I am using BasicRX daughterboards, each sampling a single real-mode signal.
From: address@hidden [mailto:address@hidden On Behalf Of Marcus D. Leech
Sent: Tuesday, June 17, 2014 10:06 AM
To: address@hidden
Subject: Re: [Discuss-gnuradio] sample time alignment in GRC
On 06/17/2014 09:58 AM, Lapointe, Benjamin - 1008 - MITLL wrote:
Hi All,
I am still having trouble time aligning sample streams from two USRP X310 devices. In GRC I noticed a random time offset from run to run in the two data streams using a WX GUI Scope Sink. Looking at recorded data in MATLAB I also see a random time offset from run to run in the two data streams (8, 18, and 24 sample offset). I verified that the two data streams that I am inputting into the X310 devices are time aligned using a physical scope.
My GRC setup:
USRP Source 1 (with internal GPSDO-MINI)
Sync = unknown PPS
Mb0: Clock Source = Default
Mb0: Time Source = Default
USRP Source 2
Sync = unknown PPS
Mb0: Clock Source = External
Mb0: Time Source = External
For looking at the data streams I have USRP Source -> Complex to Mag -> WX GUI Scope Sink.
For recording the data streams I have USRP Source -> Head (5K) -> File Sink (Unbuffered: OFF)
Ref Out SMA of USRP 1 is connected to Ref In SMA of USRP 2 with a 6” SMA cable.
PPS Trig Out SMA of USRP 1 is connected to PPS Trig In SMA of USRP 2 with a 6” SMA cable.
RF input to USRP devices is a pulsed RF signal, to make it easier to look at time offset.
GPS on USRP 1 is locked; however, I work with tall buildings completely surrounding me and so I don’t know the strength of the GPS lock.
I have an OctoClock-G on order to distribute 10 MHz Ref and 1 PPS signals, but until then..
Does anyone have any other ideas for getting time-aligned samples from run to run in GRC, or what I am doing wrong? I would expect at most a minimal constant time offset between data streams if the 10 MHz Ref and 1 PPS signals are locked.
Thanks!
-ben
From: Marcus Leech [mailto:address@hidden]
Sent: Friday, June 13, 2014 2:04 PM
To: Lapointe, Benjamin - 1008 - MITLL
Cc: address@hidden
Subject: Re: [Discuss-gnuradio] sample time alignment in GRC
Make sure that you specify that the 2nd X310 uses external clock and 1PPS, and all of them should use time synch of
Also, there has been a bug in the scope sink (dunno if fixed) where samples are *not* time-aligned in the scope sink. The except
is that a complex-pair will be time-aligned internally, but not necessarily to other streams being displayed.
on Jun 13, 2014, Lapointe, Benjamin - 1008 - MITLL <address@hidden> wrote:
Hi,
I have two USRP X310 devices that I am trying to time align in GNU Radio Companion. One X310 has a GPSDO that is sending 10 MHz reference and 1 PPS signals to the other one. The GPS is locked. Ideally I would have matched length cables for 10 MHz reference and 1 PPS, but I think my setup is close enough. (Input signal from sig gen = pulsed 10.005 MHz, input is split with matched length cables, USRP output sampling rate = 5M, USRP center frequency = 10M.)
I am using WX GUI Scope Sink to look at the magnitudes of each stream from the USRP devices. I expect to see no/minimal delay between the two signal streams, but I am seeing delays of 24, 13, 9, 0, 3, 6, 25, 24 samples from run to run between the two signal streams. The period of the signal is 50 samples, so the maximum delay difference is 25 samples. Am I missing something in my configuration? Since I am using a 10 MHz reference and 1 PPS signals, I expect time alignment between the two sample streams. Is there a GRC block for forcing time alignment?
Thanks!
-Ben
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What daughtercards are you using again?
There *will* be a random phase offset between the two "sides" here, because GRC flow-graphs can't take advantage of timed-commands to phase-align
the LOs on WBX and SBX cards.
--
Marcus Leech
Principal Investigator
Shirleys Bay Radio Astronomy Consortium
http://www.sbrac.org
smime.p7s