On 06/29/2021 04:51 PM, Larry wrote:
Hello everyone,
I am having an issue with achieving a phase-synchronous RF
configuration using an N310 with an Octoclock and a linear antenna
array. What I believe should be the expected results of this setup is
that each AD9371 should receive synchronous signals aligned in phase,
with a random 180 degree offset between each AD9371 transceiver
(please correct me if this assumption if wrong). Here is a summary of
my setup, issues, and outcomes:
1) Hardware/software setup: Using an N310 running HG image at
1-gigabit network connection on UHD version 3.15 on Ubuntu 18.04. This
is supported by an Octoclock-G serving as the 10MHz reference and PPS
source for the N310. Equal length cables are used between all channels
of the N310, to facilitate better phase synchronization. My GnuRadio
flowgraph consists of a USRP source into a simple squelch, feed
forward AGC, frequency xlating fir filter, and then converted from
complex to real going into a QT time sink.
2) Testing & results: I am attempting to receive a bursty signal using
a four element linear dipole antenna array, with the elements spaced
slightly under lambda/2 distance apart. Two main tests have been
performed; one with the N310 directly wired to another SDR that is
injecting a generated sine wave into the N310, and another test over
the air using a radio transmitter.
i. Testing with a wired connection results in the correct expected
results - phase-aligned signals, with the channel pairs on each AD9371
transceiver offset by presumably + or - 180 degrees. I can then align
using simple delays to achieve phase alignment between all channels.
This works with 2, 3, or 4 channels used.
ii. Testing over the air results in very unsynchronized signals among
all four channels. These results tend to be repeatable and consistent
in their behavior, but the channels all are received both wildly out
of phase (even channels on the same AD9371 transceiver), and even
(depending on location of the transmitter relative to the antenna
array) inverted in amplitude relative to other channels (particularly
interesting was that the imaginary component of one channel would
match the inverse of a different channel's real component). This test
has been performed at ranges exceeding 75~ feet, and as near as 5 feet
away. The results are similar in either situation. It is also worth
noting that varying the transmitter's location parallel to the antenna
array (finding a 'sweet spot', so to speak) resulted in at most 2,
possibly 3 of the channels to align properly in phase without
calibrating using delays (at least one channel would always stay
wildly different). Testing over the air using fewer than 4 channels
yields marginally improved, but overall similarly poor results.
I have tried using an external LO source for the N310 as well as
operating the Octoclock with and without GPS functionality enabled. I
have varied the sample rates, distances, and testing environments as
well as changing cables and splitters to try to rule out any hardware
component errors. These seem to have no real impact on the strange
results I get with the over the air RF configuration. Any help to
sanity check or troubleshoot my issues would be greatly appreciated.
Thank you!
Given that the N310 has NO WAY of distinguishing between signals
arriving from some wired emitter and those arriving from an antenna
array, I can't for the life of me see how this could be N310/GnuRadio
related. How would it know?