On 09/15/2017 05:38 PM, John Shields wrote:
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?
Yes. That will have the effect of asserting the "phase resynch" pin on
the SBX's synthesizer at the exact same time.
2) based on the comment about stream synch being controlled by the
scheduler, what do I need to do re: synching the CORDICs?
That should happen as a side effect of tuning at the same time.
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 ?
Do a get_time_last_pps, saved that value. Spin doing get_time_last_pps
until it is different from the saved value. Pause a few 10s of
milliseconds on each
spin.
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?
Slow phase rotation is just a an inevitable property of analog hardware
and temperature differentials. Some of the big observatories do active
mitigation by sending measurement tones down the coax toward the
antenna and measuring phase slip, and compensating.
In my experience, I don't see this on our 600Mhz interferometer after it
has equilibrated after start-up.
What is the reason for fringe-stopping? In small-scale amateur
observations, it is mostly a "frill". I have thought of using it for
interferometric
pulsar observations, where you want to keep the correlation at
maximum over a long period (at least as long as the primary field-of-view).
Kind Regards,
John
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