On 09/06/2017 04:59 PM, Cho, Daniel J (332C) via USRP-users wrote:
Hello –
I am trying to frequency sync two USRP B210s. I have a GPSDO on both
USRPs so the 10 MHz ref is unavailable (as noted in the site manual)
and I am unable to use the GPSDO yet because I do not have a GPS
antenna (it is coming in the mail). But so that I don’t just sit and
wait for the GPS antennas to arrive, I wanted to try frequency
synching the two USRP using a 1PPS reference input.
I generated a 1 PPS square wave using an arbitrary waveform generator
with a 20% duty cycle at 5V high and 0V low. I fed the square waves
into the “PPS in” input. On the GnuRadio, I set it to “unknown PPS”
and clock source to “external” for both USRPs. I ran the flowgraph
(which is just one B210 transmitting and one B210 receiving and
sending the data to a QT frequency/constellation sink) and there is
not synchronization. There is still a rotation in the constellation
plot.
How can I get the 2 USRPs frequency synched?
Thanks,
Daniel Cho
In order for two devices to be perfectly time-and-frequency synched,
they must both see *exactly* the same clock and PPS source. In your case,
what will happen is that the board, seeing no external clock, will
use its internal clock. While your time-of-day will agree on the two
devices, they'll
be running off different system clocks, so will not stay in perfect
phase-and-frequency synchronism for more than a few seconds at a time.
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