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

Marcus D. Leech patchvonbraun at gmail.com
Tue Mar 5 08:33:42 EST 2019


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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