Hi,
After lot of debugging I have some progress:
1.) Having multiple channels is not enough since it only sets _stream_now=FALSE
but not _start_time. Consequently both Source+Sink use get_time_now()+0.1 which
may be different
2.) set_unknown_pps only sets the the time to zero at next pps
3.) "Sync: Unknown PPS" (set_unknown_pps) has to be only set in either USRP
Source or Sink since they share the common motherboard. (having it in both
still seems to work but takes longer to start because setting time is done
twice, I think)
4.) set_start_time *has* to be called on *both* USRP Source/Sink. This can't be
done via grc and requires py edits.
Now comes the part that I don't understand. When I use:
self.uhd_usrp_sink_0.set_start_time(uhd.time_spec(1.0))
self.uhd_usrp_source_0.set_start_time(uhd.time_spec(1.0))
I always get a string of "L" and the flowgraph is stuck. No matter which number
I put here. After long time I figured out that the start time for USRP Sink has
to be at least 100ms later. This works:
self.uhd_usrp_sink_0.set_start_time(uhd.time_spec(1.1))
self.uhd_usrp_source_0.set_start_time(uhd.time_spec(1.0))
1.0001 does not work and neither does 1.001, 1.01 etc. But any number larger
than 1.1 does.
WHY is this the case?
Also, why do we need to wait so long (100ms does not work, I get "WARN: USRP
Source Block caught rx error code: 2" and "U")? What exactly has to happen
within this one second and what are the cases when this one second may not be
enough?
Lukas
> Hello,
>
> I am using USRP X310+UBX+gnuradio for an application where my transmitted
> signal (TX) is reflected and received (RX).
> For my tests I use the simple loopback config (TX -> 30dB Attn -> RX),
> transmit a pulse and plot both on the same plot (triggered in the TX pulse).
>
> The response comes around a whopping 38ms later!
> I added a delay block before plotting the TX signal but the delay is still
> random each time I restart the flowgraph (residual time delays up to 700us).
>
> I wonder if I can achieve sample-accurate alignment that is consistent across
> flowgraph runs (maybe even USRP power cycles).
>
> RX+TX Streaming starts at the same time to my understanding because both my
> USRP Source+Sink have two ports and I use the setting "Sync: Unknown PPS" as
> discussed some time on this list.
>
>
> Thanks,
> Lukas
>
>
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