[USRP-users] different phase difference using splitter on each run

Koyel Das (Vehere) koyel.das at vehere.com
Tue Mar 5 11:10:50 EST 2019


Are RX1 and RX2 of RF0 controlled by same master oscillator? Will connecting two antennas to RX1 and RX2 of RF0, rather than to RX1 of RF0 and to RX1 of RF1, give same phase difference between these two channels at every run?

Koyel Das
Senior – Product Engineer
Vehere | Proactive Communications Intelligence & Cyber Defence
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________________________________
From: USRP-users <usrp-users-bounces at lists.ettus.com> on behalf of Marcus D. Leech via USRP-users <usrp-users at lists.ettus.com>
Sent: Tuesday, March 5, 2019 7:03:42 PM
To: usrp-users at lists.ettus.com
Subject: Re: [USRP-users] different phase difference using splitter on each run

On 03/05/2019 06:01 AM, Koyel Das (Vehere) via USRP-users wrote:

Hi,


I am using USRP 2955 R and I have connected an antenna to one end of a two channel splitter and connected the other two ports of the splitter to the two RX1 pors of USRP. I am getting different phase difference in each run of USRP though in one particular run the phase difference is constant for all channels. The tuned frequency is 2.4 GHz and sampling rate is 1 MSps so as expected that channel phase differences are almost same. But when I stop acquiring data and restart it using gnuradio or the API, then I am getting again a new value of phase difference across all channels, which is again same across channels. Is USRP so unstable that it gives different value of phase difference for each run for the same frequency, and sampling rate? The setup in the runs are also the same.


Regards,

Koyel


Koyel Das
Senior – Product Engineer

Vehere | Proactive Communications Intelligence & Cyber Defence
M: +919051132173 | T: +91 33 40545454 | F: +91 33 40545455 | W: www.vehere.com<http://www.vehere.com/>

This isn't about "instability".  It's about the way PLL synthesizers work, and the fact that by default each channel is using its own synthesizer.

There are a few things that need to happen:

   All the channels you're receiving need to be in the same multi_usrp block, which, among other things, is a conceptual "synchronization domain".

   Use timed-commands for tuning (which is a bit awkward to arrange in Gnu Radio, you'll have to modify the generated code).  This will arrange
   for the synthesizers to be given a "resynch" signal at exactly the same time.

   OR

   Use the "shared LO" features of the TwinRX cards that are on the 2955.

See the knowledge-base document on the TwinRX and phase-synchronization here:

https://kb.ettus.com/TwinRX






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