Thanks, as always, Marcus,
Here is the relevant code I
am executing:
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())
# now put in code to make sure that both usrps have seen at least 1
pps - pulse
# self.save_last_pps=self.uhd_usrp_source_0.get_last_pps(0)
# as the USRPs have been running for hours will tackle the 'spin
until get_last_pps changes' code later
# inserted code to synch LO and, by way of the mechanism, align the
CORDICs
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+.45, 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)
when I run the FG, I get variable results - I attach 2 screenshots
which show the averaged phase difference is 74-odd degrees for one run
and the next run the result is 101-odd degrees. Clearly, I am doing
something wrong but, for the life of me, I cannot figure it out. For
completeness, I will later put this code, when it works into the
set_freq function but I don't have this parameter changeable in the FG.
Kind Regards,
John
On 18/09/17 03:20, Marcus D. Leech wrote:
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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