Hello, I am attempting to do synchronized multi-site reception using USRP N210 radios. Our first attempt was to make use of 2x radios with ettus GPSDO modules installed, feeding the radios with a common PRN sequence and correlating the outputs to evaluate timing offset. The Ettus GPSDO radios say they provide +/-60ns timing accuracy, so even in the worst case where one of the radios goes to each side of the limit, at 10 MHz we would have slightly more than 1 sample of offset between them, but in testing we observed +0 to +3 samples of offset.
To isolate the problem we repeated the test with two different N210 radios fed by a common external 10 MHz and PPS signal from a GNSS disciplined Rubidium source, and observed exactly the same +0 to +3 samples of shift between the captured sequences. Seeing the same error when using a common clock indicates the problem is very likely to be in our test software. In our software we follow https://files.ettus.com/manual/page_sync.html for synchronization: 1. We create two USRP objects, one for each radio. 2. We command the USRPs to use the external synchronization (radios report back that they are synchronized to the GPSDO in the former configuration) 3. We use a common time in the time spec commands for each configuration change (e.g. setting LO frequencies) 4. For each burst capture of 50,000 samples we set a time_spec for each USRP a long time in the future (1.0 seconds since the last capture) 5. Step 4 is repeated 1000 times giving us 1000 bursts for evaluating the synchronization The only factor I can see is that we are using WBX radio modules, and we are uncertain if this is somehow a factor in what we are seeing. Thanks in advance for feedback. -Aiden
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