Re: N310 phase sync issue with antenna array

[Marcus D. Leech]

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?