On Mon, Sep 4, 2017 at 6:14 PM, Marcus D. Leech via USRP-users < usrp-users@lists.ettus.com> wrote:
> On 09/04/2017 04:54 PM, Michael M via USRP-users wrote: > >> I'm having an issue with my Ettus E310 where I'm seeing inconsistent >> timestamping from one run to another. >> >> First, I set up a reference transmitter that is designed to output a >> waveform at the top of each second. The transmitter has been tested and >> shown to be within a few nanoseconds of GPS time with other test >> equipment. This transmitter is connected to the RX2 input of the USRP E310. >> >> I am then setting the time source on the E310 to GPSDO and the clock >> source to INTERNAL. I am waiting until the gps is locked and the reference >> is locked. Once this is done, we are getting the gps time from the E310 >> and setting the clock to what the GPS time should be at the next 1PPS. >> This is all working fine and the time looks correct. >> >> Then, I receive the signal, write it out to files with the time stamps >> and correlate with the signal that is being transmitted by the reference >> transmitter. Within one run, I'm getting a standard deviation of under >> 50ns for arrival time relative to the top of each second. If I stop >> receiving and start again, I am again getting a standard deviation of under >> 50ns for arrival time. The problem is that that arrival time varies as >> much as a couple of microseconds between separate recordings. >> >> My question is, can anyone think of what might be the cause of this >> variance from one run to another? Could it be something I'm doing wrong? >> Could the process of locking the digital PLL to the 1pps reference have >> enough jitter that once locked it is tracking a value that can be off by a >> couple microseconds? I'd like to be able to get repeatable results from >> one run to another (within 100ns). >> >> Thanks, >> >> Michael >> >> How are you starting your receive process? How is that time-coordinated? Here's a quick snippet of what we were doing. We started with the usrp_source_impl in gnuradio ( https://github.com/gnuradio/gnuradio/blob/master/gr-uhd/lib/usrp_source_impl.cc), and added the following code to set the timecode to gps time: this->set_clock_source("internal",0); //Set time source to GPSDO this->set_time_source("gpsdo",0); //Wait for GPS Lock while(this->get_mboard_sensor("gps_locked",0).value != "true") { boost::this_thread::sleep(boost::posix_time::seconds(2)); } //Wait for REF Lock while(this->get_mboard_sensor("ref_locked",0).value != "true") { boost::this_thread::sleep(boost::posix_time::seconds(2)); } //Extra Sleep never Hurts boost::this_thread::sleep(boost::posix_time::seconds(5)); //Set PPS Time long long this_pps, last_pps; this_pps = last_pps = this->get_time_last_pps(0).to_ticks(1.0); //Wait for PPS Edge while (this_pps == last_pps) { this_pps = this->get_time_last_pps(0).to_ticks(1.0); } boost::this_thread::sleep(boost::posix_time::seconds(.2)); this->set_time_next_pps(::uhd::time_spec_t(this->get_mboard_sensor("gps_time",0).to_int() + 1.0)); this_pps = last_pps = this->get_time_last_pps(0).to_ticks(1.0); //Wait for PPS Edge while (this_pps == last_pps) { this_pps = this->get_time_last_pps(0).to_ticks(1.0); } boost::this_thread::sleep(boost::posix_time::seconds(.2)); int sec_gps = this->get_mboard_sensor("gps_time",0).to_int(); long long pps_sec = this->get_time_last_pps(0).to_ticks(1.0); if (sec_gps == pps_sec) { std::cout << "SYNC GOOD" << std::endl; } else { std::cout <<"FAILED TO SYNC" <<std::endl; } Then, in the work method, we also added a check to see if 10 seconds had elapsed, in which case we tag the output to include the new timecode: (fullSecs is initialized to 0 and newGoal is initialized to true when starting) ... if(newGoal && fullSecs != 0) { newGoal = false; goalSecs = fullSecs + 10; //std::cout << "New Goal Set" << std::endl; } switch(_metadata.error_code) { case ::uhd::rx_metadata_t::ERROR_CODE_NONE: if(fullSecs >= goalSecs) { _tag_now = true; newGoal = true; } else { fullSecs = _metadata.time_spec.get_full_secs(); //std::cout << fullSecs << std::endl; } if(_tag_now) { _tag_now = false; fullSecs = _metadata.time_spec.get_full_secs(); //create a timestamp pmt for the first sample const pmt::pmt_t val = pmt::make_tuple (pmt::from_uint64(_metadata.time_spec.get_full_secs()), pmt::from_double(_metadata.time_spec.get_frac_secs())); //create a tag set for each channel for(size_t i = 0; i < _nchan; i++) { this->add_item_tag(i, nitems_written(0), TIME_KEY, val, _id); this->add_item_tag(i, nitems_written(0), RATE_KEY, pmt::from_double(_samp_rate), _id); this->add_item_tag(i, nitems_written(0), FREQ_KEY, pmt::from_double(this->get_center_freq(i)), _id); } } break; ... We then wrote a gnuradio block that writes out files every time we get a new tag (using the previously received timestamp, approximately every 10 seconds). We connected that block to the output of the usrp_source We then used external processing to do the correlations with the reference file.
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