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View all threadsPhase drift issue of N310
Hi all,
There are two daughter boards in N310, the phase difference between
channels of different dboards will drift rapidly.
In my test, a B200 transmits single tone continuous wave to a 1-to-2
splitter, and the outputs of splitter are connected to input ports of RF0
and RF2 of N310.
During the single program running, the signals received by the two channels
should keep the phase difference approximately stable. But I observed that
the phase drift of the two channels was very fast. Within a time span of 20
seconds, the phase difference varied by tens of degrees. Sense that two
dboards are using independent reference clocks, not the same reference
clock from mboard. By contrast, the phase difference between the two
channels of x310 with two ubx is stable.
Does anyone know how to explain this issue?
Best regards,
Damon
Hello,
You sort of said the answer yourself:
"two dboards are using independent reference clocks"
RX0 and RX1 use the same LO while RX2 and RX3 use a different LO.
I like to think of each N310 as two radios in the same box (when it comes
to phase issues). Additionally, I think UHD/GNUradio start up RF components
at run-time so all calibration needs to be preformed each time you run a
flow-graph or C file?
There are signal processing ways to mitigate this. Consider looking into
ESPRIT or array shape calibration research.
The hardware fix to this problem is feeding an external LO into each N310
dboard.
Attached are some old results from our lab with an 8 TX 24 RX system set up
indoors (all N310s). You can see that our DOA estimate in ESPRIT drifts
badly (10 degrees or so) over about a second of data.
The point I'm trying to make is, depending on your application, you can
avoid buying external LO's, references, ect with some math effort.
However, if you are looking for hardware I suggest Ettus Octoclocks for
splitting and the following frequency synthesizer:
https://www.valonrf.com/frequency-synthesizer-4400mhz.html
Good luck with your endeavors,
Ali
On Wed, Mar 13, 2019 at 5:52 AM guowang qiu via USRP-users <
usrp-users@lists.ettus.com> wrote:
Hi all,
There are two daughter boards in N310, the phase difference between
channels of different dboards will drift rapidly.
In my test, a B200 transmits single tone continuous wave to a 1-to-2
splitter, and the outputs of splitter are connected to input ports of RF0
and RF2 of N310.
During the single program running, the signals received by the two
channels should keep the phase difference approximately stable. But I
observed that the phase drift of the two channels was very fast. Within a
time span of 20 seconds, the phase difference varied by tens of degrees.
Sense that two dboards are using independent reference clocks, not the same
reference clock from mboard. By contrast, the phase difference between the
two channels of x310 with two ubx is stable.
Does anyone know how to explain this issue?
Best regards,
Damon
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Hi Ali,
The daughterboards have their own clock generators, but they are not
exactly 'independent'. At least they don't have to be, as they share the
same reference clock. Look at the block diagram:
https://kb.ettus.com/images/9/9d/USRP_N310_N300_DB_Schematic.pdf
and "Ref Clock" block. I don't have N310 and I know that reality can be
a bit far from expectations (i.e. look at my "What makes sense and what
doesn't in the way carrier frequency is set for TwinRX currently?" post).
But maybe the daughterboards can be configured to use that reference clock.
Best Regards,
Piotr Krysik
W dniu 13.03.2019 o 19:59, Ali Dormiani via USRP-users pisze:
Hello,
You sort of said the answer yourself:
"two dboards are using independent reference clocks"
RX0 and RX1 use the same LO while RX2 and RX3 use a different LO.
I like to think of each N310 as two radios in the same box (when it
comes to phase issues). Additionally, I think UHD/GNUradio start up RF
components at run-time so all calibration needs to be preformed each
time you run a flow-graph or C file?
There are signal processing ways to mitigate this. Consider looking
into ESPRIT or array shape calibration research.
The hardware fix to this problem is feeding an external LO into each
N310 dboard.
Attached are some old results from our lab with an 8 TX 24 RX system
set up indoors (all N310s). You can see that our DOA estimate in
ESPRIT drifts badly (10 degrees or so) over about a second of data.
The point I'm trying to make is, depending on your application, you
can avoid buying external LO's, references, ect with some math effort.
However, if you are looking for hardware I suggest Ettus Octoclocks
for splitting and the following frequency synthesizer:
https://www.valonrf.com/frequency-synthesizer-4400mhz.html
Good luck with your endeavors,
Ali
On Wed, Mar 13, 2019 at 5:52 AM guowang qiu via USRP-users
<usrp-users@lists.ettus.com mailto:usrp-users@lists.ettus.com> wrote:
Hi all,
There are two daughter boards in N310, the phase difference
between channels of different dboards will drift rapidly.
In my test, a B200 transmits single tone continuous wave to a
1-to-2 splitter, and the outputs of splitter are connected to
input ports of RF0 and RF2 of N310.
During the single program running, the signals received by the two
channels should keep the phase difference approximately stable.
But I observed that the phase drift of the two channels was very
fast. Within a time span of 20 seconds, the phase difference
varied by tens of degrees. Sense that two dboards are using
independent reference clocks, not the same reference clock from
mboard. By contrast, the phase difference between the two channels
of x310 with two ubx is stable.
Does anyone know how to explain this issue?
Best regards,
Damon
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On 03/14/2019 03:56 AM, Piotr Krysik via USRP-users wrote:
Hi Ali,
The daughterboards have their own clock generators, but they are not
exactly 'independent'. At least they don't have to be, as they share the
same reference clock. Look at the block diagram:
https://kb.ettus.com/images/9/9d/USRP_N310_N300_DB_Schematic.pdf
and "Ref Clock" block. I don't have N310 and I know that reality can be
a bit far from expectations (i.e. look at my "What makes sense and what
doesn't in the way carrier frequency is set for TwinRX currently?" post).
But maybe the daughterboards can be configured to use that reference clock.
Best Regards,
Piotr Krysik
The LMK clock generator uses the reference clock from the mainboard, so
there should not be any mutual phase-jitter/drift issues. I can test this
on my N310 in the coming day or two.
What version of UHD is in use?