[USRP-users] Receive samples at exact time_spec_t

ROBIN TORTORA tilla at comcast.net
Thu Nov 16 12:42:02 EST 2017


If you are using UHD 3.9.x or earlier, I would guess filter delay...

> On November 16, 2017 at 5:31 AM Hojoon Yang via USRP-users <usrp-users at lists.ettus.com> wrote:
> 
>     Hi.
>      
>     I use USRP B200 with external on-board GPSDO.
> 
>     1. I set the clock source and time soure to GPSDO
>         usrp->set_clock_source("gpsdo", 0);
>         usrp->set_time_source("gpsdo", 0);
>      
>     2. Set_time_next_pps and sleep 2 secs.
>         usrp->set_time_next_pps(uhd::time_spec_t(0.0), 0);
>         boost::this_thread::sleep(boost::posix_time::seconds(2));
>      
>     3. Issue the stream command to start the stream from 3.0 seconds.
>         uhd::stream_cmd_t stream_cmd(uhd::stream_cmd_t::STREAM_MODE_NUM_SAMPS_AND_DONE);
>         stream_cmd.num_samps = 100;
>         stream_cmd.stream_now = false;
>         const uhd::time_spec_t stream_time = uhd::time_spec_t(3.0);
>         stream_cmd.time_spec = stream_time;
>         rx_stream->issue_stream_cmd(stream_cmd);
>      
>     4. Start the Rx streams and check received time.
>         size_t num_rx_samps = rx_stream->recv(&buff.front(), buff.size(), md, 5.0);
>         ---------------------------------------------------------------------------------------------------
>         Received packet: 100 samples, 3 full secs, 0.000037 frac secs
>         Stream time was: 3 full secs, 0.000000 frac secs
>         Difference between stream time and first packet: 37.093750 us
>      
>     I command to start receving from 3.0 seconds, but the USRP B200 does not start at exact time(there are some delay, 37us).
>      
>     I guess it seems normal.
>      
>     (a)But is there any method to overcome this tiny difference? at least below 10 us.
>      
>     (b)Is the difference always same? If not, Could you tell me which things could change the difference? 
>      
>     (c)Can I solve the problem If I set the stream time to 3.0s - 37us?
>      
>     It will be very appreciated if someone could answer the above three questions!
>      
>     Thanks in advance.
>     _______________________________________________
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>     USRP-users at lists.ettus.com
>     http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com
> 
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