[USRP-users] LO phase alignment

John Malsbury john.malsbury at ettus.com
Fri Jun 22 14:11:38 EDT 2012


If I might chime in...

If you use the SBX retune feature, this will align all of the phase LOs.
However, variations in other parts of the signal chain (mixers, filters,
etc) will cause slight phase differences across the channels.  So, you may
still need to some level of calibration of channel-to-channel phase if you
need them very closely aligned.  The phase differences between the channels
will change across temperature and frequency, but not with each re-tune if
you use these timed commands.  Calibration requirements are ultimately
driven by the channel-to-channel phase and amplitude requirements of your
application.

Chris, can you explain a little about how this system works?  Are you doing
ranging and comms with this system?  I've seen similar, multi-channel
optical systems with other customers.

-John




On Fri, Jun 22, 2012 at 10:49 AM, Matt Ettus <matt at ettus.com> wrote:

> To clarify here, as long as all LOs are tuned at the same time, you
> will always get the same phase offset.  If you retune one, you have to
> retune the others simultaneously.
>
> Matt
>
> On Fri, Jun 22, 2012 at 1:58 AM, Christophe ALEXANDRE
> <christophe.alexandre at cnam.fr> wrote:
> > Hi Josh,
> >
> > i've read the link :
> >
> > Using timed commands, multiple frontends can be tuned at a specific time.
> > This timed-tuning ensures that the phase offsets between VCO/PLL chains
> will
> > remain constant after each re-tune. See notes below:
> >  a.. There is a random phase offset between any two frontends
> >  b.. This phase offset is different for different LO frequencies
> >  c.. This phase offset remains constant after retuning
> >   a.. Due to divider, WBX phase offset will be randomly +/- 180 deg after
> > re-tune
> >  d.. This phase offset will drift over time due to thermal and other
> > characteristics
> >  e.. Periodic calibration will be necessary for phase-coherent
> applications
> > I still don't understand the principle you use. is it a VCO special
> property
> > ?
> >
> > some questions still remain :
> >
> > Q1) if i change LO freq from f1 to f2 then back to f1, should i get the
> same
> > phase offset on f1 ?
> >
> > Q2) if i turn off the N210s then on, should i get the same offset on the
> > same freq ?
> >
> > Q3) if the Q1, Q2 answers are yes, what kind of phase offset precision
> > should i expect ?
> >
> >
> >
> > regards.
> >
> >
> > Christophe ALEXANDRE
> > Conservatoire National des Arts et Métiers (CNAM)
> > Laboratoire CEDRIC-LAETITIA
> > Département EASY
> > Accès 17-1-32, Case 2D2P10
> > 292 rue Saint Martin
> > 75141 PARIS CEDEX 03
> > FRANCE
> > email : christophe.alexandre at cnam.fr
> > tel. 0140272699
> > mob. 0651087311
> > fax. 0140272994
> >
> >
> > ----- Original Message ----- From: "Josh Blum" <josh at ettus.com>
> > To: <usrp-users at lists.ettus.com>
> > Sent: Thursday, June 21, 2012 5:53 PM
> >
> > Subject: Re: [USRP-users] LO phase alignment
> >
> >
> >>
> >>
> >> On 06/21/2012 08:48 AM, Christophe ALEXANDRE wrote:
> >>>
> >>> hi John,
> >>>
> >>> i have already read this app note (in fact, i have started with it).
> >>> I've only seen a star after phase on table 2 for SBX but no more info.
> >>>
> >>> what is the basic principle used to solve the phase alignment problem ?
> >>>
> >>
> >> Does this example help:
> >>
> >>
> http://files.ettus.com/uhd_docs/manual/html/sync.html#align-los-in-the-front-end-sbx-wbx-n-series
> >>
> >>> can i use this new feature using gnuradio ?
> >>>
> >>
> >> Yes
> >>
> >>> do you have any example (even C++ example) showing how to use it ?
> >>>
> >>
> >> Link above.
> >>
> >> -josh
> >>
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> >
> >
> >
> >
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>
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