[USRP-users] Align CORDICS in the DSP ( N200x2, MIMO, SBX, GPSDO)

John Shields john.shields at xtra.co.nz
Fri Oct 13 16:57:00 EDT 2017


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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