[Discuss-gnuradio] Python synchronisation of 2xUSRP N200 with SBX in one UHD container.

Hi,

    One has O/B GPSDO, the other connected by MIMO cable on Ubuntu 
with UHD_003.010.002.000

    From the /manual/page_sync.html page there is information on how to
align the CORDICs and LOs of SBX.

    I have modified the python generated by a GRC FG and here are the
relevant changes I made:

        self.uhd_usrp_source_0.set_clock_source('gpsdo', 0)
        self.uhd_usrp_source_0.set_time_source('gpsdo', 0)
        self.uhd_usrp_source_0.set_clock_source('mimo', 1)
        self.uhd_usrp_source_0.set_time_source('mimo', 1)
        self.uhd_usrp_source_0.set_samp_rate(samp_rate)
self.uhd_usrp_source_0.set_time_unknown_pps(uhd.time_spec())
    # inserted code to align CORDICs ?

    # inserted code to synch LO
self.tune_time=self.uhd_usrp_source_0.get_time_now(0)+uhd.time_spec_t(0.1)
# tune in 100 msecs from 'now'
        self.uhd_usrp_source_0.set_command_time(self.tune_time)
        self.uhd_usrp_source_0.set_center_freq(freq, 0)
        self.uhd_usrp_source_0.set_center_freq(freq, 1)
        self.uhd_usrp_source_0.clear_command_time()

        self.uhd_usrp_source_0.set_gain(rf_gain, 0)
        self.uhd_usrp_source_0.set_antenna('RX2', 0)

        self.uhd_usrp_source_0.set_gain(rf_gain, 1)
        self.uhd_usrp_source_0.set_antenna('RX2', 1)

While there is a discussion on the web re: the tune_time which seems to
suggest, I could just put in the, uhd.time_spec_t(0.1), there is a
comment from Josh saying to make sure the time is in the future so that
is why I added the time now.

In that same thread there were comments to the effect that stream_cmds
have not been swig-ed as the control of streaming is part of the
scheduler so:

questions:

    1) have I got the correct code to synch the LO i.e. by tuning both
devices at the same exact time?

    2) based on the comment about stream synch being controlled by the
scheduler, what do I need to do re: synching the CORDICs?

    3) I have noticed that sometimes when I print out the real_seconds
of time_now, it has a low number (likely the number of seconds the USRPs
have been running.

    -- Setting references to the internal GPSDO
    --     1) catch time transition at pps edge
    --     2) set times next pps (synchronously)

        I presume that I am reading the time_now before it is synched
at the next pps where the value of time goes to 1505510864.00000 . If
the answer to 1) is that my code is correct, how do I make sure that the
pps edge has occurred before I get_time_now ?

    4) the sync document also implies that the phase offset will drift
(due to thermal and other characteristics) so phase-coherent
applications will need re-calibration. is this done by somehow stopping
the stream(s), doing a set_freq and whatever the answer to 2) above is
and then restarting. I am trying to collect complex visibilities over a
long period of time ( i.e. fringe stopping) so wonder if the drift is
slow enough whether I need to do the "calibration" mentioned in the sync
doc?

                          Kind Regards,

                                       John

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