[USRP-users] X310, 2 channel sample time and phase alignment

Michael West michael.west at ettus.com
Tue Sep 2 19:37:48 EDT 2014


Hi Elizabeth,

First, I would strongly recommend updating to 3.7.2.  We did fix some phase
alignment issues in that version.

Second, gr-uhd does not use timed commands when changing the frequency (or
gain) and that can cause alignment issues for the X310.  Try setting the
frequency to 0.  If that aligns the signals, just edit the generated Python
code and surround the frequency tune requests (and any other radio control
commands) with set_command_time() and clear_command_time() calls to make
sure both radios are acted upon at the same time.

Best regards,
Michael E. West


On Tue, Sep 2, 2014 at 12:40 PM, Kemp, Elizabeth - 1008 - MITLL via
USRP-users <usrp-users at lists.ettus.com> wrote:

> They are the same! Thanks though.
>
> -Liz
>
> -----Original Message-----
> From: Per Zetterberg [mailto:perz at kth.se]
> Sent: Tuesday, September 02, 2014 3:22 PM
> To: Kemp, Elizabeth - 1008 - MITLL; Marcus D. Leech; Ian Buckley
> Cc: usrp-users at lists.ettus.com
> Subject: RE: [USRP-users] X310, 2 channel sample time and phase alignment
>
> Are the cable lengths the same? If not, that could be the reason.
> ________________________________________
> From: USRP-users [usrp-users-bounces at lists.ettus.com] on behalf of Kemp,
>       Elizabeth - 1008 - MITLL via USRP-users [usrp-users at lists.ettus.com]
> Sent: Tuesday, September 02, 2014 8:55 PM
> To: Marcus D. Leech; Ian Buckley
> Cc: usrp-users at lists.ettus.com
> Subject: Re: [USRP-users] X310, 2 channel sample time and phase alignment
>
> It is always the same for a given frequency.
>
> ________________________________
> From: Marcus D. Leech [mleech at ripnet.com]
> Sent: Tuesday, September 02, 2014 2:12 PM
> To: Kemp, Elizabeth - 1008 - MITLL; Ian Buckley
> Cc: usrp-users at lists.ettus.com
> Subject: Re: [USRP-users] X310, 2 channel sample time and phase alignment
>
> On 09/02/2014 01:57 PM, Kemp, Elizabeth - 1008 - MITLL wrote:
> Marcus - The two channels are on two different BASIC_RX boards. I can try
> updating to 3.7.2 and see if that changes anything.
>
> Ian - Thanks, unfortunately I am using a 0 degree splitter (it would be
> nice if that was the fix!). I also picked an example where they are almost
> 180 degrees out of phase, for some other frequencies they are actually
> pretty close. I am using a mini circuits ZX10-2-12-S+ splitter - according
> to the datasheet the phase unbalance should only be 0.01 degrees at 50 MHz.
> Is the phase offset always the same for any given frequency, or does it
> change from run to run?
>
>
>
> From: Ian Buckley [mailto:ianb at ionconcepts.com]
> Sent: Tuesday, September 02, 2014 1:37 PM
> To: Marcus D. Leech
> Cc: Kemp, Elizabeth - 1008 - MITLL; usrp-users at lists.ettus.com<mailto:
> usrp-users at lists.ettus.com>
> Subject: Re: [USRP-users] X310, 2 channel sample time and phase alignment
>
> Thats a giant delay...nothing I can think of internal to X310 could
> account for that. I don't suppose you accidentally used a 180 degree
> splitter instead of a 0 degree? I only ask because I ddi the same once and
> wasted a bunch of time...
>
> On Sep 2, 2014, at 10:28 AM, Marcus D. Leech via USRP-users <
> usrp-users at lists.ettus.com<mailto:usrp-users at lists.ettus.com>> wrote:
>
>
> On 09/02/2014 01:25 PM, Kemp, Elizabeth - 1008 - MITLL wrote:
> Thank you for the reply Marcus!
>
> I just plotted as a complex pair and it still doesn't have the proper
> phase alignment...  I'm using UHD version 3.7.1.
>
> Could you update to 3.7.2?
>
> Also, how are you setting up the two channels?  Are they on two different
> BASIC_RX?
>
>
>
> -Liz
>
>
> From: USRP-users [mailto:usrp-users-bounces at lists.ettus.com] On Behalf Of
> Marcus D. Leech via USRP-users
> Sent: Tuesday, September 02, 2014 11:49 AM
> To: usrp-users at lists.ettus.com<mailto:usrp-users at lists.ettus.com>
> Subject: Re: [USRP-users] X310, 2 channel sample time and phase alignment
>
> On 09/02/2014 11:23 AM, Kemp, Elizabeth - 1008 - MITLL via USRP-users
> wrote:
>
> Hi,
>
>
>
> I have a test set up where I am recording on both channels of an X310.  I
> have the output of a signal generator connected to the channels through a
> splitter (and equal length cables).  I am using the BasicRX Daughterboards
> (1-250 MHz).
>
>
>
> I have attached a screenshot from GRC of the WX GUI that shows a plot of
> the real samples, and the magnitude of the two channels.  As you can see in
> the magnitude plot, the two channels seem to be time-aligned, but there is
> a huge phase offset between the two channels.  I've observed that the phase
> offset varies across different frequencies.
>
>
>
> So my question is - why is there a phase offset between the 2 channels?
> Is there anything that can be done to correct it?
>
>
>
> Thanks for your help,
>
> Liz
>
>
>
>
>
> _______________________________________________
>
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>
> USRP-users at lists.ettus.com<mailto:USRP-users at lists.ettus.com>
>
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> There may not be.  In the past, the scope sink has had issues with proper
> phase display.  If you plot it as a complex pair instead, it maintains
> proper phase
>   display.
>
> Also, what version of UHD are you using?
>
>
>
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