We're working on a two-channel phase interferometry application using an
X410 with two ZBX daughterboards and am having difficulty achieving
consistent inter-channel phase measurements across application restarts.
I'm hoping someone on the list has experience with this and can point me in
the right direction. I've found quite a few emails on the mailing list from
about a year ago on the topic, but they didn't seem to have a resolution
(as far as I could tell).
Here's my current configuration:
NI USRP X410 (Rev 7)
Two ZBX daughterboards
MPM Version: 5.3
FPGA Version: 8.3 (UC_200)
FPGA git hash: c37b318.clean
I'm using UHD 4.7.0.0 with the Multi_USRP API in C++. The ports I'm
connected to are the RX1 SMA ports on the same daughterboard (A:0 and A:1).
The X410 is also connected to an external 10 MHz + 1 PPS reference from an
Octoclock. I'm feeding these receive ports with a pulsed waveform from a
signal generator connected via a splitter.
Problem:
Within a single execution of the application, the phase difference between
the two channels is extremely stable and consistent pulse-to-pulse.
However, when the application is restarted the phase offset changes to an
essentially arbitrary value. Across five consecutive runs we observed delta
phase values of approximately -25, -154, -25, 74, and -119 degrees at 3050
MHz.
The same code (timed command tune requests) and test equipment setup was
working with an X310. The phase differences were consistent over a period
of a few days of application restarts and power cycles. It seems like there
are some fundamental architectural differences between the X310 with two
UBX-160s and the X410 with two ZBXs that prevent the same task from being
accomplished.
Here are the steps we've taken thus far to try and get the same initial
phase on both channels from one run of the application to another:
1. Switched clock and time source to "external" to lock to the Octoclock's
10 MHz and 1 PPS reference and polled the ref_locked sensor before
proceeding.
2. Used set_time_next_pps() rather than set_time_now() to latch device time
on a PPS edge, followed by a sleep longer than one second to guarantee the
latch has occurred.
3. Configured subdev spec (A:0 A:1), per-channel parameters (sample rate,
bandwidth, gain, antenna), and get_rx_stream() all after the ref_locked
wait and PPS latch.
4. Issued simultaneous tune commands to both channels using
set_command_time() aligned to the next PPS edge
(get_time_now().get_full_secs() + 1.0), followed by another sleep greater
than 1 second to allow LO lock.
5. Attempted explicit LO sharing via set_rx_lo_source("internal",
"LO1"/"LO2", chan) on both channels, however set_rx_lo_export_enabled() was
not supported on this radio so we abandoned that.
The within-run phase stability is excellent (sub-degree variation
pulse-to-pulse), which suggests the hardware is working correctly. The
problem appears to be that something is initializing to an arbitrary phase
state on each UHD session that is not being reset by any of the above steps.
The ultimate question we have: with an X410 and two ZBX daughterboards is
multi-channel phase coherence across multiple UHD sessions possible? If so,
are there any steps we might be missing or out of order from what we tried
above? Feeding in a known signal every time we start a session is not an
option.
There was some chatter in the mailing list about newer versions of UHD
resolving this, but other replies from March 2025 left it ambiguous (to me
at least) as to whether this was fixed for X410s or X440s or fixed at all.
Any guidance you all might provide is greatly appreciated!
Best,
Chris
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