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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