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:mle...@ripnet.com]
*Sent:* Friday, June 13, 2014 2:04 PM
*To:* Lapointe, Benjamin - 1008 - MITLL
*Cc:* discuss-gnuradio@gnu.org
*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
"unknown PPS".
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*
<blapoi...@ll.mit.edu <mailto:blapoi...@ll.mit.edu>> 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
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