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

Marcus Müller marcus.mueller at ettus.com
Fri Oct 17 05:55:56 EDT 2014

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?


[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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