You talk about board-mounted GPSDOs in each of your B210s, but then talk
about using an Octoclock-G.
In the Octoclock-G example, you are explicitly selecting "external" for
your refclock and time source?
It is fully-expected that no two GPSDOs will precisely agree on
frequency and phase, and will have some amount of mutual phase-noise.
Even when fed with the same antenna.
On 2017-09-21 10:54, Piotr Krysik via USRP-users wrote:
> Hi all,
>
> I'm trying trying to use multiple USRPs B210 (that have GPSDOs mounted)
> for tasks requiring good phase coherence between the devices (i.e.
> phased antenna array) and there is some problem. In the recorded signal
> I have found abrupt phase jumps that make B210s unusable for performing
> phase coherent tasks (and that generally might be a problem).
>
> The devices were synchronized with use of Octoclock-G providing PPS and
> 10MHz ref signal. To check phase coherence I used a signal generator
> (generating sinusoid: -40dBm at 690MHz with USRPs tuned to 690.1MHz with
> sample rate 1MHz, PPS and ref signals taken from external ports, version
> of UHD 3.9.2). See the attached picture in which I removed expected
> phase of the sinusoid (recorded with one of the channels) from the phase
> of the recorded signal. In this example phase jumps happen almost
> periodically (about every 10 seconds).
>
> Now is the funny part: after taking out GPSDO the problem goes away
> (checked with 3 USRPs B210). Hence the very probable theory is that
> GPSDO's interference causes the problem.
>
> Have you observed this? And more importantly: do you a have solution?
> (Taking GPSDOs out from all our B210s is not one of them as there are
> many valid tasks that require both GPSDOs and good quality of signal
> signal phase measurement. I still have to check if the same thing
> happens without Octoclock-G.)
>
> (in the next episode: slow start of transmission in USRP B210 :) )
>
> --
> Best Regards,
> Piotr Krysik
>
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