my intuition instead of actually looking at the math! > > > >> >> 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 >> >> > > > _______________________________________________ > USRP-users mailing > listUSRP-users@lists.ettus.comhttp://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com > > > > _______________________________________________ > USRP-users mailing list > USRP-users@lists.ettus.com > http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com > >
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