On 10/13/2017 04:57 PM, John Shields via USRP-users wrote:
Hi,
From the Hardware Driver and USRP Manual, under the section on
Align CORDICs in the DSP, it says :
In order to achieve phase alignment between USRP devices, the CORDICS
in both devices must be aligned with respect to each other. This is
easily achieved by issuing stream commands with a time spec property,
which instructs the streaming to begin at a specified time. Since the
devices are already synchronized via the 10 MHz and PPS inputs, the
streaming will start at exactly the same time on both devices. The
CORDICs are reset at each start-of-burst command, so users should
ensure that every start-of-burst also has a time spec set.
For receive, a burst is started when the user issues a stream command.
This stream command should have a time spec set:
uhd::stream_cmd_tstream_cmd(uhd::stream_cmd_t::STREAM_MODE_NUM_SAMPS_AND_DONE);
stream_cmd.num_samps = samps_to_recv;
stream_cmd.stream_now = false;
stream_cmd.time_spec = time_to_recv;
usrp->issue_stream_cmd(stream_cmd);
The next section mentions how to LO align SBXs and I have done this in
python (and it has been checked by an expert) but I still get a
variable phase offset between both channels on the USRPs and suspect
the CORDICs are spinning relatively.
So I have a couple of questions re: CORDIC alignment:
1) if my desire is to stream for a looooong time (e.g. days) what
stream command do I give in Python?
2) I have seen one posting where the person showed that alignment of
the CORDICs should happen after a timed_cmd tune – is this correct, or
should it be before, or does it not matter?
3) has anyone been able to get N200r4x2, GPSDO, MIMO cable, 2xSBX to
align with zero phase offset or, at least, a constant phase offset
which doesn’t vary between each run of the GRC file?
Kind Regards,
John
The UHD source block in GRC does all of this, except for the
timed-command wrapper for tuning.
It starts streaming at a fixed time, and uses timestamps to time-align
samples.
So the only thing left would be PLL synthesizer ambiguity.
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