# Phase coherence for N210 and SBX **URL:** \ **Category:** GNU Radio\ **Created:** [April 23, 2015, 8:51pm UTC](https://www.ruby-forum.com/t/phase-coherence-for-n210-and-sbx/241688 "2015-04-23T20:51:54Z")\ **Posts on this page:** 13\ **Page:** 1
**Author:** ![musicdenotation](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/ea666f/32.png) [@musicdenotation](https://www.ruby-forum.com/u/musicdenotation)\ **Post date:** [April 23, 2015, 8:51pm UTC](https://www.ruby-forum.com/t/phase-coherence-for-n210-and-sbx/241688/1 "2015-04-23T20:51:54Z")
Hi, I am working on phase coherent reception using 2 USRP N210s where each is mounted with a SBX. The 2 N210s are connected via a MIMO cable. 10MHz reference clock and PPS are generated by an OctoClock-G and are fed into each N210. In this experiment, a TX antenna transmits a constant signal to 2 RX antennas. I want to obtain coherent phases at the 2 RX antennas. Attached is the flow graph I am executing. It seems that I cannot achieve coherent phase at the 2 Rx antennas. Any suggestions? Thanks. Pengyu ---
**Author:** ![Pengyu\_Z](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/p/e79b87/32.png) [@Pengyu\_Z](https://www.ruby-forum.com/u/Pengyu_Z)\ **Post date:** [April 24, 2015, 12:07am UTC](https://www.ruby-forum.com/t/phase-coherence-for-n210-and-sbx/241688/2 "2015-04-24T00:07:57Z")
I followed the suggestions and plotted the received baseband signal in the IQ constellation. The phase offset of the baseband received by the two RX antennas are still not constant… Attached are the update flow graph and python program. Would be great if you can take a look again. Thanks. ---
**Author:** ![Pengyu\_Z](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/p/e79b87/32.png) [@Pengyu\_Z](https://www.ruby-forum.com/u/Pengyu_Z)\ **Post date:** [April 23, 2015, 9:08pm UTC](https://www.ruby-forum.com/t/phase-coherence-for-n210-and-sbx/241688/3 "2015-04-23T21:08:15Z")
If you have the Octoclock-G connected to both units, then you don’t need to use the MIMO cable. Make sure that both mboards have “external” for both the reference clock and pps selections, and that “device time” is set to “unknown PPS” for both of them. On 2015-04-23 14:49, Pengyu Z. wrote: > Hi, > > I am working on phase coherent reception using 2 USRP N210s where each is > mounted with a SBX. The 2 N210s are connected via a MIMO cable. 10MHz reference > clock and PPS are generated by an OctoClock-G and are fed into each N210. In this > experiment, a TX antenna transmits a constant signal to 2 RX antennas. I want to > obtain coherent phases at the 2 RX antennas. Attached is the flow graph I am > executing. It seems that I cannot achieve coherent phase at the 2 Rx antennas. Any > suggestions? Thanks. > > Pengyu > > * * * > > Discuss-gnuradio mailing list > [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) > [Discuss-gnuradio Info Page](https://lists.gnu.org/mailman/listinfo/discuss-gnuradio) [1] Links: ---
**Author:** ![Pengyu\_Z](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/p/e79b87/32.png) [@Pengyu\_Z](https://www.ruby-forum.com/u/Pengyu_Z)\ **Post date:** [April 24, 2015, 12:19am UTC](https://www.ruby-forum.com/t/phase-coherence-for-n210-and-sbx/241688/4 "2015-04-24T00:19:44Z")
On 04/23/2015 06:06 PM, Pengyu Z. wrote: > I followed the suggestions and plotted the received baseband signal in > the IQ constellation. The phase offset of the baseband received by the > two RX antennas are still not constant… Attached are the update flow > graph and python program. Would be great if you can take a look again. > Thanks. ALSO, the WX GUI scope sink doesn’t maintain predictable phase-alignment between multiple signal sources… ---
**Author:** ![Pengyu\_Z](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/p/e79b87/32.png) [@Pengyu\_Z](https://www.ruby-forum.com/u/Pengyu_Z)\ **Post date:** [April 24, 2015, 12:14am UTC](https://www.ruby-forum.com/t/phase-coherence-for-n210-and-sbx/241688/5 "2015-04-24T00:14:20Z")
On 04/23/2015 06:06 PM, Pengyu Z. wrote: > ``` > don't need to use the MIMO cable. > > ``` > > > ``` > > cable. 10MHz reference clock and PPS are generated by an > > https://lists.gnu.org/mailman/listinfo/discuss-gnuradio > > > > ``` If you want phase-coherency, the two USRP sources _must_ be part of the same multi\_usrp object–you have them as separate sources. One can think of a multi\_usrp object as also providing a common “coherency domain”. Also, even when you do that, the “timed commands” feature in UHD, specifically designed to zero-out phase _offset_ for the SBX and WBX and UBX boards, isn’t, as far as I recall, something that gr-uhd does automatically, so it isn’t available in a GR flow-graph. Martin B. can correct me if I’m wrong. ---
**Author:** ![Pengyu\_Z](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/p/e79b87/32.png) [@Pengyu\_Z](https://www.ruby-forum.com/u/Pengyu_Z)\ **Post date:** [April 24, 2015, 12:25am UTC](https://www.ruby-forum.com/t/phase-coherence-for-n210-and-sbx/241688/6 "2015-04-24T00:25:09Z")
Thanks for the inspiring tips. I did not use the WX GUI scope for checking the phase offset. Instead, I log the baseband signal received by each RX antenna. Then, I plot the IQ constellation of the received baseband for checking their phase offset. Can you be a bit more specific when you say the “multi\_usrp object” and the “timed commands”? Are there tutorials that I can follow step by step for using the two functions you mentioned? Your comments are very helpful 🙂 ---
**Author:** ![Pengyu\_Z](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/p/e79b87/32.png) [@Pengyu\_Z](https://www.ruby-forum.com/u/Pengyu_Z)\ **Post date:** [April 24, 2015, 12:30am UTC](https://www.ruby-forum.com/t/phase-coherence-for-n210-and-sbx/241688/7 "2015-04-24T00:30:35Z")
On 04/23/2015 06:23 PM, Pengyu Z. wrote: > Thanks for the inspiring tips. I did not use the WX GUI scope for > checking the phase offset. Instead, I log the baseband signal received > by each RX antenna. Then, I plot the IQ constellation of the received > baseband for checking their phase offset. > > Can you be a bit more specific when you say the “multi\_usrp object” > and the “timed commands”? Are there tutorials that I can follow step > by step for using the two functions you mentioned? Your comments are > very helpful 🙂 A source/sink block in Gnu Radio uses an underlying multi\_usrp object, so all you have to do is configure your source block for two channels, with two devices, and use addr0=\ **[USRP Hardware Driver and Device Manual: Coding to the API](https://files.ettus.com/manual/page_coding.html)** The documents for gr-uhd are part of the Gnu Radio documentation: > **[GNU Radio Manual and C++ API Reference: gr::uhd::usrp\_source Class Reference](https://www.gnuradio.org/doc/doxygen/classgr_1_1uhd_1_1usrp__source.html)** ---
**Author:** ![Pengyu\_Z](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/p/e79b87/32.png) [@Pengyu\_Z](https://www.ruby-forum.com/u/Pengyu_Z)\ **Post date:** [April 24, 2015, 9:09am UTC](https://www.ruby-forum.com/t/phase-coherence-for-n210-and-sbx/241688/8 "2015-04-24T09:09:02Z")
Hi Marcus, hi Pengyu, I’d like to make an addition to that: modern gr-uhd uses a timed command to start streaming samples, so the CORDICs in both USRPs should be aligned. But: It doesn’t inherently use timed commands for tuning; you should do something like: usrp\_source0.set\_command\_time(uhd\_source0.get\_time\_now()+0.1) usrp\_source0.set\_center\_freq(frequency) usrp\_source0.clear\_command\_time() time.sleep(0.15) after you configured your usrp\_source and before your top\_block is .start()ed. Generally, I like [http://files.ettus.com/manual/page\_sync.html](http://files.ettus.com/manual/page_sync.html) because it explains a bit why and how things work. Greetings, Marcus ---
**Author:** ![Pengyu\_Z](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/p/e79b87/32.png) [@Pengyu\_Z](https://www.ruby-forum.com/u/Pengyu_Z)\ **Post date:** [April 24, 2015, 10:29pm UTC](https://www.ruby-forum.com/t/phase-coherence-for-n210-and-sbx/241688/9 "2015-04-24T22:29:11Z")
Hi Marcus, I updated the flow graph (attached) as you suggested. It would be great if you can help check and let me know if I am wrong… For upgrading the UHD, I followed this link. UHD is installed successfully. However, it seems that gnuradio does not call the latest UHD. Instead, it still calls uhd-host which is combined in the gnuradio package. Do you have suggestions about how to solve this problem? Thanks. Pengyu ---
**Author:** ![Pengyu\_Z](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/p/e79b87/32.png) [@Pengyu\_Z](https://www.ruby-forum.com/u/Pengyu_Z)\ **Post date:** [April 24, 2015, 11:25pm UTC](https://www.ruby-forum.com/t/phase-coherence-for-n210-and-sbx/241688/10 "2015-04-24T23:25:27Z")
I got the same UHD version (shown below) when I use apt-get for installation. I will try the PyBOMBS now… $ python top\_block.py linux; GNU C++ version 4.8.2; Boost\_105400; UHD\_003.005.005-0-unknown $ apt-get install gnuradio ---
**Author:** ![Pengyu\_Z](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/p/e79b87/32.png) [@Pengyu\_Z](https://www.ruby-forum.com/u/Pengyu_Z)\ **Post date:** [April 24, 2015, 10:50pm UTC](https://www.ruby-forum.com/t/phase-coherence-for-n210-and-sbx/241688/11 "2015-04-24T22:50:04Z")
On 04/24/2015 04:27 PM, Pengyu Z. wrote: > ``` > constellation of the received baseband for checking their > two devices, and use > > http://gnuradio.org/doc/doxygen/classgr_1_1uhd_1_1usrp__source.html > > ``` Uninstall all your uhd and gnuradio packages, and re-install, following the guide here: [http://ettus-apps.sourcerepo.com/redmine/ettus/projects/uhd/wiki/GNU\_Radio\_UHD](http://ettus-apps.sourcerepo.com/redmine/ettus/projects/uhd/wiki/GNU_Radio_UHD) ---
**Author:** ![Pengyu\_Z](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/p/e79b87/32.png) [@Pengyu\_Z](https://www.ruby-forum.com/u/Pengyu_Z)\ **Post date:** [April 25, 2015, 10:51am UTC](https://www.ruby-forum.com/t/phase-coherence-for-n210-and-sbx/241688/12 "2015-04-25T10:51:41Z")
You _must not_ do apt-get install gnuradio, because Ubuntu’s GNU Radio depends on Ubuntu’s UHD which will automatically and inevitably be installed; so you must uninstall gnuradio, uhd-host, and your manually installed UHD now, before doing things with pybombs. Greetings, Marcus ---
**Author:** ![Pengyu\_Z](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/p/e79b87/32.png) [@Pengyu\_Z](https://www.ruby-forum.com/u/Pengyu_Z)\ **Post date:** [April 24, 2015, 11:43pm UTC](https://www.ruby-forum.com/t/phase-coherence-for-n210-and-sbx/241688/13 "2015-04-24T23:43:43Z")
Hello Pengyu: I’ll send you my build instructions document. I think that will help guide you. –Neel