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_t stream_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
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