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Re: N310 phase sync issue with antenna array

From: Marcus D. Leech
Subject: Re: N310 phase sync issue with antenna array
Date: Tue, 29 Jun 2021 16:58:09 -0400
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.7.0

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?

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