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

Kemp, Elizabeth - 1008 - MITLL Elizabeth.Kemp at ll.mit.edu
Tue Sep 2 14:55:23 EDT 2014


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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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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