Hi.
We have developed a project using the USRP 2945 and UHD libraries to estimate a
signal's direction of arrival.
In this project, the four channels of the USRP 2945 are connected through a
power splitter to a function generator, and the local oscillators in the back
are connected following the user guides (LO OUT 1 IF 2 connected to LO IN 0 IF
2, and LO OUT 1 IF 1 connected to LO IN 0 IF 1).
However, it seems that, when we tune some frequencies, 400 MHz for instance, we
observe a huge decrease in power in channels 1 and 3 (50 dB).
Taking into account that, by eliminating the set_command_time instruction in
the setting tune frequency function, this problem disappears, we suspect that
this may be caused by time synchronization. Although, if we do this, 90 and 180
degrees phase offsets start appearing when we tune several frequencies.
Does anyone know what might be happening? I think the relevant code is below:
Thank you for your time.
Regards.
// SDR configuration
usrp->set_time_unknown_pps(uhd::time_spec_t());
usrp->set_rx_lo_export_enabled(true,uhd::usrp::multi_usrp::ALL_LOS,2); // 2
usrp->set_rx_lo_source("internal",uhd::usrp::multi_usrp::ALL_LOS,2); // 2
usrp->set_rx_lo_source("companion",uhd::usrp::multi_usrp::ALL_LOS,3); // 3
usrp->set_rx_lo_source("external",uhd::usrp::multi_usrp::ALL_LOS,0); // 0
usrp->set_rx_lo_source("external",uhd::usrp::multi_usrp::ALL_LOS,1); // 1
usrp->clear_command_time();
usrp->set_command_time(usrp->get_time_now()+uhd::time_spec_t(0.1));
uhd::tune_request_t tune_request(freq_);
tune_request.dsp_freq_policy = uhd::tune_request_t::POLICY_MANUAL;
for (int i = 0; i < CHANNELS; i++) {
usrp->set_rx_rate(rate_, i);
usrp->set_rx_freq(tune_request, i);
usrp->set_rx_gain(gain_, i);
usrp->set_rx_bandwidth(bandwidth_, i);
}
std::this_thread::sleep_for(std::chrono::milliseconds(110));
usrp->clear_command_time();
// A method to tune frequencies
bool Acquirer_SDR::sdr_set_tune_frequency(usrp_2945* radio, float freq, int
channels) {
bool success = true;
std::cout << "Setting tune frequency: " << freq << std::endl;
radio->usrp->clear_command_time();
radio->usrp->set_command_time(radio->usrp->get_time_now()+uhd::time_spec_t(0.1));
uhd::tune_request_t tune(freq);
tune.dsp_freq_policy = uhd::tune_request_t::POLICY_MANUAL;
for (int i = 0; i < channels; ++i) {
std::cout << " Ch" << i << " ";
radio->usrp->set_rx_freq(tune, i);
std::cout << radio->usrp->get_rx_freq(i);
if (std::abs(radio->usrp->get_rx_freq(i) - freq) > MAX_PRECISION){
std::cout << " ERR";
success = false;
}
else{
radio->freq_ = freq;
std::cout << " OK";
}
}
std::cout << "\n";
std::this_thread::sleep_for(std::chrono::milliseconds(110));
radio->usrp->clear_command_time();
return success;
}
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