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Re: [Discuss-gnuradio] phase coherence for N210 and SBX

From: Marcus D. Leech
Subject: Re: [Discuss-gnuradio] phase coherence for N210 and SBX
Date: Thu, 23 Apr 2015 18:12:20 -0400
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.2.0

On 04/23/2015 06:06 PM, Pengyu Zhang 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.

On Thu, Apr 23, 2015 at 3:06 PM, <address@hidden> wrote:

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 Zhang wrote:

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.

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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 Braun  can correct
  me if I'm wrong.

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