On 2022-09-08 04:54, Aiden Morrison via USRP-users wrote:
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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With the WBX, even with common clocks between them, there will be a
180deg phase ambiguity because of the
2XLO mixer. This is unavoidable, even when using timed-command
tuning to phase-align the synthesizers.
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