Hi,all, I'm using tow E312 to estimate the arrive time difference between received signals.The program is explained as follows: uhd::usrp::multi_usrp::sptr UsrpDev = uhd::usrp::multi_usrp::make(“”); UsrpDev->set_time_source("gpsdo"); UsrpDev->set_clock_source("internal", uhd::usrp::multi_usrp::ALL_MBOARDS); ... uhd::stream_args_t StreamArgs; StreamArgs.cpu_format ="fc32"; StreamArgs.channels.clear(); StreamArgs.channels.push_back(0); uhd::rx_streamer::sptr RxStreamDev = UsrpDev->get_rx_stream(StreamArgs); ... UsrpDev->set_rx_gain(curGain); UsrpDev->set_rx_rate(curRxRates); UsrpDev->set_rx_freq(tune_request, 0); ... UsrpDev->set_time_next_pps(uhd::time_spec_t(GpsTime + 1.0)); ... uhd::stream_cmd_t CapCmdContStart( uhd::stream_cmd_t::STREAM_MODE_START_CONTINUOUS ); CapCmdContStart.stream_now = false; CapCmdContStart.num_samps = IQ_NUMS; CapCmdContStart.time_spec = uhd::time_spec_t((double)nextBoardTime); RxStreamDev->issue_stream_cmd(CapCmdContStart); ... size_t RxNums = RxStreamDev->recv( RecvBuffVec, RecvNums, RxMetaData, timeoutD, false ); ... Right now, the data blocks received from the two receivers have strictly aligned time stamps: [image: 内嵌图片 1] However, the time difference between IQ data is irregular. In repeated tests, the range of thetime difference of the signal at the same location may reach thousands of sampling points. For example, the IQ data of the two data blocks are shown in the following figure. These two blocks are captured from the GSM downlink by two E312 in same location, and the blocks have the same start timestamps. In the figure, the red line is I, and yellow line is Q.According to the GSM frequency correction burst, we can see that the TDOA error of the two data blocks is very large. There is no GSM frequency correction burst in the first data block, but there is a GSM frequency correction burst in the second data blocks(2200~2500). [image: 内嵌图片 2] On the other hand, we use “STREAM_MODE_NUM_SAMPS_AND_DONE” commands to collect data, and the time difference is equally large and irregular. We use Linux ppstest command to test the PPS of the GPS, and the display is normal. Our problem is that , 1) does the E312 of the GPS have a dedicated 1PPS output pin available to the FPGA?; 2) can E312 be used for TDOA measurement and, if possible, how to implement it in the program? We should appreciate an early reply, a prompt reply would help us greatly. best reagrds Jon

Maybe the accuracy of PPS is not enough ?Any suggestion is welcome. thank you. best regards Jon 2017-10-16 17:17 GMT+08:00 liu Jong <jongliu1987@gmail.com>: > Hi,all, > > I'm using tow E312 to estimate the arrive time difference between received > signals.The > program is explained as follows: > > uhd::usrp::multi_usrp::sptr UsrpDev = uhd::usrp::multi_usrp::make(“”); > UsrpDev->set_time_source("gpsdo"); > UsrpDev->set_clock_source("internal", uhd::usrp::multi_usrp::ALL_MBOARDS); > ... > uhd::stream_args_t StreamArgs; > StreamArgs.cpu_format ="fc32"; > StreamArgs.channels.clear(); > StreamArgs.channels.push_back(0); > uhd::rx_streamer::sptr RxStreamDev = UsrpDev->get_rx_stream(StreamArgs); > ... > UsrpDev->set_rx_gain(curGain); > UsrpDev->set_rx_rate(curRxRates); > UsrpDev->set_rx_freq(tune_request, 0); > ... > UsrpDev->set_time_next_pps(uhd::time_spec_t(GpsTime + 1.0)); > ... > uhd::stream_cmd_t CapCmdContStart( uhd::stream_cmd_t::STREAM_MODE_START_CONTINUOUS > ); > CapCmdContStart.stream_now = false; > CapCmdContStart.num_samps = IQ_NUMS; > CapCmdContStart.time_spec > = uhd::time_spec_t((double)nextBoardTime); > RxStreamDev->issue_stream_cmd(CapCmdContStart); > ... > size_t RxNums = RxStreamDev->recv( RecvBuffVec, RecvNums, RxMetaData, > timeoutD, false ); > ... > > > Right now, the data blocks received from the two receivers have strictly > aligned time stamps: > > [image: 内嵌图片 1] > > However, the time difference between IQ data is irregular. In repeated > tests, the range of thetime difference of the signal at the same location > may reach thousands of sampling points. > For example, the IQ data of the two data blocks are shown in the following > figure. These two blocks are captured from the GSM downlink by two E312 in > same location, and the blocks > have the same start timestamps. In the figure, the red line is I, and > yellow line is Q.According to the GSM frequency correction burst, we can > see that the TDOA error of the two data > blocks is very large. There is no GSM frequency correction burst in the > first data block, but there is a GSM frequency correction burst in the > second data blocks(2200~2500). > > > [image: 内嵌图片 2] > > > On the other hand, we use “STREAM_MODE_NUM_SAMPS_AND_DONE” commands > to collect data, and the time difference is equally large and irregular. > We use Linux ppstest command to test the PPS of the GPS, and the display > is normal. > > Our problem is that , > 1) does the E312 of the GPS have a dedicated 1PPS output pin available to > the FPGA?; > 2) can E312 be used for TDOA measurement and, if possible, how to > implement it in the > program? > We should appreciate an early reply, a prompt reply would help us greatly. > > best reagrds > Jon > >