[USRP-users] USRP X300/X310 reference daisy-chain

Rob Miller rdmiller8 at hotmail.com
Tue Oct 28 15:02:08 EDT 2014


Thanks for all the great details --- they have been most helpful.  I'm currently considering just a handful of X300s, however wanted to understand the nuances associated with expanding to using 10s of X300s  --- all lab-based and certainly not mobile :)Best,Rob
> Date: Fri, 17 Oct 2014 12:13:18 +0200
> To: usrp-users at lists.ettus.com
> Subject: Re: [USRP-users] USRP X300/X310 reference daisy-chain
> From: usrp-users at lists.ettus.com
> 
> One thing I forgot to mention: the ref clock signal goes through a
> jitter cleaner, which should reduce errors when daisy-chaining. (it's an
> LMK04816, if you want to look up the datasheet).
> 
> Marcus
> 
> On 17.10.2014 11:55, Marcus Müller wrote:
> > Hi Rob,
> >
> > Michael is right, the Ref out is buffered (it's U530, which is a FIN1002
> > LVDS->LVTTL converter/buffer), and the signal is basically just an
> > output of the internal 10MHz clock.
> > So what you get by daisy chaining is USRPs that shouldn't exhibit
> > significant frequency offset[1]. What you won't get is
> > delay-compensation, since the X300 can't know your cable length. Also,
> > you'll get accumulated error -- which might not be a big problem for the
> > overall system, because the PLLs generating the ADC clocks on the
> > motherboards and the LOs on the daughterboards don't have an infinite
> > control loop bandwidth, which will antagonize random variations of pulse
> > duration, but it surely won't increase system performance.
> >
> > Personally, I find daisy chaining to be an awesome concept when it comes
> > to synchronizing "a few" devices, because you can do that without an
> > external clock distributor, knowing that errors introduced can't be that
> > serious. When it comes to synchronizing a lot of devices, I'd say a
> > clock distributor is the easiest way to go, on the fact alone that you
> > can take out any X300 without breaking clocking.
> >
> > So, as usual, I'm curious [2], so: what setup do you have in mind?
> >
> > Greetings,
> > Marcus
> >
> > [1] let's assume, for gross simplification, that you're not moving the
> > x300s relative to each other at high speeds while having them connected
> > via coax, so we can neglect Doppler ;)
> > [2] ok. I'm a nerd. Stuff me in a room with a rack full of cool devices
> > and I'm happy for a while. That might explain the curiosity when it
> > comes to connecting numbers of X300s...
> > On 17.10.2014 02:36, Michael West via USRP-users wrote:
> >> Hi Rob,
> >>
> >> Characterization of the daisy-chaining for the X3x0s has not been
> >> completed, so it is not possible to say at this time.  Officially,
> >> daisy-chaining is not yet supported.
> >>
> >> That said, I believe there is buffering on the lines so, in theory, an
> >> unlimited number of devices can be daisy-chained.  Some propagation delay
> >> from device to device is expected (varies dependent on cable length), so
> >> the limitation is primarily dependent on how tightly all the devices need
> >> to be synchronized for a particular application.  The bottom line is that
> >> daisy-chaining is still experimental, so you will have to try it to see if
> >> it works for you.
> >>
> >> Best regards,
> >> Michael E. West
> >>
> >> On Thu, Oct 16, 2014 at 3:30 PM, Rob Miller via USRP-users <
> >> usrp-users at lists.ettus.com> wrote:
> >>
> >>> Hi -
> >>> Does anyone know a practical maximum for the number of USRP X300/310s that
> >>> can have their references (10MHz/1PPS) daisy-chained together.
> >>> Best,
> >>> Rob
> >>>
> >>>
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> >>
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> 
> 
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