Hongwei,
In addition to the PLL that tracks the external 10MHz ref to produce the
internal 10 MHz ref, there are the four PLLs that track the internal 10 MHz
ref to produce the four LO signals (2Tx/2Rx) as you mentioned. But, the
phase variation from the LO PLLs is much less - I don't really know
the reason for this. This is why you see less noisy results for the cases
where the transmit and receive channels are on one device.
However, even in the one device case, if you compare the phase stability of
a system with "shared LO" such as can be implemented with X310/TwinRx,
N310, or N321 systems, you will see that the relative phase is
significantly less noisy on these devices as compared to the X310/UBX
one-device results ("stable" results) in your case.
Rob
On Thu, May 2, 2024 at 4:01 AM zhou <hwz...@yahoo.com> wrote:
> Thanks, Rob and Marcus.
>
> A single tone is repeatedly transmitted to make a continuous stream out of
> the Tx antenna. It is periodically sampled in Rx; the interval is about 1s.
> The sample time is aligned with the beginning of the transmitted signal to
> make sure we sample at the same time across antennas. We calculate the
> angle of the first complex sample of capture to evaluate the signal phase.
>
> I agree with you that there can be some phase wobble between the 10M ref
> signals applied to PLLs in two devices because of temperature or other
> random factors, however, inside a USRP, there are four independent PLLs for
> 2 Tx and 2 Rx. The 10M ref signals to them should also be wobbling. But my
> measurements show that within the same USRP, phase is pretty stable between
> Tx and Rx. Any explanation on this?
>
> Kind regards.
> Hongwei
>
>
> On Wednesday, 1 May 2024 at 20:34:15 BST, Marcus D Leech <
> patchvonbr...@gmail.com> wrote:
>
>
> It’s also why you can’t get tight instantaneous phase alignment between
> two GPSDO devices even when on the same antenna.
>
> Sent from my iPhone
>
> On May 1, 2024, at 2:12 PM, Rob Kossler <rkoss...@nd.edu> wrote:
>
>
> The 10 MHz ref supplied to each X310 device is used in a PLL in each
> device to obtain the 10MHz ref used for that device (and for disciplining
> the various LOs on the device). Thus, there is a relative phase "wobble"
> between the 10MHz ref signals used on each device as each PLL continuously
> adjusts to maintain disciplined output. Over time, this averages out to
> zero. But, instantaneously, it is not. So, my question is: how
> instantaneous is your phase measurement? If you instead calculate a phase
> averaged over numerous samples, can you get a consistent result? From your
> plot, it looks like this is true.
> Rob
>
> On Wed, May 1, 2024 at 11:04 AM zhou via USRP-users <
> usrp-users@lists.ettus.com> wrote:
>
>
>
> On Wednesday, 1 May 2024 at 15:15:12 BST, Marcus D. Leech <
> patchvonbr...@gmail.com> wrote:
>
>
> On 01/05/2024 10:11, zhou via USRP-users wrote:
>
> Hi Marcus,
>
> Thanks for your response.
>
> "Are you setting up clocking identically for both USRPs? That is setting
> the reference clock to "external" and the 1PPS source to "external" on both
> devices? Are you using a single multi_usrp object for all RX channels?"
>
> Yes, I use multi_usrp multi_usrp::make(
> 'addr0=192.168.12.2,second_addr0=192.168.13.2,addr1=192.168.14.2,second_addr1=192.168.15.2,master_clock_rate=184.32e6'
> )
>
> "external" set for both ref and pps:
> usrp->set_clock_source("external")
> usrp->set_time_source("external")
> I think this should automatically set both devices.
>
> "What type of daughtercards are in your X310?"
> UBX
>
> Kind regards.
>
> And, to clarify, this is an Octoclock-G, and not a plain Octoclock ?
>
> It is OctoClock GPSDO, and Internal is used.
>
>
>
>
> On Wednesday, 1 May 2024 at 14:19:44 BST, Marcus D. Leech
> <patchvonbr...@gmail.com> <patchvonbr...@gmail.com> wrote:
>
>
> On 01/05/2024 08:25, zhou via USRP-users wrote:
>
> Hi All,
>
> I am trying to use 4Rx and 4Tx antennas from two X310 USRPs. I hope the
> received signals have stable phase relationship but they don't seem to be.
> I am wondering why and how to fix it.
>
> I measured the phase using the connection as below:
> [image: Inline image]
> cos(t)+i*sin(t) signal is split into and received on four Rx antennas. Two
> X310s are connected to the same OctoClock for 10MHz Ref and PPS. Tx and Rx
> commands are all timed. The measurement results are as below:
>
>
> The Tx signal is continuous during test. I measured phase every second for
> 20 sec. In the 2nd USRP, the phases are stable on both antennas while it is
> not in the 1st. If I change the Tx signal to the 1st USRP, then the results
> swap - phases become stable in the 1st USRP and unstable in the 2nd.
>
> My first though was that there might be small CFO between USRPs even
> though both are connected to the OctoClock, but CFO should have caused
> linear change. Here, the phase offset is not linear and kind of random
> within 20 second measurement.
>
> What can be the reason? Any suggestion will be appreciated.
>
> Kind regards,
> H.
> Are you setting up clocking identically for both USRPs? That is setting
> the reference clock to "external" and the 1PPS source to "external" on both
> devices? Are you using a single multi_usrp object for all RX channels?
>
> What type of daughtercards are in your X310?
>
>
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